snapshot: introduce an intermediate `_refinedgroups` generator

This method will be used to improve the search for a good snapshot base. To
keep things simpler, we introduce the necessary function before doing any
delta base logic change. The next handful of commits will focus on refactoring
the code to let that new logic land as clearly as possible.

# General Idea

Right now, the search for a good delta base stop whenever we found a good one.
However, when using sparse-revlog, we should probably try a bit harder.

We do significant effort to increase delta re-use by jumping on "unrelated"
delta chains that provide better results. Moving to another chain for a better
result is good, but we have no guarantee we jump at a reasonable point in that
new chain. When we consider over the chains related to the parents, we start
from the higher-level snapshots. This is a way to consider the snapshot closer
to the current revision that has the best chance to produce a small delta. We
do benefit from this walk order when jumping to a better "unrelated" stack.

To counter-balance this, we'll introduce a way to "refine" the result. After a
good delta have been found, we'll keep searching for a better delta, using the
current best one as a starting point.

# Target Setup

The `finddeltainfo` method is responsible for the general search for a good
delta. It requests candidates base from `_candidategroups` and decides which
one are usable.

The `_candidategroups` generator act as a top-level filter, it does not care
about how we pick candidates, it just does basic filtering, excluding
revisions that have been tested already or that are an obvious misfit.

The `_rawgroups` generator is the one with the actual ancestors walking logic,
It does not care about what would do a good delta and what was already tested,
it just issues the initial candidates.

We introduce a new `_refinedgroup` function to bridge the gap between
`_candidategroups` and `_rawgroups`. It delegates the initial iteration logic
and then performing relevant refining of the valid base once found. (This
logic is yet to be added to function)

All these logics are fairly independent and easier to understand when standing
alone, not mixed with each other. It also makes it easy to test and try
different approaches for one of those four layers without affecting the other
ones.

# Technical details

To communicate `finddeltainfo` choice of "current best delta base" to the
`_refinegroup` logic, we plan to use python co-routine feature. The
`_candidategroups` and `_refinegroup` generators will become co-routine. This
will allow `_refinegroup` to detect when a good delta have been found and
triggers various refining steps.

For now, `_candidategroups` will just pass the value down the stack.

After poking at various option, the co-routine appears the best to keep each
layers focus on its duty, without the need to spread implementation details
across layers.

mercurial/revlogutils/deltas.py

@@ -1,850 +1,857 b''
 # revlogdeltas.py - Logic around delta computation for revlog
 #
 # Copyright 2005-2007 Matt 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"""
 
 from __future__ import absolute_import
 
 import collections
 import heapq
 import struct
 
 # import stuff from node for others to import from revlog
 from ..node import (
     nullrev,
 )
 from ..i18n import _
 
 from .constants import (
     REVIDX_ISCENSORED,
     REVIDX_RAWTEXT_CHANGING_FLAGS,
 )
 
 from ..thirdparty import (
     attr,
 )
 
 from .. import (
     error,
     mdiff,
 )
 
 RevlogError = error.RevlogError
 CensoredNodeError = error.CensoredNodeError
 
 # maximum <delta-chain-data>/<revision-text-length> ratio
 LIMIT_DELTA2TEXT = 2
 
 class _testrevlog(object):
     """minimalist fake revlog to use in doctests"""
 
     def __init__(self, data, density=0.5, mingap=0):
         """data is an list of revision payload boundaries"""
         self._data = data
         self._srdensitythreshold = density
         self._srmingapsize = mingap
 
     def start(self, rev):
         if rev == 0:
             return 0
         return self._data[rev - 1]
 
     def end(self, rev):
         return self._data[rev]
 
     def length(self, rev):
         return self.end(rev) - self.start(rev)
 
     def __len__(self):
         return len(self._data)
 
 def slicechunk(revlog, revs, deltainfo=None, 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.
 
     >>> 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(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]]
     """
     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,
     # * deltainfo is used to pick are good delta chain when writing.
     if not (deltainfo is None or targetsize is None):
         msg = 'cannot use `targetsize` with a `deltainfo`'
         raise error.ProgrammingError(msg)
     for chunk in _slicechunktodensity(revlog, revs,
                                       deltainfo,
                                       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.
 
     >>> revlog = _testrevlog([
     ...  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)
     ... ])
 
     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]]
     """
     assert targetsize is None or 0 <= targetsize
     if targetsize is None or segmentspan(revlog, revs) <= targetsize:
         yield revs
         return
 
     startrevidx = 0
     startdata = revlog.start(revs[0])
     endrevidx = 0
     iterrevs = enumerate(revs)
     next(iterrevs) # skip first rev.
     for idx, r in iterrevs:
         span = revlog.end(r) - startdata
         if span <= targetsize:
             endrevidx = idx
         else:
             chunk = _trimchunk(revlog, revs, startrevidx, endrevidx + 1)
             if chunk:
                 yield chunk
             startrevidx = idx
             startdata = revlog.start(r)
             endrevidx = idx
     yield _trimchunk(revlog, revs, startrevidx)
 
 def _slicechunktodensity(revlog, revs, deltainfo=None, 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.
 
