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  • Katsunori FUJIWARA's avatar
    commands: make commit acquire locks before processing (issue4368) · a01d3d32b53a
    Katsunori FUJIWARA authored
    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
    a01d3d32b53a