delta-find: use a single snapshot cache when applying a group to an object

This will avoid walking the revlog over and over again in some situations.

The difference is hard to show in our current benchmark suite, as the gain is
lower than their overall instability.

mercurial/revlogutils/deltas.py

@@ -1,1472 +1,1476 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 ..pycompat import getattr
 
 from .constants import (
     COMP_MODE_DEFAULT,
     COMP_MODE_INLINE,
     COMP_MODE_PLAIN,
     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"""
         self._data = data
         self._srdensitythreshold = density
         self._srmingapsize = mingap
         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._srdensitythreshold`. No gap smaller than
     `revlog._srmingapsize` 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._srmingapsize`.
 
     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._srmingapsize)
     # 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._srdensitythreshold, revlog._srmingapsize
     ):
         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(fh, 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, _df=fh)
         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
 
     # - '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._maxdeltachainspan
     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._sparserevlog 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._maxchainlen and revlog._maxchainlen < 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
 
 
 def _candidategroups(
     revlog,
     textlen,
     p1,
     p2,
     cachedelta,
     excluded_bases=None,
     target_rev=None,
+    snapshot_cache=None,
 ):
     """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(revlog) and revlog._storedeltachains):
         yield None
         return
 
     deltalength = revlog.length
     deltaparent = revlog.deltaparent
     sparse = revlog._sparserevlog
     good = None
 
     deltas_limit = textlen * LIMIT_DELTA2TEXT
     group_chunk_size = revlog._candidate_group_chunk_size
 
     tested = {nullrev}
     candidates = _refinedgroups(
         revlog,
         p1,
         p2,
         cachedelta,
+        snapshot_cache=snapshot_cache,
     )
     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 revlog._generaldelta and 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 excluded_bases is not None and rev in excluded_bases:
                 tested.add(rev)
                 continue
             # We are in some recomputation cases and that rev is too high in
             # the revlog
             if target_rev is not None and rev >= target_rev:
                 tested.add(rev)
                 continue
             # filter out delta base that will never produce good delta
             if deltas_limit < revlog.length(rev):
                 tested.add(rev)
                 continue
             if sparse and revlog.rawsize(rev) < (textlen // LIMIT_BASE2TEXT):
                 tested.add(rev)
                 continue
             # no delta for rawtext-changing revs (see "candelta" for why)
             if 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 = revlog._chaininfo(rev)
             # if chain will be too long, skip base
             if revlog._maxchainlen and chainlen >= revlog._maxchainlen:
                 tested.add(rev)
                 continue
             # if chain already have too much data, skip base
             if deltas_limit < chainsize:
                 tested.add(rev)
                 continue
             if sparse and revlog.upperboundcomp is not None:
                 maxcomp = revlog.upperboundcomp
                 basenotsnap = (p1, p2, nullrev)
                 if rev not in basenotsnap and revlog.issnapshot(rev):
                     snapshotdepth = 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 = revlog.rawsize(rev)
                     rawsizedistance = max(textlen - revsize, 0)
                     # use an estimate of the compression upper bound.
                     lowestrealisticdeltalen = rawsizedistance // maxcomp
 
                     # check the absolute constraint on the delta size
                     snapshotlimit = 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 = 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 _refinedgroups(revlog, p1, p2, cachedelta):
+def _refinedgroups(revlog, p1, p2, cachedelta, snapshot_cache=None):
     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 cachedelta is not None and 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 (cachedelta[0],)
         if good is not None:
             if debug_info is not None:
                 debug_info['cached-delta.accepted'] += 1
             yield None
             return
-    # XXX cache me higher
+    if snapshot_cache is None:
     snapshot_cache = SnapshotCache()
     groups = _rawgroups(
         revlog,
         p1,
         p2,
         cachedelta,
         snapshot_cache,
     )
     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 revlog._sparserevlog:
         yield None
         return
 
     # if we have a refinable value, try to refine it
     if good is not None and good not in (p1, p2) and revlog.issnapshot(good):
         # refine snapshot down
         previous = None
         while previous != good:
             previous = good
             base = revlog.deltaparent(good)
             if base == nullrev:
                 break
             good = yield (base,)
         # refine snapshot up
         if not snapshot_cache.snapshots:
             snapshot_cache.update(revlog, good + 1)
         previous = None
         while good != previous:
             previous = good
             children = tuple(sorted(c for c in 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 _rawgroups(revlog, p1, p2, cachedelta, snapshot_cache=None):
     """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.
     """
     gdelta = revlog._generaldelta
     # gate sparse behind general-delta because of issue6056
     sparse = gdelta and revlog._sparserevlog
     curr = len(revlog)
     prev = curr - 1
     deltachain = lambda rev: revlog._deltachain(rev)[0]
 
     if gdelta:
         # exclude already lazy tested base if any
         parents = [p for p in (p1, p2) if p != nullrev]
 
         if not revlog._deltabothparents 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:
         if snapshot_cache is None:
             # map: base-rev: [snapshot-revs]
             snapshot_cache = SnapshotCache()
         # 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 revlog.issnapshot(s):
                     break
                 parents_snaps[idx].add(s)
         snapfloor = min(parents_snaps[0]) + 1
         snapshot_cache.update(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 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 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(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 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,)
 
