[PATCH 2 of 2] store: speed up read and write of large fncache files

[Bryan O'Sullivan]

# HG changeset patch
# User Bryan O'Sullivan <bryano at fb.com>
# Date 1334269314 25200
# Node ID bc925c976545ed38f4a94df006d8346e3772e6c8
# Parent  524152bfd147d5375a3692a35698857b92c8c6f5
store: speed up read and write of large fncache files

In my tests of an fncache containing 300,000 entries, this improves
read time from 567ms to 307, and write time from 1328ms to 533.

These numbers aren't so great, since the fncache file is only 17MB
in size, but they're an improvement.

diff -r 524152bfd147 -r bc925c976545 mercurial/store.py
--- a/mercurial/store.py	Thu Apr 12 15:21:52 2012 -0700
+++ b/mercurial/store.py	Thu Apr 12 15:21:54 2012 -0700
@@ -317,36 +317,35 @@
 
     def _load(self):
         '''fill the entries from the fncache file'''
-        self.entries = set()
         self._dirty = False
         try:
             fp = self.opener('fncache', mode='rb')
         except IOError:
             # skip nonexistent file
+            self.entries = set()
             return
-        for n, line in enumerate(fp):
-            if (len(line) < 2) or (line[-1] != '\n'):
-                t = _('invalid entry in fncache, line %s') % (n + 1)
-                raise util.Abort(t)
-            self.entries.add(decodedir(line[:-1]))
+        self.entries = set(map(decodedir, fp.read().splitlines()))
+        if '' in self.entries:
+            fp.seek(0)
+            for n, line in enumerate(fp):
+                if not line.rstrip('\n'):
+                    t = _('invalid entry in fncache, line %s') % (n + 1)
+                    raise util.Abort(t)
         fp.close()
 
-    def rewrite(self, files):
-        fp = self.opener('fncache', mode='wb')
-        for p in files:
-            fp.write(encodedir(p) + '\n')
+    def _write(self, files, atomictemp):
+        fp = self.opener('fncache', mode='wb', atomictemp=atomictemp)
+        fp.write('\n'.join(map(encodedir, files)) + '\n')
         fp.close()
-        self.entries = set(files)
         self._dirty = False
 
+    def rewrite(self, files):
+        self._write(files, False)
+        self.entries = set(files)
+
     def write(self):
-        if not self._dirty:
-            return
-        fp = self.opener('fncache', mode='wb', atomictemp=True)
-        for p in self.entries:
-            fp.write(encodedir(p) + '\n')
-        fp.close()
-        self._dirty = False
+        if self._dirty:
+            self._write(self.entries, True)
 
     def add(self, fn):
         if self.entries is None: