filelog: cmp: don't read data if hashes are identical (issue2273)

filelog.renamed() is an expensive call as it reads the filelog if p1 == nullid.
It's more efficient to first compute the hash, and to bail early if
the computed hash is the same as the stored nodeid.

'samehashes' variable is not strictly necessary, but helps for comprehension.

mercurial/filelog.py

@@ -1,70 +1,79 b''
 # filelog.py - file history class for mercurial
 #
 # Copyright 2005-2007 Matt Mackall <[e-mail]>
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 import revlog
 
 class filelog(revlog.revlog):
     def __init__(self, opener, path):
         revlog.revlog.__init__(self, opener,
                         "/".join(("data", path + ".i")))
 
     def read(self, node):
         t = self.revision(node)
         if not t.startswith('\1\n'):
             return t
         s = t.index('\1\n', 2)
         return t[s + 2:]
 
     def _readmeta(self, node):
         t = self.revision(node)
         if not t.startswith('\1\n'):
             return {}
         s = t.index('\1\n', 2)
         mt = t[2:s]
         m = {}
         for l in mt.splitlines():
             k, v = l.split(": ", 1)
             m[k] = v
         return m
 
     def add(self, text, meta, transaction, link, p1=None, p2=None):
         if meta or text.startswith('\1\n'):
             mt = ["%s: %s\n" % (k, v) for k, v in sorted(meta.iteritems())]
             text = "\1\n%s\1\n%s" % ("".join(mt), text)
         return self.addrevision(text, transaction, link, p1, p2)
 
     def renamed(self, node):
         if self.parents(node)[0] != revlog.nullid:
             return False
         m = self._readmeta(node)
         if m and "copy" in m:
             return (m["copy"], revlog.bin(m["copyrev"]))
         return False
 
     def size(self, rev):
         """return the size of a given revision"""
 
         # for revisions with renames, we have to go the slow way
         node = self.node(rev)
         if self.renamed(node):
             return len(self.read(node))
 
         # XXX if self.read(node).startswith("\1\n"), this returns (size+4)
         return revlog.revlog.size(self, rev)
 
     def cmp(self, node, text):
         """compare text with a given file revision
 
         returns True if text is different than what is stored.
         """
 
-        # for renames, we have to go the slow way
-        if text.startswith('\1\n') or self.renamed(node):
+        t = text
+        if text.startswith('\1\n'):
+            t = '\1\n\1\n' + text
+
+        samehashes = not revlog.revlog.cmp(self, node, t)
+        if samehashes:
+            return False
+
+        # renaming a file produces a different hash, even if the data
+        # remains unchanged. Check if it's the case (slow):
+        if self.renamed(node):
             t2 = self.read(node)
             return t2 != text
 
-        return revlog.revlog.cmp(self, node, text)
+        return True