Mercurial > hg
comparison mercurial/changelog.py @ 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 | 7a93b7b3dc2d |
children | 230f73019e49 |
comparison
equal
deleted
inserted
replaced
46606:ced66295ea90 | 46607:e9901d01d135 |
---|---|
378 extra = self.extra | 378 extra = self.extra |
379 return encoding.tolocal(extra.get(b"branch")), b'close' in extra | 379 return encoding.tolocal(extra.get(b"branch")), b'close' in extra |
380 | 380 |
381 | 381 |
382 class changelog(revlog.revlog): | 382 class changelog(revlog.revlog): |
383 def __init__(self, opener, trypending=False): | 383 def __init__(self, opener, trypending=False, concurrencychecker=None): |
384 """Load a changelog revlog using an opener. | 384 """Load a changelog revlog using an opener. |
385 | 385 |
386 If ``trypending`` is true, we attempt to load the index from a | 386 If ``trypending`` is true, we attempt to load the index from a |
387 ``00changelog.i.a`` file instead of the default ``00changelog.i``. | 387 ``00changelog.i.a`` file instead of the default ``00changelog.i``. |
388 The ``00changelog.i.a`` file contains index (and possibly inline | 388 The ``00changelog.i.a`` file contains index (and possibly inline |
389 revision) data for a transaction that hasn't been finalized yet. | 389 revision) data for a transaction that hasn't been finalized yet. |
390 It exists in a separate file to facilitate readers (such as | 390 It exists in a separate file to facilitate readers (such as |
391 hooks processes) accessing data before a transaction is finalized. | 391 hooks processes) accessing data before a transaction is finalized. |
392 | |
393 ``concurrencychecker`` will be passed to the revlog init function, see | |
394 the documentation there. | |
392 """ | 395 """ |
393 if trypending and opener.exists(b'00changelog.i.a'): | 396 if trypending and opener.exists(b'00changelog.i.a'): |
394 indexfile = b'00changelog.i.a' | 397 indexfile = b'00changelog.i.a' |
395 else: | 398 else: |
396 indexfile = b'00changelog.i' | 399 indexfile = b'00changelog.i' |
402 indexfile, | 405 indexfile, |
403 datafile=datafile, | 406 datafile=datafile, |
404 checkambig=True, | 407 checkambig=True, |
405 mmaplargeindex=True, | 408 mmaplargeindex=True, |
406 persistentnodemap=opener.options.get(b'persistent-nodemap', False), | 409 persistentnodemap=opener.options.get(b'persistent-nodemap', False), |
410 concurrencychecker=concurrencychecker, | |
407 ) | 411 ) |
408 | 412 |
409 if self._initempty and (self.version & 0xFFFF == revlog.REVLOGV1): | 413 if self._initempty and (self.version & 0xFFFF == revlog.REVLOGV1): |
410 # changelogs don't benefit from generaldelta. | 414 # changelogs don't benefit from generaldelta. |
411 | 415 |