view tests/test-exchange-obsmarkers-case-D2.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

============================================
Testing obsolescence markers push: Cases D.2
============================================

Mercurial pushes obsolescences markers relevant to the "pushed-set", the set of
all changesets that requested to be "in sync" after the push (even if they are
already on both side).

This test belongs to a series of tests checking such set is properly computed
and applied. This does not tests "obsmarkers" discovery capabilities.

Category D: Partial Information Case
TestCase 2: missing prune target (prune in "pushed set")

D.2 missing prune target (prune in "pushed set")
================================================

.. {{{
..   A ø⇠✕ A'
..     |/
..     ● O
.. }}}
..
.. Marker exist from:
..
..  * A' succeed to A
..  * A' (prune)
..
.. Command runs:
..
..  * hg push
..
.. Expected exchange:
..
..  * `A ø⇠o A'`
..  * A' (prune)

Setup
-----

  $ . $TESTDIR/testlib/exchange-obsmarker-util.sh

Initial

  $ setuprepos D.2
  creating test repo for test case D.2
  - pulldest
  - main
  - pushdest
  cd into `main` and proceed with env setup
  $ cd main
  $ mkcommit A0
  $ hg up -q 0
  $ mkcommit A1
  created new head
  $ hg debugobsolete `getid 'desc(A0)'` `getid 'desc(A1)'`
  1 new obsolescence markers
  obsoleted 1 changesets
  $ hg prune --date '0 0' .
  1 new obsolescence markers
  obsoleted 1 changesets
  $ hg strip --hidden -q 'desc(A1)'
  $ hg log -G --hidden
  x  28b51eb45704 (draft): A0
  |
  @  a9bdc8b26820 (public): O
  
  $ inspect_obsmarkers
  obsstore content
  ================
  28b51eb45704506b5c603decd6bf7ac5e0f6a52f e5ea8f9c73143125d36658e90ef70c6d2027a5b7 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  e5ea8f9c73143125d36658e90ef70c6d2027a5b7 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  $ cd ..
  $ cd ..

Actual Test
-----------

  $ dotest D.2
  ## Running testcase D.2
  ## initial state
  # obstore: main
  28b51eb45704506b5c603decd6bf7ac5e0f6a52f e5ea8f9c73143125d36658e90ef70c6d2027a5b7 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  e5ea8f9c73143125d36658e90ef70c6d2027a5b7 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  # obstore: pushdest
  # obstore: pulldest
  ## pushing from main to pushdest
  pushing to pushdest
  searching for changes
  no changes found
  remote: 2 new obsolescence markers
  ## post push state
  # obstore: main
  28b51eb45704506b5c603decd6bf7ac5e0f6a52f e5ea8f9c73143125d36658e90ef70c6d2027a5b7 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  e5ea8f9c73143125d36658e90ef70c6d2027a5b7 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  # obstore: pushdest
  28b51eb45704506b5c603decd6bf7ac5e0f6a52f e5ea8f9c73143125d36658e90ef70c6d2027a5b7 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  e5ea8f9c73143125d36658e90ef70c6d2027a5b7 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  # obstore: pulldest
  ## pulling from main into pulldest
  pulling from main
  searching for changes
  no changes found
  2 new obsolescence markers
  ## post pull state
  # obstore: main
  28b51eb45704506b5c603decd6bf7ac5e0f6a52f e5ea8f9c73143125d36658e90ef70c6d2027a5b7 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  e5ea8f9c73143125d36658e90ef70c6d2027a5b7 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  # obstore: pushdest
  28b51eb45704506b5c603decd6bf7ac5e0f6a52f e5ea8f9c73143125d36658e90ef70c6d2027a5b7 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  e5ea8f9c73143125d36658e90ef70c6d2027a5b7 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  # obstore: pulldest
  28b51eb45704506b5c603decd6bf7ac5e0f6a52f e5ea8f9c73143125d36658e90ef70c6d2027a5b7 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  e5ea8f9c73143125d36658e90ef70c6d2027a5b7 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}