     ``deltainfo`` is a _deltainfo instance of a revision that we would append
     to the top of the revlog.
 
     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
 
     nextrev = len(revlog)
     nextoffset = revlog.end(nextrev - 1)
 
     if deltainfo is None:
         deltachainspan = segmentspan(revlog, revs)
         chainpayload = sum(length(r) for r in revs)
     else:
         deltachainspan = deltainfo.distance
         chainpayload = deltainfo.compresseddeltalen
 
     if deltachainspan < mingapsize:
         yield revs
         return
 
     readdata = deltachainspan
 
     if deltachainspan:
         density = chainpayload / float(deltachainspan)
     else:
         density = 1.0
 
     if density >= targetdensity:
         yield revs
         return
 
     if deltainfo is not None and deltainfo.deltalen:
         revs = list(revs)
         revs.append(nextrev)
 
     # Store the gaps in a heap to have them sorted by decreasing size
     gapsheap = []
     heapq.heapify(gapsheap)
     prevend = None
     for i, rev in enumerate(revs):
         if rev < nextrev:
             revstart = start(rev)
             revlen = length(rev)
         else:
             revstart = nextoffset
             revlen = deltainfo.deltalen
 
         # 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:
                 heapq.heappush(gapsheap, (-gapsize, i))
 
         prevend = revstart + revlen
 
     # Collect the indices of the largest holes until the density is acceptable
     indicesheap = []
     heapq.heapify(indicesheap)
     while gapsheap and density < targetdensity:
         oppgapsize, gapidx = heapq.heappop(gapsheap)
 
         heapq.heappush(indicesheap, gapidx)
 
         # the gap sizes are stored as negatives to be sorted decreasingly
         # by the heap
         readdata -= (-oppgapsize)
         if readdata > 0:
             density = chainpayload / float(readdata)
         else:
             density = 1.0
 
     # Cut the revs at collected indices
     previdx = 0
     while indicesheap:
         idx = heapq.heappop(indicesheap)
 
         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, deltainfo=None):
     """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
     if deltainfo is not None and len(revlog) <= revs[-1]:
         if len(revs) == 1:
             return deltainfo.deltalen
         offset = revlog.end(len(revlog) - 1)
         end = deltainfo.deltalen + offset
     else:
         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(">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, raw=False)
         fulltext = mdiff.patch(basetext, delta)
 
     try:
         res = revlog._processflags(fulltext, flags, 'read', raw=True)
         fulltext, validatehash = res
         if validatehash:
             revlog.checkhash(fulltext, expectednode, p1=p1, p2=p2)
         if flags & REVIDX_ISCENSORED:
             raise RevlogError(_('node %s is not censored') % expectednode)
     except 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(object):
     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 isgooddeltainfo(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.
 
     if revlog._sparserevlog:
         # As sparse-read will be used, we can consider that the distance,
         # instead of being the span of the whole chunk,
         # is the span of the largest read chunk
         base = deltainfo.base
 
         if base != nullrev:
             deltachain = revlog._deltachain(base)[0]
         else:
             deltachain = []
 
         # search for the first non-snapshot revision
         for idx, r in enumerate(deltachain):
             if not revlog.issnapshot(r):
                 break
         deltachain = deltachain[idx:]
         chunks = slicechunk(revlog, deltachain, deltainfo)
         all_span = [segmentspan(revlog, revs, deltainfo)
                     for revs in chunks]
         distance = max(all_span)
     else:
         distance = deltainfo.distance
 
     textlen = revinfo.textlen
     defaultmax = textlen * 4
     maxdist = revlog._maxdeltachainspan
     if not maxdist:
         maxdist = distance # ensure the conditional pass
     maxdist = max(maxdist, defaultmax)
     if revlog._sparserevlog and maxdist < revlog._srmingapsize:
         # In multiple place, we are ignoring irrelevant data range below a
         # certain size. Be also apply this tradeoff here and relax span
         # constraint for small enought content.
         maxdist = revlog._srmingapsize
 
