[PATCH 1 of 4 STABLE] lock: avoid unintentional lock acquisition at failure of readlock

[FUJIWARA Katsunori]

# HG changeset patch
# User FUJIWARA Katsunori <foozy at lares.dti.ne.jp>
# Date 1493454422 -32400
#      Sat Apr 29 17:27:02 2017 +0900
# Branch stable
# Node ID 7cb8ebcca7d3a542031c41e09dc3ca0e2db36dcc
# Parent  fcddcf442e4f4cd15bb9334e009fb60299e0a4e6
lock: avoid unintentional lock acquisition at failure of readlock

Acquiring lock by vfs.makelock() and getting lock holder (aka
"locker") information by vfs.readlock() aren't atomic action.
Therefore, failure of the former doesn't ensure success of the latter.

Before this patch, lock is unintentionally acquired, if
self.parentlock is None (this is default value), and lock._readlock()
returns None for ENOEXIST at vfs.readlock(), because these None are
recognized as equal to each other.

In this case, lock symlink (or file) isn't created, even though lock
is treated as acquired in memory.

To avoid this issue, this patch checks self.parentlock, only if
lock._readlock() returns valid non-None "locker".

This issue will be covered by a test added in subsequent patch,
because simple emulation of ENOEXIST at vfs.readlock() easily causes
another issue.  "raising ENOEXIST only at the first vfs.readlock()
invocation" is too complicated for unit test, IMHO.

diff --git a/mercurial/lock.py b/mercurial/lock.py
--- a/mercurial/lock.py
+++ b/mercurial/lock.py
@@ -135,7 +135,7 @@ class lock(object):
                     # is different from the pid being different because we do
                     # want the unlock and postrelease functions to be called,
                     # but the lockfile to not be removed.
-                    if locker == self.parentlock:
+                    if locker is not None and locker == self.parentlock:
                         self._parentheld = True
                         self.held = 1
                         return