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annotate mercurial/lock.py @ 1877:d314a89fa4f1

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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.
author Vadim Gelfer <vadim.gelfer@gmail.com>
date Fri, 10 Mar 2006 08:31:31 -0800
parents cd5c1db2132a
children ff5c9a92f556
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1 # lock.py - simple locking scheme for mercurial
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2 #
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3 # Copyright 2005 Matt Mackall <mpm@selenic.com>
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4 #
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5 # This software may be used and distributed according to the terms
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6 # of the GNU General Public License, incorporated herein by reference.
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7
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8 from demandload import *
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9 demandload(globals(), 'errno os socket time util')
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10
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11 class LockException(Exception):
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12 pass
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13 class LockHeld(LockException):
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14 pass
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15 class LockUnavailable(LockException):
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16 pass
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17
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18 class lock(object):
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19 # lock is symlink on platforms that support it, file on others.
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20
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21 # symlink is used because create of directory entry and contents
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22 # are atomic even over nfs.
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23
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24 # old-style lock: symlink to pid
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25 # new-style lock: symlink to hostname:pid
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26
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27 def __init__(self, file, timeout=-1, releasefn=None):
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28 self.f = file
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29 self.held = 0
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30 self.timeout = timeout
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31 self.releasefn = releasefn
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32 self.id = None
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33 self.host = None
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34 self.pid = None
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35 self.lock()
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36
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37 def __del__(self):
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38 self.release()
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39
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40 def lock(self):
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41 timeout = self.timeout
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42 while 1:
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43 try:
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44 self.trylock()
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45 return 1
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46 except LockHeld, inst:
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47 if timeout != 0:
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48 time.sleep(1)
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49 if timeout > 0:
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50 timeout -= 1
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51 continue
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52 raise inst
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53
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54 def trylock(self):
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55 if self.id is None:
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56 self.host = socket.gethostname()
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57 self.pid = os.getpid()
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58 self.id = '%s:%s' % (self.host, self.pid)
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59 while not self.held:
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60 try:
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61 util.makelock(self.id, self.f)
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62 self.held = 1
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63 except (OSError, IOError), why:
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64 if why.errno == errno.EEXIST:
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65 locker = self.testlock()
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66 if locker:
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67 raise LockHeld(locker)
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68 else:
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69 raise LockUnavailable(why)
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70
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71 def testlock(self):
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72 '''return id of locker if lock is valid, else None.'''
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73 # if old-style lock, we cannot tell what machine locker is on.
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74 # with new-style lock, if locker is on this machine, we can
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75 # see if locker is alive. if locker is on this machine but
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76 # not alive, we can safely break lock.
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77 locker = util.readlock(self.f)
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78 c = locker.find(':')
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79 if c == -1:
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80 return locker
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81 host = locker[:c]
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82 if host != self.host:
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83 return locker
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84 try:
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85 pid = int(locker[c+1:])
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86 except:
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87 return locker
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88 if util.testpid(pid):
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89 return locker
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90 # if locker dead, break lock. must do this with another lock
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91 # held, or can race and break valid lock.
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92 try:
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93 l = lock(self.f + '.break')
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94 l.trylock()
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95 os.unlink(self.f)
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96 l.release()
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97 except (LockHeld, LockUnavailable):
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98 return locker
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99
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100 def release(self):
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101 if self.held:
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102 self.held = 0
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103 if self.releasefn:
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104 self.releasefn()
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105 try:
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106 os.unlink(self.f)
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107 except: pass
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