rbc: fix superfluous rebuilding from scratch - don't abuse self._rbcnamescount

The code used self._rbcnamescount as if it was the length of self._names ...
but actually it is just the number of good entries on disk. This caused the
cache to be populated inefficiently. In some cases very inefficiently.

Instead of checking the length before lookup, just try a lookup in self._names
- that is also in most cases faster.

Comments and debug messages are tweaked to help understanding the issue
and the fix.

mercurial/branchmap.py

@@ -358,7 +358,7 b' class revbranchcache(object):'
         self._repo = repo
         self._names = [] # branch names in local encoding with static index
         self._rbcrevs = array('c') # structs of type _rbcrecfmt
-        self._rbcsnameslen = 0
+        self._rbcsnameslen = 0 # length of names read at _rbcsnameslen
         try:
             bndata = repo.vfs.read(_rbcnames)
             self._rbcsnameslen = len(bndata) # for verification before writing
@@ -380,7 +380,8 b' class revbranchcache(object):'
                                len(repo.changelog))
         if self._rbcrevslen == 0:
             self._names = []
-        self._rbcnamescount = len(self._names) # number of good names on disk
+        self._rbcnamescount = len(self._names) # number of names read at
+                                               # _rbcsnameslen
         self._namesreverse = dict((b, r) for r, b in enumerate(self._names))
 
     def _clear(self):
@@ -416,13 +417,17 b' class revbranchcache(object):'
         if cachenode == '\0\0\0\0':
             pass
         elif cachenode == reponode:
-            if branchidx < self._rbcnamescount:
+            try:
                 return self._names[branchidx], close
-            # referenced branch doesn't exist - rebuild is expensive but needed
-            self._repo.ui.debug("rebuilding corrupted revision branch cache\n")
-            self._clear()
+            except IndexError:
+                # recover from invalid reference to unknown branch
+                self._repo.ui.debug("referenced branch names not found"
+                    " - rebuilding revision branch cache from scratch\n")
+                self._clear()
         else:
             # rev/node map has changed, invalidate the cache from here up
+            self._repo.ui.debug("history modification detected - truncating "
+                "revision branch cache to revision %s\n" % rev)
             truncate = rbcrevidx + _rbcrecsize
             del self._rbcrevs[truncate:]
             self._rbcrevslen = min(self._rbcrevslen, truncate)

tests/test-branches.t

@@ -587,6 +587,8 b' recovery from invalid cache file with so'
   $ f --size .hg/cache/rbc-revs*
   .hg/cache/rbc-revs-v1: size=120
   $ hg log -r 'branch(.)' -T '{rev} ' --debug
+  history modification detected - truncating revision branch cache to revision 13
+  history modification detected - truncating revision branch cache to revision 1
   3 4 8 9 10 11 12 13 truncating cache/rbc-revs-v1 to 8
   $ rm -f .hg/cache/branch* && hg head a -T '{rev}\n' --debug
   5
@@ -632,7 +634,7 b' situation where the cache is out of sync'
 cache is rebuilt when corruption is detected
   $ echo > .hg/cache/rbc-names-v1
   $ hg log -r '5:&branch(.)' -T '{rev} ' --debug
-  rebuilding corrupted revision branch cache
+  referenced branch names not found - rebuilding revision branch cache from scratch
   8 9 10 11 12 13 truncating cache/rbc-revs-v1 to 40
   $ f --size --hexdump .hg/cache/rbc-*
   .hg/cache/rbc-names-v1: size=79
@@ -688,16 +690,13 b' Test for multiple incorrect branch cache'
   0010: 56 46 78 69 00 00 00 01                         |VFxi....|
   $ : > .hg/cache/rbc-revs-v1
 
+No superfluous rebuilding of cache:
   $ hg log -r "branch(null)&branch(branch)" --debug
-  rebuilding corrupted revision branch cache
-  rebuilding corrupted revision branch cache
-  truncating cache/rbc-revs-v1 to 8
-BUG: the cache was declared corrupt multiple times and not fully rebuilt:
   $ f --size --hexdump .hg/cache/rbc-*
   .hg/cache/rbc-names-v1: size=14
   0000: 64 65 66 61 75 6c 74 00 62 72 61 6e 63 68       |default.branch|
   .hg/cache/rbc-revs-v1: size=24
-  0000: 00 00 00 00 00 00 00 00 fa 4c 04 e5 00 00 00 00 |.........L......|
+  0000: 66 e5 f5 aa 00 00 00 00 fa 4c 04 e5 00 00 00 00 |f........L......|
   0010: 56 46 78 69 00 00 00 01                         |VFxi....|
 
   $ cd ..

tests/test-rebase-conflicts.t

@@ -302,6 +302,7 b' Check that the right ancestors is used w'
   bundle2-input-part: total payload size 1713
   bundle2-input-bundle: 0 parts total
   invalid branchheads cache (served): tip differs
+  history modification detected - truncating revision branch cache to revision 9
   rebase completed
   updating the branch cache
   truncating cache/rbc-revs-v1 to 72