# HG changeset patch
# User Boris Feld <boris.feld@octobus.net>
# Date 1545039997 -3600
# Node ID ba09db267cb68e54523e64f213d4f1eea50be9ac
# Parent  42f59d3f714dd283d6edcc4d72083ce38857f9cb
delta: ignore base whose chains already don't match expectations

If we know the existing chain does not match our criteria, there is no point
to build a delta to append. This is especially useful when dealing with a full
text much smaller than its parent. In that case, the parent chain is probably
already too large.

example affected manifest write
before: 1.421005s
after:  0.815520s (-42%)

diff -r 42f59d3f714d -r ba09db267cb6 mercurial/revlogutils/deltas.py
--- a/mercurial/revlogutils/deltas.py	Mon Dec 17 10:42:19 2018 +0100
+++ b/mercurial/revlogutils/deltas.py	Mon Dec 17 10:46:37 2018 +0100
@@ -655,6 +655,17 @@
             # no delta for rawtext-changing revs (see "candelta" for why)
             if revlog.flags(rev) & REVIDX_RAWTEXT_CHANGING_FLAGS:
                 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:
+                continue
+            # if chain already have too much data, skip base
+            if deltas_limit < chainsize:
+                continue
             group.append(rev)
         if group:
             # XXX: in the sparse revlog case, group can become large,