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Switch to using a separate lockfile per target.

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The previous method, using fcntl byterange locks, was very efficient and
avoided unnecessarily filesystem metadata churn (ie. creating/deleting
inodes).  Unfortunately, MacOS X (at least version 10.6.5) apparently has a
race condition in its fcntl locking that makes it unusably unreliable
(http://apenwarr.ca/log/?m=201012#13).

My tests indicate that if you only ever lock a *single* byterange on a file,
the race condition doesn't cause a problem.  So let's just use one lockfile
per target.  Now "redo -j20 test" passes for me on both MacOS and Linux.

This doesn't measurably affect the speed on Linux, at least, in my tests.

The bad news: it's hard to safely *delete* those lockfiles when we're done
with them, so they tend to accumulate in the .redo dir.
This commit is contained in:
Avery Pennarun 2010-12-14 02:25:17 -08:00
commit 95680ed7ef
2 changed files with 13 additions and 10 deletions
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18
state.py
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@ -19,9 +19,8 @@ def _connect(dbfile):
_db = None
_lockfile = None
def db():
global _db, _lockfile
global _db
if _db:
return _db
@ -35,10 +34,6 @@ def db():
else:
raise
_lockfile = os.open(os.path.join(vars.BASE, '.redo/locks'),
os.O_RDWR | os.O_CREAT, 0666)
close_on_exec(_lockfile, True)
must_create = not os.path.exists(dbfile)
if not must_create:
_db = _connect(dbfile)
@ -296,19 +291,22 @@ class Lock:
def __init__(self, fid):
self.owned = False
self.fid = fid
assert(_lockfile >= 0)
self.lockfile = os.open(os.path.join(vars.BASE, '.redo/lock.%d' % fid),
os.O_RDWR | os.O_CREAT, 0666)
close_on_exec(self.lockfile, True)
assert(_locks.get(fid,0) == 0)
_locks[fid] = 1
def __del__(self):
_locks[self.fid] = 0
os.close(self.lockfile)
if self.owned:
self.unlock()
def trylock(self):
assert(not self.owned)
try:
fcntl.lockf(_lockfile, fcntl.LOCK_EX|fcntl.LOCK_NB, 1, self.fid)
fcntl.lockf(self.lockfile, fcntl.LOCK_EX|fcntl.LOCK_NB, 0, 0)
except IOError, e:
if e.errno in (errno.EAGAIN, errno.EACCES):
pass # someone else has it locked
@ -319,12 +317,12 @@ class Lock:
def waitlock(self):
assert(not self.owned)
fcntl.lockf(_lockfile, fcntl.LOCK_EX, 1, self.fid)
fcntl.lockf(self.lockfile, fcntl.LOCK_EX, 0, 0)
self.owned = True
def unlock(self):
if not self.owned:
raise Exception("can't unlock %r - we don't own it"
% self.lockname)
fcntl.lockf(_lockfile, fcntl.LOCK_UN, 1, self.fid)
fcntl.lockf(self.lockfile, fcntl.LOCK_UN, 0, 0)
self.owned = False