delta-find: move is_good_delta_info on the _DeltaSearch class

There is a lot of format specific code in `is_good_delta_info`, moving it on
_DeltaSearch will allow to split this into subclass soon.

mercurial/revlogutils/deltas.py

@@ -1,1676 +1,1678 b''
 # revlogdeltas.py - Logic around delta computation for revlog
 #
 # Copyright 2005-2007 Olivia Mackall <[e-mail]>
 # Copyright 2018 Octobus <[e-mail]>
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 """Helper class to compute deltas stored inside revlogs"""
 
 
 import collections
 import struct
 
 # import stuff from node for others to import from revlog
 from ..node import nullrev
 from ..i18n import _
 
 from .constants import (
     COMP_MODE_DEFAULT,
     COMP_MODE_INLINE,
     COMP_MODE_PLAIN,
     DELTA_BASE_REUSE_FORCE,
     DELTA_BASE_REUSE_NO,
     KIND_CHANGELOG,
     KIND_FILELOG,
     KIND_MANIFESTLOG,
     REVIDX_ISCENSORED,
     REVIDX_RAWTEXT_CHANGING_FLAGS,
 )
 
 from ..thirdparty import attr
 
 from .. import (
     error,
     mdiff,
     util,
 )
 
 from . import flagutil
 
 # maximum <delta-chain-data>/<revision-text-length> ratio
 LIMIT_DELTA2TEXT = 2
 
 
 class _testrevlog:
     """minimalist fake revlog to use in doctests"""
 
     def __init__(self, data, density=0.5, mingap=0, snapshot=()):
         """data is an list of revision payload boundaries"""
         from .. import revlog
 
         self._data = data
         self.data_config = revlog.DataConfig()
         self.data_config.sr_density_threshold = density
         self.data_config.sr_min_gap_size = mingap
         self.delta_config = revlog.DeltaConfig()
         self.feature_config = revlog.FeatureConfig()
         self._snapshot = set(snapshot)
         self.index = None
 
     def start(self, rev):
         if rev == nullrev:
             return 0
         if rev == 0:
             return 0
         return self._data[rev - 1]
 
     def end(self, rev):
         if rev == nullrev:
             return 0
         return self._data[rev]
 
     def length(self, rev):
         return self.end(rev) - self.start(rev)
 
     def __len__(self):
         return len(self._data)
 
     def issnapshot(self, rev):
         if rev == nullrev:
             return True
         return rev in self._snapshot
 
 
 def slicechunk(revlog, revs, targetsize=None):
     """slice revs to reduce the amount of unrelated data to be read from disk.
 
     ``revs`` is sliced into groups that should be read in one time.
     Assume that revs are sorted.
 
     The initial chunk is sliced until the overall density (payload/chunks-span
     ratio) is above `revlog.data_config.sr_density_threshold`. No gap smaller
     than `revlog.data_config.sr_min_gap_size` is skipped.
 
     If `targetsize` is set, no chunk larger than `targetsize` will be yield.
     For consistency with other slicing choice, this limit won't go lower than
     `revlog.data_config.sr_min_gap_size`.
 
     If individual revisions chunk are larger than this limit, they will still
     be raised individually.
 
     >>> data = [
     ...  5,  #00 (5)
     ...  10, #01 (5)
     ...  12, #02 (2)
     ...  12, #03 (empty)
     ...  27, #04 (15)
     ...  31, #05 (4)
     ...  31, #06 (empty)
     ...  42, #07 (11)
     ...  47, #08 (5)
     ...  47, #09 (empty)
     ...  48, #10 (1)
     ...  51, #11 (3)
     ...  74, #12 (23)
     ...  85, #13 (11)
     ...  86, #14 (1)
     ...  91, #15 (5)
     ... ]
     >>> revlog = _testrevlog(data, snapshot=range(16))
 
     >>> list(slicechunk(revlog, list(range(16))))
     [[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]]
     >>> list(slicechunk(revlog, [0, 15]))
     [[0], [15]]
     >>> list(slicechunk(revlog, [0, 11, 15]))
     [[0], [11], [15]]
     >>> list(slicechunk(revlog, [0, 11, 13, 15]))
     [[0], [11, 13, 15]]
     >>> list(slicechunk(revlog, [1, 2, 3, 5, 8, 10, 11, 14]))
     [[1, 2], [5, 8, 10, 11], [14]]
 
     Slicing with a maximum chunk size
     >>> list(slicechunk(revlog, [0, 11, 13, 15], targetsize=15))
     [[0], [11], [13], [15]]
     >>> list(slicechunk(revlog, [0, 11, 13, 15], targetsize=20))
     [[0], [11], [13, 15]]
 
     Slicing involving nullrev
     >>> list(slicechunk(revlog, [-1, 0, 11, 13, 15], targetsize=20))
     [[-1, 0], [11], [13, 15]]
     >>> list(slicechunk(revlog, [-1, 13, 15], targetsize=5))
     [[-1], [13], [15]]
     """
     if targetsize is not None:
         targetsize = max(targetsize, revlog.data_config.sr_min_gap_size)
     # targetsize should not be specified when evaluating delta candidates:
     # * targetsize is used to ensure we stay within specification when reading,
     densityslicing = getattr(revlog.index, 'slicechunktodensity', None)
     if densityslicing is None:
         densityslicing = lambda x, y, z: _slicechunktodensity(revlog, x, y, z)
     for chunk in densityslicing(
         revs,
         revlog.data_config.sr_density_threshold,
         revlog.data_config.sr_min_gap_size,
     ):
         for subchunk in _slicechunktosize(revlog, chunk, targetsize):
             yield subchunk
 
 
 def _slicechunktosize(revlog, revs, targetsize=None):
     """slice revs to match the target size
 
     This is intended to be used on chunk that density slicing selected by that
     are still too large compared to the read garantee of revlog. This might
     happens when "minimal gap size" interrupted the slicing or when chain are
     built in a way that create large blocks next to each other.
 
     >>> data = [
     ...  3,  #0 (3)
     ...  5,  #1 (2)
     ...  6,  #2 (1)
     ...  8,  #3 (2)
     ...  8,  #4 (empty)
     ...  11, #5 (3)
     ...  12, #6 (1)
     ...  13, #7 (1)
     ...  14, #8 (1)
     ... ]
 
     == All snapshots cases ==
     >>> revlog = _testrevlog(data, snapshot=range(9))
 
     Cases where chunk is already small enough
     >>> list(_slicechunktosize(revlog, [0], 3))
     [[0]]
     >>> list(_slicechunktosize(revlog, [6, 7], 3))
     [[6, 7]]
     >>> list(_slicechunktosize(revlog, [0], None))
     [[0]]
     >>> list(_slicechunktosize(revlog, [6, 7], None))
     [[6, 7]]
 
     cases where we need actual slicing
     >>> list(_slicechunktosize(revlog, [0, 1], 3))
     [[0], [1]]
     >>> list(_slicechunktosize(revlog, [1, 3], 3))
     [[1], [3]]
     >>> list(_slicechunktosize(revlog, [1, 2, 3], 3))
     [[1, 2], [3]]
     >>> list(_slicechunktosize(revlog, [3, 5], 3))
     [[3], [5]]
     >>> list(_slicechunktosize(revlog, [3, 4, 5], 3))
     [[3], [5]]
     >>> list(_slicechunktosize(revlog, [5, 6, 7, 8], 3))
     [[5], [6, 7, 8]]
     >>> list(_slicechunktosize(revlog, [0, 1, 2, 3, 4, 5, 6, 7, 8], 3))
     [[0], [1, 2], [3], [5], [6, 7, 8]]
 
