Add a features list to branches.cache to detect caches of old hg versions.

The leading space in the written file makes sure that the feature list never
can match an existing version, even if the first feature can be read as hex.
Additionally old hg versions display the id with --debug, too.

mercurial/localrepo.py

@@ -17,6 +17,7 b' demandload(globals(), "os revlog time ut'
 class localrepository(repo.repository):
     capabilities = ('lookup', 'changegroupsubset')
     supported = ('revlogv1', 'store')
+    branchcache_features = ('unnamed',)
 
     def __del__(self):
         self.transhandle = None
@@ -376,6 +377,22 b' class localrepository(repo.repository):'
             f = self.opener("branches.cache")
             lines = f.read().split('\n')
             f.close()
+            features = lines.pop(0).strip()
+            if not features.startswith('features: '):
+                raise ValueError(_('branch cache: no features specified'))
+            features = features.split(' ', 1)[1].split()
+            missing_features = []
+            for feature in self.branchcache_features:
+                try:
+                    features.remove(feature)
+                except ValueError, inst:
+                    missing_features.append(feature)
+            if missing_features:
+                raise ValueError(_('branch cache: missing features: %s')
+                                 % ', '.join(missing_features))
+            if features:
+                raise ValueError(_('branch cache: unknown features: %s')
+                                 % ', '.join(features))
             last, lrev = lines.pop(0).split(" ", 1)
             last, lrev = bin(last), int(lrev)
             if not (lrev < self.changelog.count() and
@@ -397,6 +414,7 b' class localrepository(repo.repository):'
     def _writebranchcache(self, branches, tip, tiprev):
         try:
             f = self.opener("branches.cache", "w")
+            f.write(" features: %s\n" % ' '.join(self.branchcache_features))
             f.write("%s %s\n" % (hex(tip), tiprev))
             for label, node in branches.iteritems():
                 f.write("%s %s\n" % (hex(node), label))

tests/test-mq-caches.out

@@ -7,6 +7,7 b' No .hg/branches.cache'
 # some regular revisions
 Patch queue now empty
 tip: 1
+ features: unnamed
 3f910abad313ff802d3a23a7529433872df9b3ae 1
 3f910abad313ff802d3a23a7529433872df9b3ae bar
 9539f35bdc80732cc9a3f84e46508f1ed1ec8cff foo
@@ -15,10 +16,12 b' 9539f35bdc80732cc9a3f84e46508f1ed1ec8cff'
 applying p1
 Now at: p1
 tip: 2
+ features: unnamed
 3f910abad313ff802d3a23a7529433872df9b3ae 1
 3f910abad313ff802d3a23a7529433872df9b3ae bar
 9539f35bdc80732cc9a3f84e46508f1ed1ec8cff foo
 tip: 3
+ features: unnamed
 3f910abad313ff802d3a23a7529433872df9b3ae 1
 3f910abad313ff802d3a23a7529433872df9b3ae bar
 9539f35bdc80732cc9a3f84e46508f1ed1ec8cff foo
@@ -27,6 +30,7 b' branch bar: 2'
 
 # removing the cache
 tip: 3
+ features: unnamed
 3f910abad313ff802d3a23a7529433872df9b3ae 1
 3f910abad313ff802d3a23a7529433872df9b3ae bar
 9539f35bdc80732cc9a3f84e46508f1ed1ec8cff foo
@@ -35,6 +39,7 b' branch bar: 2'
 
 # importing rev 1 (the cache now ends in one of the patches)
 tip: 3
+ features: unnamed
 3f910abad313ff802d3a23a7529433872df9b3ae 1
 3f910abad313ff802d3a23a7529433872df9b3ae bar
 9539f35bdc80732cc9a3f84e46508f1ed1ec8cff foo
@@ -49,5 +54,6 b' applying p1'
 applying p2
 Now at: p2
 tip: 3
+ features: unnamed
 9539f35bdc80732cc9a3f84e46508f1ed1ec8cff 0
 9539f35bdc80732cc9a3f84e46508f1ed1ec8cff foo

tests/test-newbranch.out

@@ -74,6 +74,7 b' modify a branch'
 
 
 4:4909a3732169
+ features: unnamed
 4909a3732169c0c20011c4f4b8fdff4e3d89b23f 4
 bf1bc2f45e834c75404d0ddab57d53beab56e2f8 
 4909a3732169c0c20011c4f4b8fdff4e3d89b23f foo