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revlog: add a mechanism to verify expected file position before appending
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Authored by spectral on Wed, Feb 3, 8:16 PM.

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Summary

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.

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Event Timeline

spectral created this revision.Wed, Feb 3, 8:16 PM

This still adds all of the function call overhead even when the feature is not used. I also don't like that this check is done repeatedly e.g. during an unbundle. I don't think I would mind checking the size once per revlog on the first write, but not repeatedly.

spectral updated this revision to Diff 25481.Thu, Feb 4, 2:46 PM

This still adds all of the function call overhead even when the feature is not used. I also don't like that this check is done repeatedly e.g. during an unbundle. I don't think I would mind checking the size once per revlog on the first write, but not repeatedly.

Switched to just being a boolean check instead of a dummy function. Also cleaned up the formatting issues, turns out I didn't have black installed correctly.

I agree that in a "tight" loop like during a pull it's much less useful, and would be open to having the checks disabled for those operations if/when they're ever enabled by default. However, there are commands like split or rebase (with a merge tool specified) that can have several pauses while hg is waiting on the user (writing a description, choosing what to split, resolving conflicts) where the files have already been written to once, but the user had a chance to "forget" about what they were doing. If there was an easy way of having any user interaction reset the state so that the next write caused the checks to happen again, I might be open to doing that.

Sorry, I'm not super familiar with the phabricator workflow, should I be doing something more here?

This revision was not accepted when it landed; it landed in state Needs Review.
This revision was automatically updated to reflect the committed changes.

Seems like this patch made test-git-interop.t more broken than it was before. I'll append the following patch in flight to make it just as broken as it was before:

diff --git a/hgext/git/__init__.py b/hgext/git/__init__.py
--- a/hgext/git/__init__.py
+++ b/hgext/git/__init__.py
@@ -90,7 +90,7 @@ class gitstore(object):  # store.basicst
             return os.path.join(self.path, b'..', b'.hg', f)
         raise NotImplementedError(b'Need to pick file for %s.' % f)

-    def changelog(self, trypending):
+    def changelog(self, trypending, concurrencychecker):
         # TODO we don't have a plan for trypending in hg's git support yet
         return gitlog.changelog(self.git, self._db)