     Case with too large individual chunk (must return valid chunk)
     >>> list(_slicechunktosize(revlog, [0, 1], 2))
     [[0], [1]]
     >>> list(_slicechunktosize(revlog, [1, 3], 1))
     [[1], [3]]
     >>> list(_slicechunktosize(revlog, [3, 4, 5], 2))
     [[3], [5]]
 
     == No Snapshot cases ==
     >>> revlog = _testrevlog(data)
 
     Cases where chunk is already small enough
     >>> list(_slicechunktosize(revlog, [0], 3))
     [[0]]
     >>> list(_slicechunktosize(revlog, [6, 7], 3))
     [[6, 7]]
     >>> list(_slicechunktosize(revlog, [0], None))
     [[0]]
     >>> list(_slicechunktosize(revlog, [6, 7], None))
     [[6, 7]]
 
     cases where we need actual slicing
     >>> list(_slicechunktosize(revlog, [0, 1], 3))
     [[0], [1]]
     >>> list(_slicechunktosize(revlog, [1, 3], 3))
     [[1], [3]]
     >>> list(_slicechunktosize(revlog, [1, 2, 3], 3))
     [[1], [2, 3]]
     >>> list(_slicechunktosize(revlog, [3, 5], 3))
     [[3], [5]]
     >>> list(_slicechunktosize(revlog, [3, 4, 5], 3))
     [[3], [4, 5]]
     >>> list(_slicechunktosize(revlog, [5, 6, 7, 8], 3))
     [[5], [6, 7, 8]]
     >>> list(_slicechunktosize(revlog, [0, 1, 2, 3, 4, 5, 6, 7, 8], 3))
     [[0], [1, 2], [3], [5], [6, 7, 8]]
 
     Case with too large individual chunk (must return valid chunk)
     >>> list(_slicechunktosize(revlog, [0, 1], 2))
     [[0], [1]]
     >>> list(_slicechunktosize(revlog, [1, 3], 1))
     [[1], [3]]
     >>> list(_slicechunktosize(revlog, [3, 4, 5], 2))
     [[3], [5]]
 
     == mixed case ==
     >>> revlog = _testrevlog(data, snapshot=[0, 1, 2])
     >>> list(_slicechunktosize(revlog, list(range(9)), 5))
     [[0, 1], [2], [3, 4, 5], [6, 7, 8]]
     """
     assert targetsize is None or 0 <= targetsize
     startdata = revlog.start(revs[0])
     enddata = revlog.end(revs[-1])
     fullspan = enddata - startdata
     if targetsize is None or fullspan <= targetsize:
         yield revs
         return
 
     startrevidx = 0
     endrevidx = 1
     iterrevs = enumerate(revs)
     next(iterrevs)  # skip first rev.
     # first step: get snapshots out of the way
     for idx, r in iterrevs:
         span = revlog.end(r) - startdata
         snapshot = revlog.issnapshot(r)
         if span <= targetsize and snapshot:
             endrevidx = idx + 1
         else:
             chunk = _trimchunk(revlog, revs, startrevidx, endrevidx)
             if chunk:
                 yield chunk
             startrevidx = idx
             startdata = revlog.start(r)
             endrevidx = idx + 1
         if not snapshot:
             break
 
     # for the others, we use binary slicing to quickly converge toward valid
     # chunks (otherwise, we might end up looking for start/end of many
     # revisions). This logic is not looking for the perfect slicing point, it
     # focuses on quickly converging toward valid chunks.
     nbitem = len(revs)
     while (enddata - startdata) > targetsize:
         endrevidx = nbitem
         if nbitem - startrevidx <= 1:
             break  # protect against individual chunk larger than limit
         localenddata = revlog.end(revs[endrevidx - 1])
         span = localenddata - startdata
         while span > targetsize:
             if endrevidx - startrevidx <= 1:
                 break  # protect against individual chunk larger than limit
             endrevidx -= (endrevidx - startrevidx) // 2
             localenddata = revlog.end(revs[endrevidx - 1])
             span = localenddata - startdata
         chunk = _trimchunk(revlog, revs, startrevidx, endrevidx)
         if chunk:
             yield chunk
         startrevidx = endrevidx
         startdata = revlog.start(revs[startrevidx])
 
     chunk = _trimchunk(revlog, revs, startrevidx)
     if chunk:
         yield chunk
 
 
 def _slicechunktodensity(revlog, revs, targetdensity=0.5, mingapsize=0):
     """slice revs to reduce the amount of unrelated data to be read from disk.
 
     ``revs`` is sliced into groups that should be read in one time.
     Assume that revs are sorted.
 
     The initial chunk is sliced until the overall density (payload/chunks-span
     ratio) is above `targetdensity`. No gap smaller than `mingapsize` is
     skipped.
 
     >>> revlog = _testrevlog([
     ...  5,  #00 (5)
     ...  10, #01 (5)
     ...  12, #02 (2)
     ...  12, #03 (empty)
     ...  27, #04 (15)
     ...  31, #05 (4)
     ...  31, #06 (empty)
     ...  42, #07 (11)
     ...  47, #08 (5)
     ...  47, #09 (empty)
     ...  48, #10 (1)
     ...  51, #11 (3)
     ...  74, #12 (23)
     ...  85, #13 (11)
     ...  86, #14 (1)
     ...  91, #15 (5)
     ... ])
 
     >>> list(_slicechunktodensity(revlog, list(range(16))))
     [[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]]
     >>> list(_slicechunktodensity(revlog, [0, 15]))
     [[0], [15]]
     >>> list(_slicechunktodensity(revlog, [0, 11, 15]))
     [[0], [11], [15]]
     >>> list(_slicechunktodensity(revlog, [0, 11, 13, 15]))
     [[0], [11, 13, 15]]
     >>> list(_slicechunktodensity(revlog, [1, 2, 3, 5, 8, 10, 11, 14]))
     [[1, 2], [5, 8, 10, 11], [14]]
     >>> list(_slicechunktodensity(revlog, [1, 2, 3, 5, 8, 10, 11, 14],
     ...                           mingapsize=20))
     [[1, 2, 3, 5, 8, 10, 11], [14]]
     >>> list(_slicechunktodensity(revlog, [1, 2, 3, 5, 8, 10, 11, 14],
     ...                           targetdensity=0.95))
     [[1, 2], [5], [8, 10, 11], [14]]
     >>> list(_slicechunktodensity(revlog, [1, 2, 3, 5, 8, 10, 11, 14],
     ...                           targetdensity=0.95, mingapsize=12))
     [[1, 2], [5, 8, 10, 11], [14]]
     """
     start = revlog.start
     length = revlog.length
 
     if len(revs) <= 1:
         yield revs
         return
 
     deltachainspan = segmentspan(revlog, revs)
 
     if deltachainspan < mingapsize:
         yield revs
         return
 
     readdata = deltachainspan
     chainpayload = sum(length(r) for r in revs)
 
     if deltachainspan:
         density = chainpayload / float(deltachainspan)
     else:
         density = 1.0
 
     if density >= targetdensity:
         yield revs
         return
 
     # Store the gaps in a heap to have them sorted by decreasing size
     gaps = []
     prevend = None
     for i, rev in enumerate(revs):
         revstart = start(rev)
         revlen = length(rev)
 
         # Skip empty revisions to form larger holes
         if revlen == 0:
             continue
 
         if prevend is not None:
             gapsize = revstart - prevend
             # only consider holes that are large enough
             if gapsize > mingapsize:
                 gaps.append((gapsize, i))
 
         prevend = revstart + revlen
     # sort the gaps to pop them from largest to small
     gaps.sort()
 