 
 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 util.safehasattr(revlog.index, b'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:
     def __init__(
         self,
         revlog,
         write_debug=None,
         debug_search=False,
         debug_info=None,
     ):
         self.revlog = revlog
         self._write_debug = write_debug
         self._debug_search = debug_search
         self._debug_info = debug_info
+        self._snapshot_cache = SnapshotCache()
 
     def buildtext(self, revinfo, fh):
         """Builds a fulltext version of a revision
 
         revinfo: revisioninfo instance that contains all needed info
         fh:      file handle to either the .i or the .d revlog file,
                  depending on whether it is inlined or not
         """
         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(
             fh,
             revlog,
             baserev,
             delta,
             revinfo.p1,
             revinfo.p2,
             revinfo.flags,
             revinfo.node,
         )
         return fulltext
 
     def _builddeltadiff(self, base, revinfo, fh):
         revlog = self.revlog
         t = self.buildtext(revinfo, fh)
         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, _df=fh)
             delta = mdiff.textdiff(ptext, t)
 
         return delta
 
     def _builddeltainfo(self, revinfo, base, fh):
         # can we use the cached delta?
         revlog = self.revlog
         debug_search = self._write_debug is not None and self._debug_search
         chainbase = revlog.chainbase(base)
         if revlog._generaldelta:
             deltabase = base
         else:
             deltabase = chainbase
         snapshotdepth = None
         if revlog._sparserevlog and deltabase == nullrev:
             snapshotdepth = 0
         elif revlog._sparserevlog 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._lazydelta and currentbase == base:
                 delta = revinfo.cachedelta[1]
         if delta is None:
             delta = self._builddeltadiff(base, revinfo, fh)
         if 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.upperboundcomp is not None and snapshotdepth:
             lowestrealisticdeltalen = len(delta) // revlog.upperboundcomp
             snapshotlimit = revinfo.textlen >> snapshotdepth
             if debug_search:
                 msg = b"DBG-DELTAS-SEARCH:     projected-lower-size=%d\n"
                 msg %= lowestrealisticdeltalen
                 self._write_debug(msg)
             if snapshotlimit < lowestrealisticdeltalen:
                 if debug_search:
                     msg = b"DBG-DELTAS-SEARCH:     DISCARDED (snapshot limit)\n"
                     self._write_debug(msg)
                 return None
             if revlog.length(base) < lowestrealisticdeltalen:
                 if debug_search:
                     msg = b"DBG-DELTAS-SEARCH:     DISCARDED (prev size)\n"
                     self._write_debug(msg)
                 return None
         header, data = revlog.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, fh, revinfo, curr):
         rawtext = self.buildtext(revinfo, fh)
         data = self.revlog.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, fh, excluded_bases=None, target_rev=None):
         """Find an acceptable delta against a candidate revision
 
         revinfo: information about the revision (instance of _revisioninfo)
         fh:      file handle to either the .i or the .d revlog file,
                  depending on whether it is inlined or not
 
         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)
 
         if not revinfo.textlen:
             return self._fullsnapshotinfo(fh, revinfo, target_rev)
 
         if excluded_bases is None:
             excluded_bases = set()
 
         # 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 revinfo.flags & REVIDX_RAWTEXT_CHANGING_FLAGS:
             return self._fullsnapshotinfo(fh, revinfo, target_rev)
 
         gather_debug = (
             self._write_debug is not None or self._debug_info is not None
         )
         debug_search = self._write_debug is not None and self._debug_search
 
         if gather_debug:
             start = util.timer()
 
         # 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'
 
         cachedelta = revinfo.cachedelta
         p1 = revinfo.p1
         p2 = revinfo.p2
         revlog = self.revlog
 
         deltainfo = None
         p1r, p2r = revlog.rev(p1), revlog.rev(p2)
 
         if gather_debug:
             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
         if debug_search:
             msg = b"DBG-DELTAS-SEARCH: SEARCH rev=%d\n"
             msg %= target_rev
             self._write_debug(msg)
 
         groups = _candidategroups(
             self.revlog,
             revinfo.textlen,
             p1r,
             p2r,
             cachedelta,
             excluded_bases,
             target_rev,
+            snapshot_cache=self._snapshot_cache,
         )
         candidaterevs = next(groups)
         while candidaterevs is not None:
             dbg_try_rounds += 1
             if 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 p1 in candidaterevs or p2 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 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 debug_search:
                     msg = b"DBG-DELTAS-SEARCH:   CANDIDATE: rev=%d\n"
                     msg %= candidaterev
                     self._write_debug(msg)
                     candidate_type = None
                     if candidaterev == p1:
                         candidate_type = b"p1"
                     elif candidaterev == p2:
                         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 debug_search:
                     delta_start = util.timer()
                 candidatedelta = self._builddeltainfo(revinfo, candidaterev, fh)
                 if 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 debug_search:
                             msg = b"DBG-DELTAS-SEARCH:     DELTA: length=%d (GOOD)\n"
                             msg %= candidatedelta.deltalen
                             self._write_debug(msg)
                         nominateddeltas.append(candidatedelta)
                     elif debug_search:
                         msg = b"DBG-DELTAS-SEARCH:     DELTA: length=%d (BAD)\n"
                         msg %= candidatedelta.deltalen
                         self._write_debug(msg)
                 elif 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(fh, 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()
             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,
                 'revision': target_rev,
                 'delta-base': deltainfo.base,  # pytype: disable=attribute-error
                 'search_round_count': dbg_try_rounds,
                 'using-cached-base': used_cached,
                 'delta_try_count': dbg_try_count,
                 'type': dbg_type,
                 'p1-chain-len': p1_chain_len,
                 'p2-chain-len': p2_chain_len,
             }
             if (
                 deltainfo.snapshotdepth  # pytype: disable=attribute-error
                 is not None
             ):
                 dbg[
                     'snapshot-depth'
                 ] = deltainfo.snapshotdepth  # pytype: disable=attribute-error
             else:
                 dbg['snapshot-depth'] = 0
             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
 
             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)
         return deltainfo
 
 
 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