[PATCH 8 of 8] delta: reuse _findsnapshot call from previous stage

[Boris Feld]

# HG changeset patch
# User Boris Feld <boris.feld at octobus.net>
# Date 1545041567 -3600
#      Mon Dec 17 11:12:47 2018 +0100
# Node ID 244ae450e30f3e352a56c62ade6a5759cb23e05f
# Parent  3294c80c1c71ab738a46bba671ee8e094a479fc6
# EXP-Topic sparse-revlog-corner-cases
# Available At https://bitbucket.org/octobus/mercurial-devel/
#              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r 244ae450e30f
delta: reuse _findsnapshot call from previous stage

Two different stage of the sparse-revlog logic needs the _findsnapshot data.
To avoid recomputing it twice, make it possible to reuse the first computation
in the second step.

example affected manifest write
before: 0.067141s
after:  0.064252s (-5%)

(total gain since start of series: 95%)

diff --git a/mercurial/revlogutils/deltas.py b/mercurial/revlogutils/deltas.py
--- a/mercurial/revlogutils/deltas.py
+++ b/mercurial/revlogutils/deltas.py
@@ -701,7 +701,8 @@ def _refinedgroups(revlog, p1, p2, cache
         if good is not None:
             yield None
             return
-    for candidates in _rawgroups(revlog, p1, p2, cachedelta):
+    snapshots = collections.defaultdict(list)
+    for candidates in _rawgroups(revlog, p1, p2, cachedelta, snapshots):
         good = yield candidates
         if good is not None:
             break
@@ -722,12 +723,8 @@ def _refinedgroups(revlog, p1, p2, cache
                 break
             good = yield (base,)
         # refine snapshot up
-        #
-        # XXX the _findsnapshots call can be expensive and is "duplicated" with
-        # the one done in `_rawgroups`. Once we start working on performance,
-        # we should make the two logics share this computation.
-        snapshots = collections.defaultdict(list)
-        _findsnapshots(revlog, snapshots, good + 1)
+        if not snapshots:
+            _findsnapshots(revlog, snapshots, good + 1)
         previous = None
         while good != previous:
             previous = good
@@ -737,7 +734,7 @@ def _refinedgroups(revlog, p1, p2, cache
     # we have found nothing
     yield None
 
-def _rawgroups(revlog, p1, p2, cachedelta):
+def _rawgroups(revlog, p1, p2, cachedelta, snapshots=None):
     """Provides group of revision to be tested as delta base
 
     This lower level function focus on emitting delta theorically interresting
@@ -767,7 +764,9 @@ def _rawgroups(revlog, p1, p2, cachedelt
             yield parents
 
     if sparse and parents:
-        snapshots = collections.defaultdict(list) # map: base-rev: snapshot-rev
+        if snapshots is None:
+            # map: base-rev: snapshot-rev
+            snapshots = collections.defaultdict(list)
         # See if we can use an existing snapshot in the parent chains to use as
         # a base for a new intermediate-snapshot
         #