     # Collect the indices of the largest holes until the density is acceptable
     selected = []
     while gaps and density < targetdensity:
         gapsize, gapidx = gaps.pop()
 
         selected.append(gapidx)
 
         # the gap sizes are stored as negatives to be sorted decreasingly
         # by the heap
         readdata -= gapsize
         if readdata > 0:
             density = chainpayload / float(readdata)
         else:
             density = 1.0
     selected.sort()
 
     # Cut the revs at collected indices
     previdx = 0
     for idx in selected:
 
         chunk = _trimchunk(revlog, revs, previdx, idx)
         if chunk:
             yield chunk
 
         previdx = idx
 
     chunk = _trimchunk(revlog, revs, previdx)
     if chunk:
         yield chunk
 
 
 def _trimchunk(revlog, revs, startidx, endidx=None):
     """returns revs[startidx:endidx] without empty trailing revs
 
     Doctest Setup
     >>> revlog = _testrevlog([
     ...  5,  #0
     ...  10, #1
     ...  12, #2
     ...  12, #3 (empty)
     ...  17, #4
     ...  21, #5
     ...  21, #6 (empty)
     ... ])
 
     Contiguous cases:
     >>> _trimchunk(revlog, [0, 1, 2, 3, 4, 5, 6], 0)
     [0, 1, 2, 3, 4, 5]
     >>> _trimchunk(revlog, [0, 1, 2, 3, 4, 5, 6], 0, 5)
     [0, 1, 2, 3, 4]
     >>> _trimchunk(revlog, [0, 1, 2, 3, 4, 5, 6], 0, 4)
     [0, 1, 2]
     >>> _trimchunk(revlog, [0, 1, 2, 3, 4, 5, 6], 2, 4)
     [2]
     >>> _trimchunk(revlog, [0, 1, 2, 3, 4, 5, 6], 3)
     [3, 4, 5]
     >>> _trimchunk(revlog, [0, 1, 2, 3, 4, 5, 6], 3, 5)
     [3, 4]
 
     Discontiguous cases:
     >>> _trimchunk(revlog, [1, 3, 5, 6], 0)
     [1, 3, 5]
     >>> _trimchunk(revlog, [1, 3, 5, 6], 0, 2)
     [1]
     >>> _trimchunk(revlog, [1, 3, 5, 6], 1, 3)
     [3, 5]
     >>> _trimchunk(revlog, [1, 3, 5, 6], 1)
     [3, 5]
     """
     length = revlog.length
 
     if endidx is None:
         endidx = len(revs)
 
     # If we have a non-emtpy delta candidate, there are nothing to trim
     if revs[endidx - 1] < len(revlog):
         # Trim empty revs at the end, except the very first revision of a chain
         while (
             endidx > 1 and endidx > startidx and length(revs[endidx - 1]) == 0
         ):
             endidx -= 1
 
     return revs[startidx:endidx]
 
 
 def segmentspan(revlog, revs):
     """Get the byte span of a segment of revisions
 
     revs is a sorted array of revision numbers
 
     >>> revlog = _testrevlog([
     ...  5,  #0
     ...  10, #1
     ...  12, #2
     ...  12, #3 (empty)
     ...  17, #4
     ... ])
 
     >>> segmentspan(revlog, [0, 1, 2, 3, 4])
     17
     >>> segmentspan(revlog, [0, 4])
     17
     >>> segmentspan(revlog, [3, 4])
     5
     >>> segmentspan(revlog, [1, 2, 3,])
     7
     >>> segmentspan(revlog, [1, 3])
     7
     """
     if not revs:
         return 0
     end = revlog.end(revs[-1])
     return end - revlog.start(revs[0])
 
 
 def _textfromdelta(revlog, baserev, delta, p1, p2, flags, expectednode):
     """build full text from a (base, delta) pair and other metadata"""
     # special case deltas which replace entire base; no need to decode
     # base revision. this neatly avoids censored bases, which throw when
     # they're decoded.
     hlen = struct.calcsize(b">lll")
     if delta[:hlen] == mdiff.replacediffheader(
         revlog.rawsize(baserev), len(delta) - hlen
     ):
         fulltext = delta[hlen:]
     else:
         # deltabase is rawtext before changed by flag processors, which is
         # equivalent to non-raw text
         basetext = revlog.revision(baserev)
         fulltext = mdiff.patch(basetext, delta)
 
     try:
         validatehash = flagutil.processflagsraw(revlog, fulltext, flags)
         if validatehash:
             revlog.checkhash(fulltext, expectednode, p1=p1, p2=p2)
         if flags & REVIDX_ISCENSORED:
             raise error.StorageError(
                 _(b'node %s is not censored') % expectednode
             )
     except error.CensoredNodeError:
         # must pass the censored index flag to add censored revisions
         if not flags & REVIDX_ISCENSORED:
             raise
     return fulltext
 
 
 @attr.s(slots=True, frozen=True)
 class _deltainfo:
     distance = attr.ib()
     deltalen = attr.ib()
     data = attr.ib()
     base = attr.ib()
     chainbase = attr.ib()
     chainlen = attr.ib()
     compresseddeltalen = attr.ib()
     snapshotdepth = attr.ib()
 
 
 def drop_u_compression(delta):
     """turn into a "u" (no-compression) into no-compression without header
 
     This is useful for revlog format that has better compression method.
     """
     assert delta.data[0] == b'u', delta.data[0]
     return _deltainfo(
         delta.distance,
         delta.deltalen - 1,
         (b'', delta.data[1]),
         delta.base,
         delta.chainbase,
         delta.chainlen,
         delta.compresseddeltalen,
         delta.snapshotdepth,
     )
 
 
-def is_good_delta_info(revlog, deltainfo, revinfo):
-    """Returns True if the given delta is good. Good means that it is within
-    the disk span, disk size, and chain length bounds that we know to be
-    performant."""
-    if deltainfo is None:
-        return False
-
-    # the DELTA_BASE_REUSE_FORCE case should have been taken care of sooner so
-    # we should never end up asking such question. Adding the assert as a
-    # safe-guard to detect anything that would be fishy in this regard.
-    assert (
-        revinfo.cachedelta is None
-        or revinfo.cachedelta[2] != DELTA_BASE_REUSE_FORCE
-        or not revlog.delta_config.general_delta
-    )
-
-    # - 'deltainfo.distance' is the distance from the base revision --
-    #   bounding it limits the amount of I/O we need to do.
-    # - 'deltainfo.compresseddeltalen' is the sum of the total size of
-    #   deltas we need to apply -- bounding it limits the amount of CPU
-    #   we consume.
-
-    textlen = revinfo.textlen
-    defaultmax = textlen * 4
-    maxdist = revlog.delta_config.max_deltachain_span
-    if not maxdist:
-        maxdist = deltainfo.distance  # ensure the conditional pass
-    maxdist = max(maxdist, defaultmax)
-
-    # Bad delta from read span:
-    #
-    #   If the span of data read is larger than the maximum allowed.
-    #
-    #   In the sparse-revlog case, we rely on the associated "sparse reading"
-    #   to avoid issue related to the span of data. In theory, it would be
-    #   possible to build pathological revlog where delta pattern would lead
-    #   to too many reads. However, they do not happen in practice at all. So
-    #   we skip the span check entirely.
-    if not revlog.delta_config.sparse_revlog and maxdist < deltainfo.distance:
-        return False
-
-    # Bad delta from new delta size:
-    #
-    #   If the delta size is larger than the target text, storing the
-    #   delta will be inefficient.
-    if textlen < deltainfo.deltalen:
-        return False
-
-    # Bad delta from cumulated payload size:
-    #
-    #   If the sum of delta get larger than K * target text length.
-    if textlen * LIMIT_DELTA2TEXT < deltainfo.compresseddeltalen:
-        return False
-
-    # Bad delta from chain length:
-    #
-    #   If the number of delta in the chain gets too high.
-    if (
-        revlog.delta_config.max_chain_len
-        and revlog.delta_config.max_chain_len < deltainfo.chainlen
-    ):
-        return False
-
-    # bad delta from intermediate snapshot size limit
-    #
-    #   If an intermediate snapshot size is higher than the limit.  The
-    #   limit exist to prevent endless chain of intermediate delta to be
-    #   created.
-    if (
-        deltainfo.snapshotdepth is not None
-        and (textlen >> deltainfo.snapshotdepth) < deltainfo.deltalen
-    ):
-        return False
-
-    # bad delta if new intermediate snapshot is larger than the previous
-    # snapshot
-    if (
-        deltainfo.snapshotdepth
-        and revlog.length(deltainfo.base) < deltainfo.deltalen
-    ):
-        return False
-
-    return True
-
-
 # If a revision's full text is that much bigger than a base candidate full
 # text's, it is very unlikely that it will produce a valid delta. We no longer
 # consider these candidates.
 LIMIT_BASE2TEXT = 500
 
 
 class _DeltaSearch:
     """perform the search of a good delta for a single revlog revision
 
     note: some of the deltacomputer.finddeltainfo logic should probably move
     here.
     """
 
     def __init__(
         self,
         revlog,
         revinfo,
         p1,
         p2,
         cachedelta,
         excluded_bases=None,
         target_rev=None,
         snapshot_cache=None,
     ):
         # the DELTA_BASE_REUSE_FORCE case should have been taken care of sooner
         # so we should never end up asking such question. Adding the assert as
         # a safe-guard to detect anything that would be fishy in this regard.
         assert (
             cachedelta is None
             or cachedelta[2] != DELTA_BASE_REUSE_FORCE
             or not revlog.delta_config.general_delta
         )
         self.revlog = revlog
         self.revinfo = revinfo
         self.textlen = revinfo.textlen
         self.p1 = p1
         self.p2 = p2
         self.cachedelta = cachedelta
         self.excluded_bases = excluded_bases
         if target_rev is None:
             self.target_rev = len(self.revlog)
         self.target_rev = target_rev
         if snapshot_cache is None:
             # map: base-rev: [snapshot-revs]
             snapshot_cache = SnapshotCache()
         self.snapshot_cache = snapshot_cache
 
         self.tested = {nullrev}
 
     def candidate_groups(self):
         """Provides group of revision to be tested as delta base
 
         This top level function focus on emitting groups with unique and
         worthwhile content. See _raw_candidate_groups for details about the
         group order.
         """
         # should we try to build a delta?
         if not (len(self.revlog) and self.revlog._storedeltachains):
             yield None
             return
 
         if not self.revlog.delta_config.general_delta:
             # before general delta, there is only one possible delta base
             yield (self.target_rev - 1,)
             yield None
             return
 
         deltalength = self.revlog.length
         deltaparent = self.revlog.deltaparent
         sparse = self.revlog.delta_config.sparse_revlog
         good = None
 
         deltas_limit = self.textlen * LIMIT_DELTA2TEXT
         group_chunk_size = self.revlog.delta_config.candidate_group_chunk_size
 
         tested = self.tested  # prefetch for speed and code compactness
         candidates = self._refined_groups()
         while True:
             temptative = candidates.send(good)
             if temptative is None:
                 break
             group = []
             for rev in temptative:
                 # skip over empty delta (no need to include them in a chain)
                 while not (rev == nullrev or rev in tested or deltalength(rev)):
                     tested.add(rev)
                     rev = deltaparent(rev)
                 # no need to try a delta against nullrev, this will be done as
                 # a last resort.
                 if rev == nullrev:
                     continue
                 # filter out revision we tested already
                 if rev in tested:
                     continue
 
                 # an higher authority deamed the base unworthy (e.g. censored)
                 if (
                     self.excluded_bases is not None
                     and rev in self.excluded_bases
                 ):
                     tested.add(rev)
                     continue
                 # We are in some recomputation cases and that rev is too high
                 # in the revlog
                 if self.target_rev is not None and rev >= self.target_rev:
                     tested.add(rev)
                     continue
                 # filter out delta base that will never produce good delta
                 #
                 # if the delta of that base is already bigger than the limit
                 # for the delta chain size, doing a delta is hopeless.
                 if deltas_limit < self.revlog.length(rev):
                     tested.add(rev)
                     continue
                 if sparse and self.revlog.rawsize(rev) < (
                     self.textlen // LIMIT_BASE2TEXT
                 ):
                     tested.add(rev)
                     continue
                 # no delta for rawtext-changing revs (see "candelta" for why)
                 if self.revlog.flags(rev) & REVIDX_RAWTEXT_CHANGING_FLAGS:
                     tested.add(rev)
                     continue
 
                 # If we reach here, we are about to build and test a delta.
                 # The delta building process will compute the chaininfo in all
                 # case, since that computation is cached, it is fine to access
                 # it here too.
                 chainlen, chainsize = self.revlog._chaininfo(rev)
                 # if chain will be too long, skip base
                 if (
                     self.revlog.delta_config.max_chain_len
                     and chainlen >= self.revlog.delta_config.max_chain_len
                 ):
                     tested.add(rev)
                     continue
                 # if chain already have too much data, skip base
                 if deltas_limit < chainsize:
                     tested.add(rev)
                     continue
                 if (
                     sparse
                     and self.revlog.delta_config.upper_bound_comp is not None
                 ):
                     maxcomp = self.revlog.delta_config.upper_bound_comp
                     basenotsnap = (self.p1, self.p2, nullrev)
                     if rev not in basenotsnap and self.revlog.issnapshot(rev):
                         snapshotdepth = self.revlog.snapshotdepth(rev)
                         # If text is significantly larger than the base, we can
                         # expect the resulting delta to be proportional to the size
                         # difference
                         revsize = self.revlog.rawsize(rev)
                         rawsizedistance = max(self.textlen - revsize, 0)
                         # use an estimate of the compression upper bound.
                         lowestrealisticdeltalen = rawsizedistance // maxcomp
 
                         # check the absolute constraint on the delta size
                         snapshotlimit = self.textlen >> snapshotdepth
                         if snapshotlimit < lowestrealisticdeltalen:
                             # delta lower bound is larger than accepted upper
                             # bound
                             tested.add(rev)
                             continue
 
                         # check the relative constraint on the delta size
                         revlength = self.revlog.length(rev)
                         if revlength < lowestrealisticdeltalen:
                             # delta probable lower bound is larger than target
                             # base
                             tested.add(rev)
                             continue
 
                 group.append(rev)
             if group:
                 # When the size of the candidate group is big, it can result in
                 # a quite significant performance impact. To reduce this, we
                 # can send them in smaller batches until the new batch does not
                 # provide any improvements.
                 #
                 # This might reduce the overall efficiency of the compression
                 # in some corner cases, but that should also prevent very
                 # pathological cases from being an issue. (eg. 20 000
                 # candidates).
                 #
                 # XXX note that the ordering of the group becomes important as
                 # it now impacts the final result. The current order is
                 # unprocessed and can be improved.
                 if group_chunk_size == 0:
                     tested.update(group)
                     good = yield tuple(group)
                 else:
                     prev_good = good
                     for start in range(0, len(group), group_chunk_size):
                         sub_group = group[start : start + group_chunk_size]
                         tested.update(sub_group)
                         good = yield tuple(sub_group)
                         if prev_good == good:
                             break
 
         yield None
 
     def _refined_groups(self):
         good = None
         # First we try to reuse a the delta contained in the bundle.  (or from
         # the source revlog)
         #
         # This logic only applies to general delta repositories and can be
         # disabled through configuration. Disabling reuse source delta is
         # useful when we want to make sure we recomputed "optimal" deltas.
         debug_info = None
         if (
             self.cachedelta is not None
             and self.cachedelta[2] > DELTA_BASE_REUSE_NO
         ):
             # Assume what we received from the server is a good choice
             # build delta will reuse the cache
             if debug_info is not None:
                 debug_info['cached-delta.tested'] += 1
             good = yield (self.cachedelta[0],)
             if good is not None:
                 if debug_info is not None:
                     debug_info['cached-delta.accepted'] += 1
                 yield None
                 return
         groups = self._raw_groups()
         for candidates in groups:
             good = yield candidates
             if good is not None:
                 break
 
