[PATCH 3 of 5 RFC] obsstore: fast path to check if obsstore is empty

[Yuya Nishihara]

# HG changeset patch
# User Yuya Nishihara <yuya at tcha.org>
# Date 1442134861 -32400
#      Sun Sep 13 18:01:01 2015 +0900
# Node ID 13a9dacedbbee19365ddd911020a74e8ba392f0e
# Parent  3e6c365f6630059e4c1ee4fbf7ce2aa557e1821f
obsstore: fast path to check if obsstore is empty

diff --git a/mercurial/obsolete.py b/mercurial/obsolete.py
--- a/mercurial/obsolete.py
+++ b/mercurial/obsolete.py
@@ -67,7 +67,7 @@ The header is followed by the markers. M
 comment associated with each format for details.
 
 """
-import struct
+import errno, struct
 import util, base85, node, parsers
 import phases
 from i18n import _
@@ -531,6 +531,15 @@ class obsstore(object):
         return len(self._all)
 
     def __nonzero__(self):
+        if not self._cached('_all'):
+            try:
+                return self.svfs.stat('obsstore').st_size > 1
+            except OSError as inst:
+                if inst.errno != errno.ENOENT:
+                    raise
+                # just build an empty _all list if no obsstore exists, which
+                # avoids further stat() syscalls
+                pass
         return bool(self._all)
 
     def create(self, transaction, prec, succs=(), flag=0, parents=None,