revlog: store flag processors per revlog

Previously, revlog flag processing would consult a global dict
when processing flags. This was simple. But it had the undesired
side-effect that any extension could load flag processors once
and those flag processors would be available to any revlog that was
subsequent loaded in the process. e.g. in hgweb, if the narrow
extension were loaded for repo A but not repo B, repo B would be
able to decode ellipsis flags even though it shouldn't be able to.

Making the flag processors dict per-revlog allows us to have per-revlog
controls over what flag processors are available, thus preserving
desired granular access to flag processors depending on the revlog's
needs.

If a flag processor is globally registered, it is still globally
available. So this commit should not meaningfully change behavior.

Differential Revision: https://phab.mercurial-scm.org/D4646

mercurial/revlog.py

@@ -415,6 +415,10 b' class revlog(object):'
         self._srdensitythreshold = 0.50
         self._srmingapsize = 262144
 
+        # Make copy of flag processors so each revlog instance can support
+        # custom flags.
+        self._flagprocessors = dict(_flagprocessors)
+
         mmapindexthreshold = None
         v = REVLOG_DEFAULT_VERSION
         opts = getattr(opener, 'options', None)
@@ -1707,11 +1711,11 b' class revlog(object):'
             if flag & flags:
                 vhash = True
 
-                if flag not in _flagprocessors:
+                if flag not in self._flagprocessors:
                     message = _("missing processor for flag '%#x'") % (flag)
                     raise RevlogError(message)
 
-                processor = _flagprocessors[flag]
+                processor = self._flagprocessors[flag]
                 if processor is not None:
                     readtransform, writetransform, rawtransform = processor