         # If sparse revlog is enabled, we can try to refine the available
         # deltas
         if not self.revlog.delta_config.sparse_revlog:
             yield None
             return
 
         # if we have a refinable value, try to refine it
         if (
             good is not None
             and good not in (self.p1, self.p2)
             and self.revlog.issnapshot(good)
         ):
             # refine snapshot down
             previous = None
             while previous != good:
                 previous = good
                 base = self.revlog.deltaparent(good)
                 if base == nullrev:
                     break
                 good = yield (base,)
             # refine snapshot up
             if not self.snapshot_cache.snapshots:
                 self.snapshot_cache.update(self.revlog, good + 1)
             previous = None
             while good != previous:
                 previous = good
                 children = tuple(
                     sorted(c for c in self.snapshot_cache.snapshots[good])
                 )
                 good = yield children
 
         if debug_info is not None:
             if good is None:
                 debug_info['no-solution'] += 1
 
         yield None
 
     def _raw_groups(self):
         """Provides group of revision to be tested as delta base
 
         This lower level function focus on emitting delta theorically
         interresting without looking it any practical details.
 
         The group order aims at providing fast or small candidates first.
         """
         # Why search for delta base if we cannot use a delta base ?
         assert self.revlog.delta_config.general_delta
         # also see issue6056
         sparse = self.revlog.delta_config.sparse_revlog
         curr = len(self.revlog)
         prev = curr - 1
         deltachain = lambda rev: self.revlog._deltachain(rev)[0]
 
         # exclude already lazy tested base if any
         parents = [p for p in (self.p1, self.p2) if p != nullrev]
 
         if (
             not self.revlog.delta_config.delta_both_parents
             and len(parents) == 2
         ):
             parents.sort()
             # To minimize the chance of having to build a fulltext,
             # pick first whichever parent is closest to us (max rev)
             yield (parents[1],)
             # then the other one (min rev) if the first did not fit
             yield (parents[0],)
         elif len(parents) > 0:
             # Test all parents (1 or 2), and keep the best candidate
             yield parents
 
         if sparse and parents:
             # See if we can use an existing snapshot in the parent chains to
             # use as a base for a new intermediate-snapshot
             #
             # search for snapshot in parents delta chain map: snapshot-level:
             # snapshot-rev
             parents_snaps = collections.defaultdict(set)
             candidate_chains = [deltachain(p) for p in parents]
             for chain in candidate_chains:
                 for idx, s in enumerate(chain):
                     if not self.revlog.issnapshot(s):
                         break
                     parents_snaps[idx].add(s)
             snapfloor = min(parents_snaps[0]) + 1
             self.snapshot_cache.update(self.revlog, snapfloor)
             # search for the highest "unrelated" revision
             #
             # Adding snapshots used by "unrelated" revision increase the odd we
             # reuse an independant, yet better snapshot chain.
             #
             # XXX instead of building a set of revisions, we could lazily
             # enumerate over the chains. That would be more efficient, however
             # we stick to simple code for now.
             all_revs = set()
             for chain in candidate_chains:
                 all_revs.update(chain)
             other = None
             for r in self.revlog.revs(prev, snapfloor):
                 if r not in all_revs:
                     other = r
                     break
             if other is not None:
                 # To avoid unfair competition, we won't use unrelated
                 # intermediate snapshot that are deeper than the ones from the
                 # parent delta chain.
                 max_depth = max(parents_snaps.keys())
                 chain = deltachain(other)
                 for depth, s in enumerate(chain):
                     if s < snapfloor:
                         continue
                     if max_depth < depth:
                         break
                     if not self.revlog.issnapshot(s):
                         break
                     parents_snaps[depth].add(s)
             # Test them as possible intermediate snapshot base We test them
             # from highest to lowest level. High level one are more likely to
             # result in small delta
             floor = None
             for idx, snaps in sorted(parents_snaps.items(), reverse=True):
                 siblings = set()
                 for s in snaps:
                     siblings.update(self.snapshot_cache.snapshots[s])
                 # Before considering making a new intermediate snapshot, we
                 # check if an existing snapshot, children of base we consider,
                 # would be suitable.
                 #
                 # It give a change to reuse a delta chain "unrelated" to the
                 # current revision instead of starting our own. Without such
                 # re-use, topological branches would keep reopening new chains.
                 # Creating more and more snapshot as the repository grow.
 
                 if floor is not None:
                     # We only do this for siblings created after the one in our
                     # parent's delta chain. Those created before has less
                     # chances to be valid base since our ancestors had to
                     # create a new snapshot.
                     siblings = [r for r in siblings if floor < r]
                 yield tuple(sorted(siblings))
                 # then test the base from our parent's delta chain.
                 yield tuple(sorted(snaps))
                 floor = min(snaps)
             # No suitable base found in the parent chain, search if any full
             # snapshots emitted since parent's base would be a suitable base
             # for an intermediate snapshot.
             #
             # It give a chance to reuse a delta chain unrelated to the current
             # revisions instead of starting our own. Without such re-use,
             # topological branches would keep reopening new full chains.
             # Creating more and more snapshot as the repository grow.
             full = [
                 r
                 for r in self.snapshot_cache.snapshots[nullrev]
                 if snapfloor <= r
             ]
             yield tuple(sorted(full))
 
         if not sparse:
             # other approach failed try against prev to hopefully save us a
             # fulltext.
             yield (prev,)
 
