[PATCH 5 of 7] branchmap: acquires lock before writting the rev branch cache

[Pierre-Yves David]

# HG changeset patch
# User Pierre-Yves David <pierre-yves.david at ens-lyon.org>
# Date 1470401836 -7200
#      Fri Aug 05 14:57:16 2016 +0200
# Node ID 51046e1ff9b1d5817a95163a25ca3dc975db7e8f
# Parent  0b54d56b4217578079157532deea74045046d4c4
# EXP-Topic vfsward
branchmap: acquires lock before writting the rev branch cache

We now attempt to acquire a lock and write the branch cache within that lock.
This would prevent cache corruption when multiple processes try to write the cache
at the same time.

diff -r 0b54d56b4217 -r 51046e1ff9b1 mercurial/branchmap.py
--- a/mercurial/branchmap.py	Fri Aug 05 14:54:46 2016 +0200
+++ b/mercurial/branchmap.py	Fri Aug 05 14:57:16 2016 +0200
@@ -470,8 +470,10 @@ class revbranchcache(object):
     def write(self, tr=None):
         """Save branch cache if it is dirty."""
         repo = self._repo
-        if True:
+        wlock = None
+        try:
             if self._rbcnamescount < len(self._names):
+                wlock = repo.wlock(wait=False)
                 try:
                     if self._rbcnamescount != 0:
                         f = repo.vfs.open(_rbcnames, 'ab')
@@ -501,6 +503,7 @@ class revbranchcache(object):
 
             start = self._rbcrevslen * _rbcrecsize
             if start != len(self._rbcrevs):
+                wlock = repo.wlock(wait=False)
                 revs = min(len(repo.changelog),
                            len(self._rbcrevs) // _rbcrecsize)
                 try:
@@ -521,3 +524,8 @@ class revbranchcache(object):
                                   inst)
                     return
                 self._rbcrevslen = revs
+        except error.LockHeld as inst:
+            repo.ui.debug("couldn't write revision branch cache: %s\n" % inst)
+        finally:
+            if wlock is not None:
+                wlock.release()