view tests/test-merge-revert2.t @ 43709:039fbd14d4e2

lock: fix race in lock-breaking code

With low frequency, I see hg pulls fail with output like:
abort: no such file or directory: .hg/store/lock

I think what happens is, in lock.py, in:

    def _testlock(self, locker):
        if not self._lockshouldbebroken(locker):
            return locker

        # if locker dead, break lock.  must do this with another lock
        # held, or can race and break valid lock.
        try:
            with lock(self.vfs, self.f + b'.break', timeout=0):
                self.vfs.unlink(self.f)
        except error.LockError:
            return locker

if a lock is breakable on disk, and two hg processes concurrently get
to the "if locker dead" comment, a possible interleaving is: process1
finishes executing the function and then process2 finishes executing
the function. If that happens, process2 will either get ENOENT in
self.vfs.unlink (resulting in the spurious failure above), or break a
valid lock and potentially cause repository corruption.

The fix is simple enough: make sure the lock is breakable _inside_ the
critical section, because only then can we know that no other process
can invalidate our knowledge on the lock on disk.

I don't think there are tests for this. I've tested this manually
with:

diff --git a/mercurial/lock.py b/mercurial/lock.py
--- a/mercurial/lock.py
+++ b/mercurial/lock.py
@@ -351,6 +351,8 @@ class lock(object):
         if not self._lockshouldbebroken(locker):
             return locker

+        import random
+        time.sleep(1. + random.random())
         # if locker dead, break lock.  must do this with another lock
         # held, or can race and break valid lock.
         try:
@@ -358,6 +360,7 @@ class lock(object):
                 self.vfs.unlink(self.f)
         except error.LockError:
             return locker
+        time.sleep(1)

     def testlock(self):
         """return id of locker if lock is valid, else None.

and I see this change of behavior before/after this commit:

 $ $hg init repo
 $ cd repo
 $ ln -s $HOSTNAME/effffffc:987654321 .hg/wlock
 $ touch a
 $ $hg commit -Am_ & $hg commit -Am _; wait
-abort: No such file or directory: '/tmp/repo/.hg/wlock'
 adding a
+warning: ignoring unknown working parent 679a8959a8ca!
+nothing changed

Differential Revision: https://phab.mercurial-scm.org/D7199

  $ hg init

  $ echo "added file1" > file1
  $ echo "another line of text" >> file1
  $ echo "added file2" > file2
  $ hg add file1 file2
  $ hg commit -m "added file1 and file2"

  $ echo "changed file1" >> file1
  $ hg commit -m "changed file1"

  $ hg -q log
  1:dfab7f3c2efb
  0:c3fa057dd86f
  $ hg id
  dfab7f3c2efb tip

  $ hg update -C 0
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg id
  c3fa057dd86f

  $ echo "changed file1" >> file1
  $ hg id
  c3fa057dd86f+

  $ hg revert --no-backup --all
  reverting file1
  $ hg diff
  $ hg status
  $ hg id
  c3fa057dd86f

  $ hg update
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg diff
  $ hg status
  $ hg id
  dfab7f3c2efb tip

  $ hg update -C 0
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ echo "changed file1 different" >> file1

  $ hg update
  merging file1
  warning: conflicts while merging file1! (edit, then use 'hg resolve --mark')
  0 files updated, 0 files merged, 0 files removed, 1 files unresolved
  use 'hg resolve' to retry unresolved file merges
  [1]

  $ hg diff --nodates
  diff -r dfab7f3c2efb file1
  --- a/file1
  +++ b/file1
  @@ -1,3 +1,7 @@
   added file1
   another line of text
  +<<<<<<< working copy: c3fa057dd86f - test: added file1 and file2
  +changed file1 different
  +=======
   changed file1
  +>>>>>>> destination:  dfab7f3c2efb - test: changed file1

  $ hg status
  M file1
  ? file1.orig
  $ hg id
  dfab7f3c2efb+ tip

  $ hg revert --no-backup --all
  reverting file1
  $ hg diff
  $ hg status
  ? file1.orig
  $ hg id
  dfab7f3c2efb tip

  $ hg revert -r tip --no-backup --all
  $ hg diff
  $ hg status
  ? file1.orig
  $ hg id
  dfab7f3c2efb tip

  $ hg update -C
  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg diff
  $ hg status
  ? file1.orig
  $ hg id
  dfab7f3c2efb tip