+    def is_good_delta_info(self, deltainfo):
+        """Returns True if the given delta is good. Good means that it is
+        within the disk span, disk size, and chain length bounds that we know
+        to be performant."""
+        if deltainfo is None:
+            return False
+
+        # the DELTA_BASE_REUSE_FORCE case should have been taken care of sooner
+        # so we should never end up asking such question. Adding the assert as
+        # a safe-guard to detect anything that would be fishy in this regard.
+        assert (
+            self.revinfo.cachedelta is None
+            or self.revinfo.cachedelta[2] != DELTA_BASE_REUSE_FORCE
+            or not self.revlog.delta_config.general_delta
+        )
+
+        # - 'deltainfo.distance' is the distance from the base revision --
+        #   bounding it limits the amount of I/O we need to do.
+        # - 'deltainfo.compresseddeltalen' is the sum of the total size of
+        #   deltas we need to apply -- bounding it limits the amount of CPU
+        #   we consume.
+
+        textlen = self.revinfo.textlen
+        defaultmax = textlen * 4
+        maxdist = self.revlog.delta_config.max_deltachain_span
+        if not maxdist:
+            maxdist = deltainfo.distance  # ensure the conditional pass
+        maxdist = max(maxdist, defaultmax)
+
+        # Bad delta from read span:
+        #
+        #   If the span of data read is larger than the maximum allowed.
+        #
+        #   In the sparse-revlog case, we rely on the associated "sparse
+        #   reading" to avoid issue related to the span of data. In theory, it
+        #   would be possible to build pathological revlog where delta pattern
+        #   would lead to too many reads. However, they do not happen in
+        #   practice at all. So we skip the span check entirely.
+        if (
+            not self.revlog.delta_config.sparse_revlog
+            and maxdist < deltainfo.distance
+        ):
+            return False
+
+        # Bad delta from new delta size:
+        #
+        #   If the delta size is larger than the target text, storing the delta
+        #   will be inefficient.
+        if textlen < deltainfo.deltalen:
+            return False
+
+        # Bad delta from cumulated payload size:
+        #
+        #   If the sum of delta get larger than K * target text length.
+        if textlen * LIMIT_DELTA2TEXT < deltainfo.compresseddeltalen:
+            return False
+
+        # Bad delta from chain length:
+        #
+        #   If the number of delta in the chain gets too high.
+        if (
+            self.revlog.delta_config.max_chain_len
+            and self.revlog.delta_config.max_chain_len < deltainfo.chainlen
+        ):
+            return False
+
+        # bad delta from intermediate snapshot size limit
+        #
+        #   If an intermediate snapshot size is higher than the limit.  The
+        #   limit exist to prevent endless chain of intermediate delta to be
+        #   created.
+        if (
+            deltainfo.snapshotdepth is not None
+            and (textlen >> deltainfo.snapshotdepth) < deltainfo.deltalen
+        ):
+            return False
+
+        # bad delta if new intermediate snapshot is larger than the previous
+        # snapshot
+        if (
+            deltainfo.snapshotdepth
+            and self.revlog.length(deltainfo.base) < deltainfo.deltalen
+        ):
+            return False
+
+        return True
+
 
 class SnapshotCache:
     __slots__ = ('snapshots', '_start_rev', '_end_rev')
 
     def __init__(self):
         self.snapshots = collections.defaultdict(set)
         self._start_rev = None
         self._end_rev = None
 
     def update(self, revlog, start_rev=0):
         """find snapshots from start_rev to tip"""
         nb_revs = len(revlog)
         end_rev = nb_revs - 1
         if start_rev > end_rev:
             return  # range is empty
 
         if self._start_rev is None:
             assert self._end_rev is None
             self._update(revlog, start_rev, end_rev)
         elif not (self._start_rev <= start_rev and end_rev <= self._end_rev):
             if start_rev < self._start_rev:
                 self._update(revlog, start_rev, self._start_rev - 1)
             if self._end_rev < end_rev:
                 self._update(revlog, self._end_rev + 1, end_rev)
 
         if self._start_rev is None:
             assert self._end_rev is None
             self._end_rev = end_rev
             self._start_rev = start_rev
         else:
             self._start_rev = min(self._start_rev, start_rev)
             self._end_rev = max(self._end_rev, end_rev)
         assert self._start_rev <= self._end_rev, (
             self._start_rev,
             self._end_rev,
         )
 
     def _update(self, revlog, start_rev, end_rev):
         """internal method that actually do update content"""
         assert self._start_rev is None or (
             start_rev < self._start_rev or start_rev > self._end_rev
         ), (self._start_rev, self._end_rev, start_rev, end_rev)
         assert self._start_rev is None or (
             end_rev < self._start_rev or end_rev > self._end_rev
         ), (self._start_rev, self._end_rev, start_rev, end_rev)
         cache = self.snapshots
         if hasattr(revlog.index, 'findsnapshots'):
             revlog.index.findsnapshots(cache, start_rev, end_rev)
         else:
             deltaparent = revlog.deltaparent
             issnapshot = revlog.issnapshot
             for rev in revlog.revs(start_rev, end_rev):
                 if issnapshot(rev):
                     cache[deltaparent(rev)].add(rev)
 
 
 class deltacomputer:
     """object capable of computing delta and finding delta for multiple revision
 
     This object is meant to compute and find multiple delta applied to the same
     revlog.
     """
 
     def __init__(
         self,
         revlog,
         write_debug=None,
         debug_search=False,
         debug_info=None,
     ):
         self.revlog = revlog
         self._write_debug = write_debug
         if write_debug is None:
             self._debug_search = False
         else:
             self._debug_search = debug_search
         self._debug_info = debug_info
         self._snapshot_cache = SnapshotCache()
 
     @property
     def _gather_debug(self):
         return self._write_debug is not None or self._debug_info is not None
 
     def buildtext(self, revinfo):
         """Builds a fulltext version of a revision
 
         revinfo: revisioninfo instance that contains all needed info
         """
         btext = revinfo.btext
         if btext[0] is not None:
             return btext[0]
 
         revlog = self.revlog
         cachedelta = revinfo.cachedelta
         baserev = cachedelta[0]
         delta = cachedelta[1]
 
         fulltext = btext[0] = _textfromdelta(
             revlog,
             baserev,
             delta,
             revinfo.p1,
             revinfo.p2,
             revinfo.flags,
             revinfo.node,
         )
         return fulltext
 
     def _builddeltadiff(self, base, revinfo):
         revlog = self.revlog
         t = self.buildtext(revinfo)
         if revlog.iscensored(base):
             # deltas based on a censored revision must replace the
             # full content in one patch, so delta works everywhere
             header = mdiff.replacediffheader(revlog.rawsize(base), len(t))
             delta = header + t
         else:
             ptext = revlog.rawdata(base)
             delta = mdiff.textdiff(ptext, t)
 
         return delta
 
     def _builddeltainfo(self, revinfo, base, target_rev=None):
         # can we use the cached delta?
         revlog = self.revlog
         chainbase = revlog.chainbase(base)
         if revlog.delta_config.general_delta:
             deltabase = base
         else:
             if target_rev is not None and base != target_rev - 1:
                 msg = (
                     b'general delta cannot use delta for something else '
                     b'than `prev`: %d<-%d'
                 )
                 msg %= (base, target_rev)
                 raise error.ProgrammingError(msg)
             deltabase = chainbase
         snapshotdepth = None
         if revlog.delta_config.sparse_revlog and deltabase == nullrev:
             snapshotdepth = 0
         elif revlog.delta_config.sparse_revlog and revlog.issnapshot(deltabase):
             # A delta chain should always be one full snapshot,
             # zero or more semi-snapshots, and zero or more deltas
             p1, p2 = revlog.rev(revinfo.p1), revlog.rev(revinfo.p2)
             if deltabase not in (p1, p2) and revlog.issnapshot(deltabase):
                 snapshotdepth = len(revlog._deltachain(deltabase)[0])
         delta = None
         if revinfo.cachedelta:
             cachebase = revinfo.cachedelta[0]
             # check if the diff still apply
             currentbase = cachebase
             while (
                 currentbase != nullrev
                 and currentbase != base
                 and self.revlog.length(currentbase) == 0
             ):
                 currentbase = self.revlog.deltaparent(currentbase)
             if self.revlog.delta_config.lazy_delta and currentbase == base:
                 delta = revinfo.cachedelta[1]
         if delta is None:
             delta = self._builddeltadiff(base, revinfo)
         if self._debug_search:
             msg = b"DBG-DELTAS-SEARCH:     uncompressed-delta-size=%d\n"
             msg %= len(delta)
             self._write_debug(msg)
         # snapshotdept need to be neither None nor 0 level snapshot
         if revlog.delta_config.upper_bound_comp is not None and snapshotdepth:
             lowestrealisticdeltalen = (
                 len(delta) // revlog.delta_config.upper_bound_comp
             )
             snapshotlimit = revinfo.textlen >> snapshotdepth
             if self._debug_search:
                 msg = b"DBG-DELTAS-SEARCH:     projected-lower-size=%d\n"
                 msg %= lowestrealisticdeltalen
                 self._write_debug(msg)
             if snapshotlimit < lowestrealisticdeltalen:
                 if self._debug_search:
                     msg = b"DBG-DELTAS-SEARCH:     DISCARDED (snapshot limit)\n"
                     self._write_debug(msg)
                 return None
             if revlog.length(base) < lowestrealisticdeltalen:
                 if self._debug_search:
                     msg = b"DBG-DELTAS-SEARCH:     DISCARDED (prev size)\n"
                     self._write_debug(msg)
                 return None
         header, data = revlog._inner.compress(delta)
         deltalen = len(header) + len(data)
         offset = revlog.end(len(revlog) - 1)
         dist = deltalen + offset - revlog.start(chainbase)
         chainlen, compresseddeltalen = revlog._chaininfo(base)
         chainlen += 1
         compresseddeltalen += deltalen
 
