mercurial/lock.py
author Vadim Gelfer <vadim.gelfer@gmail.com>
Fri, 10 Mar 2006 08:31:31 -0800
changeset 1877 d314a89fa4f1
parent 1836 cd5c1db2132a
child 2016 ff5c9a92f556
permissions -rw-r--r--
change lock format to let us detect and break stale locks. old style: symlink to pid new style: symlink to hostname:pid if lock code finds new-style lock, it breaks lock if locking pid is on same machine and pid is not alive. otherwise, lock is left alone. this makes locking code safe with old-style locks and with locks on other machines. new code makes server part of mercurial more robust in case machine crashes, power fails, or crazy user does kill -9.
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# lock.py - simple locking scheme for mercurial
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#
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# Copyright 2005 Matt Mackall <mpm@selenic.com>
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#
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# This software may be used and distributed according to the terms
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# of the GNU General Public License, incorporated herein by reference.
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from demandload import *
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demandload(globals(), 'errno os socket time util')
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class LockException(Exception):
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    pass
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class LockHeld(LockException):
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    pass
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class LockUnavailable(LockException):
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    pass
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class lock(object):
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    # lock is symlink on platforms that support it, file on others.
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    # symlink is used because create of directory entry and contents
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    # are atomic even over nfs.
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    # old-style lock: symlink to pid
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    # new-style lock: symlink to hostname:pid
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    def __init__(self, file, timeout=-1, releasefn=None):
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        self.f = file
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        self.held = 0
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        self.timeout = timeout
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        self.releasefn = releasefn
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        self.id = None
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        self.host = None
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        self.pid = None
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        self.lock()
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    def __del__(self):
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        self.release()
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    def lock(self):
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        timeout = self.timeout
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        while 1:
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            try:
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                self.trylock()
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                return 1
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            except LockHeld, inst:
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                if timeout != 0:
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                    time.sleep(1)
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                    if timeout > 0:
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                        timeout -= 1
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                    continue
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                raise inst
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    def trylock(self):
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        if self.id is None:
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            self.host = socket.gethostname()
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            self.pid = os.getpid()
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            self.id = '%s:%s' % (self.host, self.pid)
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        while not self.held:
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            try:
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                util.makelock(self.id, self.f)
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                self.held = 1
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            except (OSError, IOError), why:
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                if why.errno == errno.EEXIST:
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                    locker = self.testlock()
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                    if locker:
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                        raise LockHeld(locker)
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                else:
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                    raise LockUnavailable(why)
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    def testlock(self):
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        '''return id of locker if lock is valid, else None.'''
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        # if old-style lock, we cannot tell what machine locker is on.
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        # with new-style lock, if locker is on this machine, we can
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        # see if locker is alive.  if locker is on this machine but
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        # not alive, we can safely break lock.
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        locker = util.readlock(self.f)
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        c = locker.find(':')
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        if c == -1:
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            return locker
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        host = locker[:c]
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        if host != self.host:
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            return locker
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        try:
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            pid = int(locker[c+1:])
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        except:
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            return locker
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        if util.testpid(pid):
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            return locker
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        # if locker dead, break lock.  must do this with another lock
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        # held, or can race and break valid lock.
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        try:
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            l = lock(self.f + '.break')
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            l.trylock()
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            os.unlink(self.f)
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            l.release()
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        except (LockHeld, LockUnavailable):
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            return locker
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    def release(self):
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        if self.held:
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            self.held = 0
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            if self.releasefn:
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                self.releasefn()
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            try:
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                os.unlink(self.f)
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            except: pass
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