branchcache: add debug output whenever cache files use truncate

The cache files are usually append only but will automatically be truncated and
recover in exceptional situations. Add a debug notice when such exceptional
situations are encountered.

mercurial/branchmap.py

@@ -414,6 +414,7 b' class revbranchcache(object):'
                         f.write('\0')
                     else:
                         f.close()
+                        repo.ui.debug("%s changed - rewriting it\n" % _rbcnames)
                         self._rbcnamescount = 0
                         self._rbcrevslen = 0
                 if self._rbcnamescount == 0:
@@ -438,6 +439,7 b' class revbranchcache(object):'
                 # see issue3543.  SEEK_END was added in 2.5
                 f.seek(0, 2) #os.SEEK_END
                 if f.tell() != start:
+                    repo.ui.debug("truncating %s to %s\n" % (_rbcrevs, start))
                     f.seek(start)
                     f.truncate()
                 end = self._rbcrevslen * _rbcrecsize

tests/test-branches.t

@@ -550,6 +550,7 b' revision branch cache is created when bu'
 recovery from invalid cache revs file with trailing data
   $ echo >> .hg/cache/rbc-revs-v1
   $ rm -f .hg/cache/branch* && hg head a -T '{rev}\n' --debug
+  truncating cache/rbc-revs-v1 to 120
   5
   $ f --size .hg/cache/rbc-revs*
   .hg/cache/rbc-revs-v1: size=120
@@ -559,6 +560,7 b' recovery from invalid cache file with pa'
   $ f --size .hg/cache/rbc-revs*
   .hg/cache/rbc-revs-v1: size=119
   $ rm -f .hg/cache/branch* && hg head a -T '{rev}\n' --debug
+  truncating cache/rbc-revs-v1 to 112
   5
   $ f --size .hg/cache/rbc-revs*
   .hg/cache/rbc-revs-v1: size=120
@@ -580,6 +582,7 b' recovery from invalid cache file with so'
   $ hg log -r 'branch(.)' -T '{rev} '
   3 4 8 9 10 11 12 13  (no-eol)
   $ rm -f .hg/cache/branch* && hg head a -T '{rev}\n' --debug
+  truncating cache/rbc-revs-v1 to 8
   5
   $ f --size --hexdump --bytes=16 .hg/cache/rbc-revs*
   .hg/cache/rbc-revs-v1: size=120

tests/test-rebase-conflicts.t

@@ -322,5 +322,6 b' Check that the right ancestors is used w'
   files: 1/1 chunks (100.00%)
   added 2 changesets with 2 changes to 1 files
   invalid branchheads cache (served): tip differs
+  truncating cache/rbc-revs-v1 to 72
   rebase completed
   updating the branch cache