Mercurial > hg
view tests/test-double-merge.t @ 46607:e9901d01d135
revlog: add a mechanism to verify expected file position before appending
If someone uses `hg debuglocks`, or some non-hg process writes to the .hg
directory without respecting the locks, or if the repo's on a networked
filesystem, it's possible for the revlog code to write out corrupted data.
The form of this corruption can vary depending on what data was written and how
that happened. We are in the "networked filesystem" case (though I've had users
also do this to themselves with the "`hg debuglocks`" scenario), and most often
see this with the changelog. What ends up happening is we produce two items
(let's call them rev1 and rev2) in the .i file that have the same linkrev,
baserev, and offset into the .d file, while the data in the .d file is appended
properly. rev2's compressed_size is accurate for rev2, but when we go to
decompress the data in the .d file, we use the offset that's recorded in the
index file, which is the same as rev1, and attempt to decompress
rev2.compressed_size bytes of rev1's data. This usually does not succeed. :)
When using inline data, this also fails, though I haven't investigated why too
closely. This shows up as a "patch decode" error. I believe what's happening
there is that we're basically ignoring the offset field, getting the data
properly, but since baserev != rev, it thinks this is a delta based on rev
(instead of a full text) and can't actually apply it as such.
For now, I'm going to make this an optional component and default it to entirely
off. I may increase the default severity of this in the future, once I've
enabled it for my users and we gain more experience with it. Luckily, most of my
users have a versioned filesystem and can roll back to before the corruption has
been written, it's just a hassle to do so and not everyone knows how (so it's a
support burden). Users on other filesystems will not have that luxury, and this
can cause them to have a corrupted repository that they are unlikely to know how
to resolve, and they'll see this as a data-loss event. Refusing to create the
corruption is a much better user experience.
This mechanism is not perfect. There may be false-negatives (racy writes that
are not detected). There should not be any false-positives (non-racy writes that
are detected as such). This is not a mechanism that makes putting a repo on a
networked filesystem "safe" or "supported", just *less* likely to cause
corruption.
Differential Revision: https://phab.mercurial-scm.org/D9952
author | Kyle Lippincott <spectral@google.com> |
---|---|
date | Wed, 03 Feb 2021 16:33:10 -0800 |
parents | 69429972ff1f |
children | 38941a28406a |
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$ hg init repo $ cd repo $ echo line 1 > foo $ hg ci -qAm 'add foo' copy foo to bar and change both files $ hg cp foo bar $ echo line 2-1 >> foo $ echo line 2-2 >> bar $ hg ci -m 'cp foo bar; change both' in another branch, change foo in a way that doesn't conflict with the other changes $ hg up -qC 0 $ echo line 0 > foo $ hg cat foo >> foo $ hg ci -m 'change foo' created new head we get conflicts that shouldn't be there $ hg merge -P changeset: 1:484bf6903104 user: test date: Thu Jan 01 00:00:00 1970 +0000 summary: cp foo bar; change both $ hg merge --debug unmatched files in other: bar all copies found (* = to merge, ! = divergent, % = renamed and deleted): on remote side: src: 'foo' -> dst: 'bar' * checking for directory renames resolving manifests branchmerge: True, force: False, partial: False ancestor: e6dc8efe11cc, local: 6a0df1dad128+, remote: 484bf6903104 starting 4 threads for background file closing (?) preserving foo for resolve of bar preserving foo for resolve of foo bar: remote copied from foo -> m (premerge) picked tool ':merge' for bar (binary False symlink False changedelete False) merging foo and bar to bar my bar@6a0df1dad128+ other bar@484bf6903104 ancestor foo@e6dc8efe11cc premerge successful foo: versions differ -> m (premerge) picked tool ':merge' for foo (binary False symlink False changedelete False) merging foo my foo@6a0df1dad128+ other foo@484bf6903104 ancestor foo@e6dc8efe11cc premerge successful 0 files updated, 2 files merged, 0 files removed, 0 files unresolved (branch merge, don't forget to commit) contents of foo $ cat foo line 0 line 1 line 2-1 contents of bar $ cat bar line 0 line 1 line 2-2 $ cd ..