[PATCH 5 of 6] tags: support setting hgtags fnodes cache entries

[Gregory Szorc]

# HG changeset patch
# User Gregory Szorc <gregory.szorc at gmail.com>
# Date 1432597735 25200
#      Mon May 25 16:48:55 2015 -0700
# Node ID 8f34e8f9636d47c5094844c4e8bbf7353b95d75a
# Parent  a95c5fe846acd1336c11f591273bc82db5b31ddc
tags: support setting hgtags fnodes cache entries

An upcoming patch will teach bundle2 to transfer .hgtags fnodes
values. To support this, we need to support inserting values into
the cache.

Add functionality to do that.

diff --git a/mercurial/tags.py b/mercurial/tags.py
--- a/mercurial/tags.py
+++ b/mercurial/tags.py
@@ -483,16 +483,29 @@ class hgtagsfnodescache(object):
         except error.LookupError:
             # No .hgtags file on this revision.
             fnode = nullid
 
+        self._writeentry(offset, properprefix, fnode)
+        return fnode
+
+    def setfnode(self, node, fnode):
+        """Set the .hgtags filenode for a given changeset."""
+        assert len(fnode) == 20
+        ctx = self._repo[node]
+
+        # Do a lookup first to avoid writing if nothing has changed.
+        if self.getfnode(ctx.node(), computemissing=False) == fnode:
+            return
+
+        self._writeentry(ctx.rev() * _fnodesrecsize, node[0:4], fnode)
+
+    def _writeentry(self, offset, prefix, fnode):
         # Slices on array instances only accept other array.
-        entry = array('c', properprefix + fnode)
+        entry = array('c', prefix + fnode)
         self._raw[offset:offset + _fnodesrecsize] = entry
         # self._dirtyoffset could be None.
         self._dirtyoffset = min(self._dirtyoffset, offset) or 0
 
-        return fnode
-
     def write(self):
         """Perform all necessary writes to cache file.
 
         This may no-op if no writes are needed or if a write lock could