Mercurial > hg
view tests/test-ui-verbosity.py.out @ 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> |
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date | Wed, 03 Feb 2021 16:33:10 -0800 |
parents | 807f3f5c60e9 |
children |
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hgrc settings command line options final result quiet verbo debug quiet verbo debug quiet verbo debug 0 False False False False False False -> False False False 1 True False False False False False -> True False False 2 False True False False False False -> False True False 3 True True False False False False -> False False False 4 False False True False False False -> False True True 5 True False True False False False -> False True True 6 False True True False False False -> False True True 7 True True True False False False -> False True True 8 False False False True False False -> True False False 9 True False False True False False -> True False False 10 False True False True False False -> True False False 11 True True False True False False -> True False False 12 False False True True False False -> True False False 13 True False True True False False -> True False False 14 False True True True False False -> True False False 15 True True True True False False -> True False False 16 False False False False True False -> False True False 17 True False False False True False -> False True False 18 False True False False True False -> False True False 19 True True False False True False -> False True False 20 False False True False True False -> False True False 21 True False True False True False -> False True False 22 False True True False True False -> False True False 23 True True True False True False -> False True False 24 False False False True True False -> False False False 25 True False False True True False -> False False False 26 False True False True True False -> False False False 27 True True False True True False -> False False False 28 False False True True True False -> False False False 29 True False True True True False -> False False False 30 False True True True True False -> False False False 31 True True True True True False -> False False False 32 False False False False False True -> False True True 33 True False False False False True -> False True True 34 False True False False False True -> False True True 35 True True False False False True -> False True True 36 False False True False False True -> False True True 37 True False True False False True -> False True True 38 False True True False False True -> False True True 39 True True True False False True -> False True True 40 False False False True False True -> False True True 41 True False False True False True -> False True True 42 False True False True False True -> False True True 43 True True False True False True -> False True True 44 False False True True False True -> False True True 45 True False True True False True -> False True True 46 False True True True False True -> False True True 47 True True True True False True -> False True True 48 False False False False True True -> False True True 49 True False False False True True -> False True True 50 False True False False True True -> False True True 51 True True False False True True -> False True True 52 False False True False True True -> False True True 53 True False True False True True -> False True True 54 False True True False True True -> False True True 55 True True True False True True -> False True True 56 False False False True True True -> False True True 57 True False False True True True -> False True True 58 False True False True True True -> False True True 59 True True False True True True -> False True True 60 False False True True True True -> False True True 61 True False True True True True -> False True True 62 False True True True True True -> False True True 63 True True True True True True -> False True True