[PATCH 3 of 3 STABLE] bookmarks: actual fix for race condition deleting bookmark
pierre-yves.david at ens-lyon.org
Thu Aug 1 13:55:40 EDT 2019
# 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
@@ -1227,6 +1227,55 @@ class localrepository(object):
@mixedrepostorecache(('bookmarks', 'plain'), ('bookmarks.current', 'plain'),
('bookmarks', ''), ('00changelog.i', ''))
+ # 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.
diff --git 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-B 3:f26c3b5167d1 (false !)
+ book-B 3:f26c3b5167d1
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