diff mercurial/commands.py @ 27192:a01d3d32b53a

commands: make commit acquire locks before processing (issue4368)

Before this patch, "hg commit" (process A) executes steps below:

  1. get current branch heads via 'repo.branchheads()'
     - cache 'repo.changelog'
  2. invoke 'repo.commit()'
  3. acquire wlock
     - invalidate 'repo.dirstate'
  4. access 'repo.dirstate'
     - re-read '.hg/dirstate'
     - check validity of parent revisions with 'repo.changelog'
  5. invoke 'repo.commitctx()'
  6. acquire store lock (slock)
     - invalidate 'repo.changelog'
  7. do committing
  8. release slock
  9. release wlock
 10. check new branch head (via 'cmdutil.commitstatus()')

If acquisition of wlock at (3) above waits for another "hg commit"
(process B) or so running parallelly to release wlock, process A
causes creating orphan revision, because:

  - '.hg/dirstate' refers the revision, which is newly added by
    process B, as its parent

  - but already cached 'repo.changelog' doesn't contain such revision

  - therefore, validating parents of '.hg/dirstate' at (4) above
    replaces such revision with 'nullid'

Then, process A creates "orphan" revision, of which parent is "null"
revision.

In addition to it, "created new head" may be shown at the end of
process A unintentionally, if store is updated parallelly, because
both getting branch heads (1) and checking new branch head (10) are
executed outside slock scope.

To avoid this issue, this patch makes "hg commit" acquire wlock and
slock before processing.

This patch resolves the issue between "hg commit" processes, but not
one between "hg commit" and other commands. Subsequent patches resolve
the latter.

Even after this patch, there are still corner case problems below:

  - filecache may overlook changes of '.hg/dirstate', and it causes
    similar issue (see below for detail)

    https://bz.mercurial-scm.org/show_bug.cgi?id=4368#c10

  - 3rd party extension may cause similar issue, if it directly uses
    'repo.commit()' without acquisition of wlock and slock

    This can be fixed by acquisition of slock at the beginning of
    'repo.commit()', but it seems suitable for "default" branch

    In fact, acquisition of slock itself is already introduced at
    "default" branch by 4414d500604f, but acquisition is not at the
    beginning of 'repo.commit()'.

This patch also changes some tests:

  - test-fncache.t needs this tricky wrapping, to release (= forced
    failure of) wlock certainly

  - order of "hg commit" output is changed by widening scope of locks,
    because some hooks are fired after releasing wlock

--- a/mercurial/commands.py	Tue Dec 01 16:15:59 2015 -0800
+++ b/mercurial/commands.py	Wed Dec 02 03:12:07 2015 +0900
@@ -1576,6 +1576,15 @@
 
     Returns 0 on success, 1 if nothing changed.
     """
+    wlock = lock = None
+    try:
+        wlock = repo.wlock()
+        lock = repo.lock()
+        return _docommit(ui, repo, *pats, **opts)
+    finally:
+        release(lock, wlock)
+
+def _docommit(ui, repo, *pats, **opts):
     if opts.get('interactive'):
         opts.pop('interactive')
         cmdutil.dorecord(ui, repo, commit, None, False,