delta: exclude base candidate much smaller than the target

If a revision's full text is that much bigger than a base candidate full text,
we no longer consider that candidate.

This solves a pathological case we encountered on a very specify repository.
It contains a long series of changesets with a very small manifest (one file)
co-existing with others changesets using a very large manifest.

Without this filtering, we ended up considering a large number of tiny full
snapshots as a potential base. It resulted in very large delta (the size of
the full text) and mercurial spending 99% of its time compressing these
deltas.

The timing of a commit moved from about 400s to about 10s (still slow, but not
ridiculously slow).

mercurial/revlogutils/deltas.py

@@ -601,6 +601,11 b' def isgooddeltainfo(revlog, deltainfo, r'
 
     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 = 50
+
 def _candidategroups(revlog, textlen, p1, p2, cachedelta):
     """Provides group of revision to be tested as delta base
 
@@ -614,6 +619,7 b' def _candidategroups(revlog, textlen, p1'
 
     deltalength = revlog.length
     deltaparent = revlog.deltaparent
+    sparse = revlog._sparserevlog
     good = None
 
     deltas_limit = textlen * LIMIT_DELTA2TEXT
@@ -644,6 +650,8 b' def _candidategroups(revlog, textlen, p1'
             # filter out delta base that will never produce good delta
             if deltas_limit < revlog.length(rev):
                 continue
+            if sparse and revlog.rawsize(rev) < (textlen // LIMIT_BASE2TEXT):
+                continue
             # no delta for rawtext-changing revs (see "candelta" for why)
             if revlog.flags(rev) & REVIDX_RAWTEXT_CHANGING_FLAGS:
                 continue

tests/test-clone-uncompressed.t

@@ -184,8 +184,8 b' Basic clone'
 #if stream-bundle2
   $ hg clone --stream -U http://localhost:$HGPORT clone1
   streaming all changes
-  1030 files to transfer, 96.4 KB of data
-  transferred 96.4 KB in * seconds (* */sec) (glob)
+  1030 files to transfer, 96.5 KB of data
+  transferred 96.5 KB in * seconds (* */sec) (glob)
 
   $ ls -1 clone1/.hg/cache
   branch2-served
@@ -201,11 +201,11 b' getbundle requests with stream=1 are unc'
   
 
   $ f --size --hex --bytes 256 body
-  body: size=112245
+  body: size=112262
   0000: 04 6e 6f 6e 65 48 47 32 30 00 00 00 00 00 00 00 |.noneHG20.......|
   0010: 7f 07 53 54 52 45 41 4d 32 00 00 00 00 03 00 09 |..STREAM2.......|
   0020: 05 09 04 0c 44 62 79 74 65 63 6f 75 6e 74 39 38 |....Dbytecount98|
-  0030: 37 35 38 66 69 6c 65 63 6f 75 6e 74 31 30 33 30 |758filecount1030|
+  0030: 37 37 35 66 69 6c 65 63 6f 75 6e 74 31 30 33 30 |775filecount1030|
   0040: 72 65 71 75 69 72 65 6d 65 6e 74 73 64 6f 74 65 |requirementsdote|
   0050: 6e 63 6f 64 65 25 32 43 66 6e 63 61 63 68 65 25 |ncode%2Cfncache%|
   0060: 32 43 67 65 6e 65 72 61 6c 64 65 6c 74 61 25 32 |2Cgeneraldelta%2|
@@ -232,8 +232,8 b' getbundle requests with stream=1 are unc'
 #if stream-bundle2
   $ hg clone --uncompressed -U http://localhost:$HGPORT clone1-uncompressed
   streaming all changes
-  1030 files to transfer, 96.4 KB of data
-  transferred 96.4 KB in * seconds (* */sec) (glob)
+  1030 files to transfer, 96.5 KB of data
+  transferred 96.5 KB in * seconds (* */sec) (glob)
 #endif
 
 Clone with background file closing enabled
@@ -274,12 +274,12 b' Clone with background file closing enabl'
   bundle2-input-bundle: with-transaction
   bundle2-input-part: "stream2" (params: 3 mandatory) supported
   applying stream bundle
-  1030 files to transfer, 96.4 KB of data
+  1030 files to transfer, 96.5 KB of data
   starting 4 threads for background file closing
   starting 4 threads for background file closing
   updating the branch cache
-  transferred 96.4 KB in * seconds (* */sec) (glob)
-  bundle2-input-part: total payload size 112077
+  transferred 96.5 KB in * seconds (* */sec) (glob)
+  bundle2-input-part: total payload size 112094
   bundle2-input-part: "listkeys" (params: 1 mandatory) supported
   bundle2-input-bundle: 1 parts total
   checking for updated bookmarks
@@ -318,8 +318,8 b' Streaming of secrets can be overridden b'
 #if stream-bundle2
   $ hg clone --stream -U http://localhost:$HGPORT secret-allowed
   streaming all changes
-  1030 files to transfer, 96.4 KB of data
-  transferred 96.4 KB in * seconds (* */sec) (glob)
+  1030 files to transfer, 96.5 KB of data
+  transferred 96.5 KB in * seconds (* */sec) (glob)
 #endif
 
   $ killdaemons.py