revisionbranchcache: fall back to slow path if starting readonly (issue4531)

Transitioning to Mercurial versions with revision branch cache could be slow as
long as all operations were readonly (revset queries) and the cache would be
populated but not written back.

Instead, fall back to using the consistently slow path when readonly and the
cache doesn't exist yet. That avoids the overhead of populating the cache
without writing it back.

If not readonly, it will still populate all missing entries initially. That
avoids repeated writing of the cache file with small updates, and it also makes
sure a fully populated cache available for the readonly operations.

mercurial/branchmap.py

@@ -330,7 +330,7 b' class revbranchcache(object):'
     and will grow with it but be 1/8th of its size.
     """
 
-    def __init__(self, repo):
+    def __init__(self, repo, readonly=True):
         assert repo.filtername is None
         self._names = [] # branch names in local encoding with static index
         self._rbcrevs = array('c') # structs of type _rbcrecfmt
@@ -342,6 +342,10 b' class revbranchcache(object):'
         except (IOError, OSError), inst:
             repo.ui.debug("couldn't read revision branch cache names: %s\n" %
                           inst)
+            if readonly:
+                # don't try to use cache - fall back to the slow path
+                self.branchinfo = self._branchinfo
+
         if self._names:
             try:
                 data = repo.vfs.read(_rbcrevs)

mercurial/revset.py

@@ -527,7 +527,7 b' def branch(repo, subset, x):'
     import branchmap
     urepo = repo.unfiltered()
     ucl = urepo.changelog
-    getbi = branchmap.revbranchcache(urepo).branchinfo
+    getbi = branchmap.revbranchcache(urepo, readonly=True).branchinfo
 
     try:
         b = getstring(x, '')