     # Bad delta from read span:
     #
     #   If the span of data read is larger than the maximum allowed.
     if maxdist < 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
 
 def _candidategroups(revlog, textlen, p1, p2, cachedelta):
     """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):
         return
 
     deltalength = revlog.length
     deltaparent = revlog.deltaparent
 
     deltas_limit = textlen * LIMIT_DELTA2TEXT
 
     tested = set([nullrev])
-    for temptative in _rawgroups(revlog, p1, p2, cachedelta):
+    for temptative in _refinedgroups(revlog, p1, p2, cachedelta):
         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)):
                 rev = deltaparent(rev)
                 tested.add(rev)
             # filter out revision we tested already
             if rev in tested:
                 continue
             tested.add(rev)
             # filter out delta base that will never produce good delta
             if deltas_limit < revlog.length(rev):
                 continue
             # no need to try a delta against nullrev, this will be done as a
             # last resort.
             if rev == nullrev:
                 continue
             # no delta for rawtext-changing revs (see "candelta" for why)
             if revlog.flags(rev) & REVIDX_RAWTEXT_CHANGING_FLAGS:
                 continue
             group.append(rev)
         if group:
             # XXX: in the sparse revlog case, group can become large,
             #      impacting performances. Some bounding or slicing mecanism
             #      would help to reduce this impact.
             yield tuple(group)
 
 def _findsnapshots(revlog, cache, start_rev):
     """find snapshot from start_rev to tip"""
     deltaparent = revlog.deltaparent
     issnapshot = revlog.issnapshot
     for rev in revlog.revs(start_rev):
         if issnapshot(rev):
             cache[deltaparent(rev)].append(rev)
 
+def _refinedgroups(revlog, p1, p2, cachedelta):
+    good = None
+    for candidates in _rawgroups(revlog, p1, p2, cachedelta):
+        good = yield candidates
+        if good is not None:
+            break
+
 def _rawgroups(revlog, p1, p2, cachedelta):
     """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
     sparse = revlog._sparserevlog
     curr = len(revlog)
     prev = curr - 1
     deltachain = lambda rev: revlog._deltachain(rev)[0]
 
     # 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 of source delta is useful when
     # we want to make sure we recomputed "optimal" deltas.
     if cachedelta and gdelta and revlog._lazydeltabase:
         # Assume what we received from the server is a good choice
         # build delta will reuse the cache
         yield (cachedelta[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:
         snapshots = collections.defaultdict(list) # map: base-rev: snapshot-rev
         # 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)
         for p in parents:
             for idx, s in enumerate(deltachain(p)):
                 if not revlog.issnapshot(s):
                     break
                 parents_snaps[idx].add(s)
         snapfloor = min(parents_snaps[0]) + 1
         _findsnapshots(revlog, snapshots, snapfloor)
         # 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(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.
         yield tuple(snapshots[nullrev])
 
     # other approach failed try against prev to hopefully save us a
     # fulltext.
     yield (prev,)
 
 class deltacomputer(object):
     def __init__(self, revlog):
         self.revlog = revlog
 
     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.revision(base, _df=fh, raw=True)
             delta = mdiff.textdiff(ptext, t)
 
         return delta
 
     def _builddeltainfo(self, revinfo, base, fh):
         # can we use the cached delta?
         if revinfo.cachedelta and revinfo.cachedelta[0] == base:
             delta = revinfo.cachedelta[1]
         else:
             delta = self._builddeltadiff(base, revinfo, fh)
         revlog = self.revlog
         header, data = revlog.compress(delta)
         deltalen = len(header) + len(data)
         chainbase = revlog.chainbase(base)
         offset = revlog.end(len(revlog) - 1)
         dist = deltalen + offset - revlog.start(chainbase)
         if revlog._generaldelta:
             deltabase = base
         else:
             deltabase = chainbase
         chainlen, compresseddeltalen = revlog._chaininfo(base)
         chainlen += 1
         compresseddeltalen += deltalen
 
         revlog = self.revlog
         snapshotdepth = None
         if 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])
 
         return _deltainfo(dist, deltalen, (header, data), deltabase,
                           chainbase, chainlen, compresseddeltalen,
                           snapshotdepth)
 
     def _fullsnapshotinfo(self, fh, revinfo):
         curr = len(self.revlog)
         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):
         """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.
         """
         if not revinfo.textlen:
             return self._fullsnapshotinfo(fh, revinfo)
 
         # 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)
 
         cachedelta = revinfo.cachedelta
         p1 = revinfo.p1
         p2 = revinfo.p2
         revlog = self.revlog
 
         deltainfo = None
         p1r, p2r = revlog.rev(p1), revlog.rev(p2)
         groups = _candidategroups(self.revlog, revinfo.textlen,
                                              p1r, p2r, cachedelta)
         for candidaterevs in groups:
             nominateddeltas = []
             for candidaterev in candidaterevs:
                 candidatedelta = self._builddeltainfo(revinfo, candidaterev, fh)
                 if isgooddeltainfo(self.revlog, candidatedelta, revinfo):
                     nominateddeltas.append(candidatedelta)
             if nominateddeltas:
                 deltainfo = min(nominateddeltas, key=lambda x: x.deltalen)
                 break
 
         if deltainfo is None:
             deltainfo = self._fullsnapshotinfo(fh, revinfo)
         return deltainfo