         return _deltainfo(
             dist,
             deltalen,
             (header, data),
             deltabase,
             chainbase,
             chainlen,
             compresseddeltalen,
             snapshotdepth,
         )
 
     def _fullsnapshotinfo(self, revinfo, curr):
         rawtext = self.buildtext(revinfo)
         data = self.revlog._inner.compress(rawtext)
         compresseddeltalen = deltalen = dist = len(data[1]) + len(data[0])
         deltabase = chainbase = curr
         snapshotdepth = 0
         chainlen = 1
 
         return _deltainfo(
             dist,
             deltalen,
             data,
             deltabase,
             chainbase,
             chainlen,
             compresseddeltalen,
             snapshotdepth,
         )
 
     def finddeltainfo(self, revinfo, excluded_bases=None, target_rev=None):
         """Find an acceptable delta against a candidate revision
 
         revinfo: information about the revision (instance of _revisioninfo)
 
         Returns the first acceptable candidate revision, as ordered by
         _candidategroups
 
         If no suitable deltabase is found, we return delta info for a full
         snapshot.
 
         `excluded_bases` is an optional set of revision that cannot be used as
         a delta base. Use this to recompute delta suitable in censor or strip
         context.
         """
         if target_rev is None:
             target_rev = len(self.revlog)
 
         gather_debug = self._gather_debug
         cachedelta = revinfo.cachedelta
         revlog = self.revlog
         p1r = p2r = None
 
         if excluded_bases is None:
             excluded_bases = set()
 
         if gather_debug:
             start = util.timer()
             dbg = self._one_dbg_data()
             dbg['revision'] = target_rev
             p1r = revlog.rev(revinfo.p1)
             p2r = revlog.rev(revinfo.p2)
             if p1r != nullrev:
                 p1_chain_len = revlog._chaininfo(p1r)[0]
             else:
                 p1_chain_len = -1
             if p2r != nullrev:
                 p2_chain_len = revlog._chaininfo(p2r)[0]
             else:
                 p2_chain_len = -1
             dbg['p1-chain-len'] = p1_chain_len
             dbg['p2-chain-len'] = p2_chain_len
 
         # 1) if the revision is empty, no amount of delta can beat it
         #
         # 2) no delta for flag processor revision (see "candelta" for why)
         # not calling candelta since only one revision needs test, also to
         # avoid overhead fetching flags again.
         if not revinfo.textlen or revinfo.flags & REVIDX_RAWTEXT_CHANGING_FLAGS:
             deltainfo = self._fullsnapshotinfo(revinfo, target_rev)
             if gather_debug:
                 end = util.timer()
                 dbg['duration'] = end - start
                 dbg[
                     'delta-base'
                 ] = deltainfo.base  # pytype: disable=attribute-error
                 dbg['search_round_count'] = 0
                 dbg['using-cached-base'] = False
                 dbg['delta_try_count'] = 0
                 dbg['type'] = b"full"
                 dbg['snapshot-depth'] = 0
                 self._dbg_process_data(dbg)
             return deltainfo
 
         deltainfo = None
 
         # If this source delta are to be forcibly reuse, let us comply early.
         if (
             revlog.delta_config.general_delta
             and revinfo.cachedelta is not None
             and revinfo.cachedelta[2] == DELTA_BASE_REUSE_FORCE
         ):
             base = revinfo.cachedelta[0]
             if base == nullrev:
                 dbg_type = b"full"
                 deltainfo = self._fullsnapshotinfo(revinfo, target_rev)
                 if gather_debug:
                     snapshotdepth = 0
             elif base not in excluded_bases:
                 delta = revinfo.cachedelta[1]
                 header, data = revlog.compress(delta)
                 deltalen = len(header) + len(data)
                 if gather_debug:
                     offset = revlog.end(len(revlog) - 1)
                     chainbase = revlog.chainbase(base)
                     distance = deltalen + offset - revlog.start(chainbase)
                     chainlen, compresseddeltalen = revlog._chaininfo(base)
                     chainlen += 1
                     compresseddeltalen += deltalen
                     if base == p1r or base == p2r:
                         dbg_type = b"delta"
                         snapshotdepth = None
                     elif not revlog.issnapshot(base):
                         snapshotdepth = None
                     else:
                         dbg_type = b"snapshot"
                         snapshotdepth = revlog.snapshotdepth(base) + 1
                 else:
                     distance = None
                     chainbase = None
                     chainlen = None
                     compresseddeltalen = None
                     snapshotdepth = None
                 deltainfo = _deltainfo(
                     distance=distance,
                     deltalen=deltalen,
                     data=(header, data),
                     base=base,
                     chainbase=chainbase,
                     chainlen=chainlen,
                     compresseddeltalen=compresseddeltalen,
                     snapshotdepth=snapshotdepth,
                 )
 
             if deltainfo is not None:
                 if gather_debug:
                     end = util.timer()
                     dbg['duration'] = end - start
                     dbg[
                         'delta-base'
                     ] = deltainfo.base  # pytype: disable=attribute-error
                     dbg['search_round_count'] = 0
                     dbg['using-cached-base'] = True
                     dbg['delta_try_count'] = 0
                     dbg['type'] = b"full"
                     if snapshotdepth is None:
                         dbg['snapshot-depth'] = 0
                     else:
                         dbg['snapshot-depth'] = snapshotdepth
                     self._dbg_process_data(dbg)
                 return deltainfo
 
         # count the number of different delta we tried (for debug purpose)
         dbg_try_count = 0
         # count the number of "search round" we did. (for debug purpose)
         dbg_try_rounds = 0
         dbg_type = b'unknown'
 
         if p1r is None:
             p1r = revlog.rev(revinfo.p1)
             p2r = revlog.rev(revinfo.p2)
 
         if self._debug_search:
             msg = b"DBG-DELTAS-SEARCH: SEARCH rev=%d\n"
             msg %= target_rev
             self._write_debug(msg)
 
         search = _DeltaSearch(
             self.revlog,
             revinfo,
             p1r,
             p2r,
             cachedelta,
             excluded_bases,
             target_rev,
             snapshot_cache=self._snapshot_cache,
         )
 
         groups = search.candidate_groups()
         candidaterevs = next(groups)
         while candidaterevs is not None:
             dbg_try_rounds += 1
             if self._debug_search:
                 prev = None
                 if deltainfo is not None:
                     prev = deltainfo.base
 
