[PATCH 3 of 3 STABLE] bookmarks: actual fix for race condition deleting bookmark

[Augie Fackler]

queued for stable, thanks

> On Aug 1, 2019, at 13:55, Pierre-Yves David <pierre-yves.david at ens-lyon.org> wrote:
> 
> # HG changeset patch
> # User Pierre-Yves David <pierre-yves.david at octobus.net>
> # Date 1561081840 -7200
> #      Fri Jun 21 03:50:40 2019 +0200
> # Branch stable
> # Node ID 56f34aeebffda197bacd77cc2345386d9dd1f073
> # Parent  df77a77a8b1744d93de9823fb8ac2d5c5d8c07d8
> # EXP-Topic debug-book
> # Available At https://bitbucket.org/octobus/mercurial-devel/
> #              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r 56f34aeebffd
> bookmarks: actual fix for race condition deleting bookmark
> 
> This is a simple but efficient fix to prevent the issue tested in
> `test-bookmarks-corner-case.t`. It might be worth pursuing a more generic
> approach where filecache learn to depend on each other, but that would not be
> suitable for stable.
> 
> The issue is complicated enough that I documented the race and its current
> solution as inline comment. See this comment for details on the fix.
> 
> diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
> --- a/mercurial/localrepo.py
> +++ b/mercurial/localrepo.py
> @@ -1227,6 +1227,55 @@ class localrepository(object):
>     @mixedrepostorecache(('bookmarks', 'plain'), ('bookmarks.current', 'plain'),
>                          ('bookmarks', ''), ('00changelog.i', ''))
>     def _bookmarks(self):
> +        # Since the multiple files involved in the transaction cannot be
> +        # written atomically (with current repository format), there is a race
> +        # condition here.
> +        #
> +        # 1) changelog content A is read
> +        # 2) outside transaction update changelog to content B
> +        # 3) outside transaction update bookmark file referring to content B
> +        # 4) bookmarks file content is read and filtered against changelog-A
> +        #
> +        # When this happens, bookmarks against nodes missing from A are dropped.
> +        #
> +        # Having this happening during read is not great, but it become worse
> +        # when this happen during write because the bookmarks to the "unknown"
> +        # nodes will be dropped for good. However, writes happen within locks.
> +        # This locking makes it possible to have a race free consistent read.
> +        # For this purpose data read from disc before locking  are
> +        # "invalidated" right after the locks are taken. This invalidations are
> +        # "light", the `filecache` mechanism keep the data in memory and will
> +        # reuse them if the underlying files did not changed. Not parsing the
> +        # same data multiple times helps performances.
> +        #
> +        # Unfortunately in the case describe above, the files tracked by the
> +        # bookmarks file cache might not have changed, but the in-memory
> +        # content is still "wrong" because we used an older changelog content
> +        # to process the on-disk data. So after locking, the changelog would be
> +        # refreshed but `_bookmarks` would be preserved.
> +        # Adding `00changelog.i` to the list of tracked file is not
> +        # enough, because at the time we build the content for `_bookmarks` in
> +        # (4), the changelog file has already diverged from the content used
> +        # for loading `changelog` in (1)
> +        #
> +        # To prevent the issue, we force the changelog to be explicitly
> +        # reloaded while computing `_bookmarks`. The data race can still happen
> +        # without the lock (with a narrower window), but it would no longer go
> +        # undetected during the lock time refresh.
> +        #
> +        # The new schedule is as follow
> +        #
> +        # 1) filecache logic detect that `_bookmarks` needs to be computed
> +        # 2) cachestat for `bookmarks` and `changelog` are captured (for book)
> +        # 3) We force `changelog` filecache to be tested
> +        # 4) cachestat for `changelog` are captured (for changelog)
> +        # 5) `_bookmarks` is computed and cached
> +        #
> +        # The step in (3) ensure we have a changelog at least as recent as the
> +        # cache stat computed in (1). As a result at locking time:
> +        #  * if the changelog did not changed since (1) -> we can reuse the data
> +        #  * otherwise -> the bookmarks get refreshed.
> +        self._refreshchangelog()
>         return bookmarks.bmstore(self)
> 
>     def _refreshchangelog(self):
> diff --git a/tests/test-bookmarks-corner-case.t b/tests/test-bookmarks-corner-case.t
> --- a/tests/test-bookmarks-corner-case.t
> +++ b/tests/test-bookmarks-corner-case.t
> @@ -200,6 +200,7 @@ Check raced push output.
>   $ cat push-output.txt
>   pushing to ssh://user@dummy/bookrace-server
>   searching for changes
> +  remote has heads on branch 'default' that are not known locally: f26c3b5167d1
>   remote: setting raced push up
>   remote: adding changesets
>   remote: adding manifests
> @@ -219,7 +220,7 @@ Check result of the push.
>   |  summary:     A1
>   |
>   | o  changeset:   3:f26c3b5167d1
> -  | |  bookmark:    book-B (false !)
> +  | |  bookmark:    book-B
>   | |  user:        test
>   | |  date:        Thu Jan 01 00:00:00 1970 +0000
>   | |  summary:     B1
> @@ -242,4 +243,4 @@ Check result of the push.
> 
>   $ hg -R bookrace-server book
>      book-A                    4:9ce3b28c16de
> -     book-B                    3:f26c3b5167d1 (false !)
> +     book-B                    3:f26c3b5167d1
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