                 if (
                     cachedelta is not None
                     and len(candidaterevs) == 1
                     and cachedelta[0] in candidaterevs
                 ):
                     round_type = b"cached-delta"
                 elif p1r in candidaterevs or p2r in candidaterevs:
                     round_type = b"parents"
                 elif prev is not None and all(c < prev for c in candidaterevs):
                     round_type = b"refine-down"
                 elif prev is not None and all(c > prev for c in candidaterevs):
                     round_type = b"refine-up"
                 else:
                     round_type = b"search-down"
                 msg = b"DBG-DELTAS-SEARCH: ROUND #%d - %d candidates - %s\n"
                 msg %= (dbg_try_rounds, len(candidaterevs), round_type)
                 self._write_debug(msg)
             nominateddeltas = []
             if deltainfo is not None:
                 if self._debug_search:
                     msg = (
                         b"DBG-DELTAS-SEARCH:   CONTENDER: rev=%d - length=%d\n"
                     )
                     msg %= (deltainfo.base, deltainfo.deltalen)
                     self._write_debug(msg)
                 # if we already found a good delta,
                 # challenge it against refined candidates
                 nominateddeltas.append(deltainfo)
             for candidaterev in candidaterevs:
                 if self._debug_search:
                     msg = b"DBG-DELTAS-SEARCH:   CANDIDATE: rev=%d\n"
                     msg %= candidaterev
                     self._write_debug(msg)
                     candidate_type = None
                     if candidaterev == p1r:
                         candidate_type = b"p1"
                     elif candidaterev == p2r:
                         candidate_type = b"p2"
                     elif self.revlog.issnapshot(candidaterev):
                         candidate_type = b"snapshot-%d"
                         candidate_type %= self.revlog.snapshotdepth(
                             candidaterev
                         )
 
                     if candidate_type is not None:
                         msg = b"DBG-DELTAS-SEARCH:     type=%s\n"
                         msg %= candidate_type
                         self._write_debug(msg)
                     msg = b"DBG-DELTAS-SEARCH:     size=%d\n"
                     msg %= self.revlog.length(candidaterev)
                     self._write_debug(msg)
                     msg = b"DBG-DELTAS-SEARCH:     base=%d\n"
                     msg %= self.revlog.deltaparent(candidaterev)
                     self._write_debug(msg)
 
                 dbg_try_count += 1
 
                 if self._debug_search:
                     delta_start = util.timer()
                 candidatedelta = self._builddeltainfo(
                     revinfo,
                     candidaterev,
                     target_rev=target_rev,
                 )
                 if self._debug_search:
                     delta_end = util.timer()
                     msg = b"DBG-DELTAS-SEARCH:     delta-search-time=%f\n"
                     msg %= delta_end - delta_start
                     self._write_debug(msg)
                 if candidatedelta is not None:
-                    if is_good_delta_info(self.revlog, candidatedelta, revinfo):
+                    if search.is_good_delta_info(candidatedelta):
                         if self._debug_search:
                             msg = b"DBG-DELTAS-SEARCH:     DELTA: length=%d (GOOD)\n"
                             msg %= candidatedelta.deltalen
                             self._write_debug(msg)
                         nominateddeltas.append(candidatedelta)
                     elif self._debug_search:
                         msg = b"DBG-DELTAS-SEARCH:     DELTA: length=%d (BAD)\n"
                         msg %= candidatedelta.deltalen
                         self._write_debug(msg)
                 elif self._debug_search:
                     msg = b"DBG-DELTAS-SEARCH:     NO-DELTA\n"
                     self._write_debug(msg)
             if nominateddeltas:
                 deltainfo = min(nominateddeltas, key=lambda x: x.deltalen)
             if deltainfo is not None:
                 candidaterevs = groups.send(deltainfo.base)
             else:
                 candidaterevs = next(groups)
 
         if deltainfo is None:
             dbg_type = b"full"
             deltainfo = self._fullsnapshotinfo(revinfo, target_rev)
         elif deltainfo.snapshotdepth:  # pytype: disable=attribute-error
             dbg_type = b"snapshot"
         else:
             dbg_type = b"delta"
 
         if gather_debug:
             end = util.timer()
             if dbg_type == b'full':
                 used_cached = (
                     cachedelta is not None
                     and dbg_try_rounds == 0
                     and dbg_try_count == 0
                     and cachedelta[0] == nullrev
                 )
             else:
                 used_cached = (
                     cachedelta is not None
                     and dbg_try_rounds == 1
                     and dbg_try_count == 1
                     and deltainfo.base == cachedelta[0]
                 )
             dbg['duration'] = end - start
             dbg[
                 'delta-base'
             ] = deltainfo.base  # pytype: disable=attribute-error
             dbg['search_round_count'] = dbg_try_rounds
             dbg['using-cached-base'] = used_cached
             dbg['delta_try_count'] = dbg_try_count
             dbg['type'] = dbg_type
             if (
                 deltainfo.snapshotdepth  # pytype: disable=attribute-error
                 is not None
             ):
                 dbg[
                     'snapshot-depth'
                 ] = deltainfo.snapshotdepth  # pytype: disable=attribute-error
             else:
                 dbg['snapshot-depth'] = 0
             self._dbg_process_data(dbg)
         return deltainfo
 
     def _one_dbg_data(self):
         dbg = {
             'duration': None,
             'revision': None,
             'delta-base': None,
             'search_round_count': None,
             'using-cached-base': None,
             'delta_try_count': None,
             'type': None,
             'p1-chain-len': None,
             'p2-chain-len': None,
             'snapshot-depth': None,
             'target-revlog': None,
         }
         target_revlog = b"UNKNOWN"
         target_type = self.revlog.target[0]
         target_key = self.revlog.target[1]
         if target_type == KIND_CHANGELOG:
             target_revlog = b'CHANGELOG:'
         elif target_type == KIND_MANIFESTLOG:
             target_revlog = b'MANIFESTLOG:'
             if target_key:
                 target_revlog += b'%s:' % target_key
         elif target_type == KIND_FILELOG:
             target_revlog = b'FILELOG:'
             if target_key:
                 target_revlog += b'%s:' % target_key
         dbg['target-revlog'] = target_revlog
         return dbg
 
     def _dbg_process_data(self, dbg):
         if self._debug_info is not None:
             self._debug_info.append(dbg)
 
         if self._write_debug is not None:
             msg = (
                 b"DBG-DELTAS:"
                 b" %-12s"
                 b" rev=%d:"
                 b" delta-base=%d"
                 b" is-cached=%d"
                 b" - search-rounds=%d"
                 b" try-count=%d"
                 b" - delta-type=%-6s"
                 b" snap-depth=%d"
                 b" - p1-chain-length=%d"
                 b" p2-chain-length=%d"
                 b" - duration=%f"
                 b"\n"
             )
             msg %= (
                 dbg["target-revlog"],
                 dbg["revision"],
                 dbg["delta-base"],
                 dbg["using-cached-base"],
                 dbg["search_round_count"],
                 dbg["delta_try_count"],
                 dbg["type"],
                 dbg["snapshot-depth"],
                 dbg["p1-chain-len"],
                 dbg["p2-chain-len"],
                 dbg["duration"],
             )
             self._write_debug(msg)
 
 
 def delta_compression(default_compression_header, deltainfo):
     """return (COMPRESSION_MODE, deltainfo)
 
     used by revlog v2+ format to dispatch between PLAIN and DEFAULT
     compression.
     """
     h, d = deltainfo.data
     compression_mode = COMP_MODE_INLINE
     if not h and not d:
         # not data to store at all... declare them uncompressed
         compression_mode = COMP_MODE_PLAIN
     elif not h:
         t = d[0:1]
         if t == b'\0':
             compression_mode = COMP_MODE_PLAIN
         elif t == default_compression_header:
             compression_mode = COMP_MODE_DEFAULT
     elif h == b'u':
         # we have a more efficient way to declare uncompressed
         h = b''
         compression_mode = COMP_MODE_PLAIN
         deltainfo = drop_u_compression(deltainfo)
     return compression_mode, deltainfo