* Deal with clock skew, both forwards and backwards, when logging
information to the git-annex branch.
* GIT_ANNEX_VECTOR_CLOCK can now be set to a fixed value (eg 1)
rather than needing to be advanced each time a new change is made.
* Misuse of GIT_ANNEX_VECTOR_CLOCK will no longer confuse git-annex.
When changing a file in the git-annex branch, the vector clock to use is now
determined by first looking at the current time (or GIT_ANNEX_VECTOR_CLOCK
when set), and comparing it to the newest vector clock already in use in
that file. If a newer time stamp was already in use, advance it forward by
a second instead.
When the clock is set to a time in the past, this avoids logging with
an old timestamp, which would risk that log line later being ignored in favor
of "newer" line that is really not newer.
When a log entry has been made with a clock that was set far ahead in the
future, this avoids newer information being logged with an older timestamp
and so being ignored in favor of that future-timestamped information.
Once all clocks get fixed, this will result in the vector clocks being
incremented, until finally enough time has passed that time gets back ahead
of the vector clock value, and then it will return to usual operation.
(This latter situation is not ideal, but it seems the best that can be done.
The issue with it is, since all writers will be incrementing the last
vector clock they saw, there's no way to tell when one writer made a write
significantly later in time than another, so the earlier write might
arbitrarily be picked when merging. This problem is why git-annex uses
timestamps in the first place, rather than pure vector clocks.)
Advancing forward by 1 second is somewhat arbitrary. setDead
advances a timestamp by just 1 picosecond, and the vector clock could
too. But then it would interfere with setDead, which wants to be
overrulled by any change. So it could use 2 picoseconds or something,
but that seems weird. It could just as well advance it forward by a
minute or whatever, but then it would be harder for real time to catch
up with the vector clock when forward clock slew had happened.
A complication is that many log files contain several different peices of
information, and it may be best to only use vector clocks for the same peice
of information. For example, a key's location log file contains
InfoPresent/InfoMissing for each UUID, and it only looks at the vector
clocks for the UUID that is being changed, and not other UUIDs.
Although exactly where the dividing line is can be hard to determine.
Consider metadata logs, where a field "tag" can have multiple values set
at different times. Should it advance forward past the last tag?
Probably. What about when a different field is set, should it look at
the clocks of other fields? Perhaps not, but currently it does, and
this does not seems like it will cause any problems.
Another one I'm not entirely sure about is the export log, which is
keyed by (fromuuid, touuid). So if multiple repos are exporting to the
same remote, different vector clocks can be used for that remote.
It looks like that's probably ok, because it does not try to determine
what order things occurred when there was an export conflict.
Sponsored-by: Jochen Bartl on Patreon
14683da9eb caused a test suite failure.
When the content of a key is not present, a LinkAnnexFailed is returned,
but replaceFile then tried to move the file into place, and since it was
not written, that crashed.
Sponsored-by: Boyd Stephen Smith Jr. on Patreon
git-annex get when run as the first git-annex command in a new repo did not
populate unlocked files. (Reversion in version 8.20210621)
I am not entirely happy with this, because I don't understand how
428c91606b caused the problem in the first
place, and I don't fully understand how skipping calling scanAnnexedFiles
during autoinit avoids the problem.
Kept the explicit call to scanAnnexedFiles during git-annex init,
so that when reconcileStaged is expensive, it can be made to run then,
rather than at some later point when the information is needed.
Sponsored-by: Brock Spratlen on Patreon
The pass was needed to populate files when annex.thin was set,
but in commit 73e0cbbb19,
reconcileStaged started to do that. So, this second pass is not needed
any longer.
I'm now reasonably sure I've identified both cases where this can
happen. v8 upgrades and certian filesystems eg NFS. Both are handled as
well as can be, though it may involve some extra checksumming work.
Eg, showImprecise 1 1.99 returned "1.1" rather than "2". The 9 rounded
upward to 10, and that was wrongly used as the decimal, rather than
carrying the 1.
Sponsored-by: Jack Hill on Patreon
An easy way to see this in action is to have an unlocked file, and touch the
object file.
While all code that compares inode caches for object files needs to be
prepared for this kind of problem and fall back to verification, having
fsck notice it and correct it is cheap (as long as fsck is being run
anyway) and ensures that if it happens for some unusual reason, there's a
way for the user to notice that it's happening.
Not that, when annex.thin is in use, the earlier call to isUnmodified
(and also potentially earlier calls to inAnnex in eg, verifyLocationLog)
will fix up the same problem silently. That might prevent the warning
being displayed, although probably it still will be, because the
Database.Keys write of the InodeCache will be queued but will not have
happened yet. I can't see a way to improve this, but it's not great.
Sponsored-by: Dartmouth College's Datalad project
This avoids it calling enteringStage VerifyStage when it's used in
places that only fall back to verification rarely, and which might be
called while in TransferStage and be going to perform a transfer after
the verification.
This is a result of an audit of every use of getInodeCaches,
to find places that misbehave when the annex object is not in the inode
cache, despite pointer files for the same key being in the inode cache.
Unfortunately, that is the case for objects that were in v7 repos that
upgraded to v8. Added a note about this gotcha to getInodeCaches.
Database.Keys.reconcileStaged, then annex.thin is set, would fail to
populate pointer files in this situation. Changed it to check if the
annex object is unmodified the same way inAnnex does, falling back to a
checksum if the inode cache is not recorded.
Sponsored-by: Dartmouth College's Datalad project
The goal is that Database.Keys be able to use it; it can't use
Annex.Content.Presence due to an import loop.
Several other things also needed to be moved to Annex.Verify as a
conseqence.
Some uses of linkFromAnnex are inside replaceWorkTreeFile, which was
already safe, but others use it directly on the work tree file, which
was race-prone. Eg, if the work tree file was first removed, then
linkFromAnnex called to populate it, the user could have re-written it in
the interim.
This came to light during an audit of all calls of addInodeCaches,
looking for such races. All the other uses of it seem ok.
Sponsored-by: Brett Eisenberg on Patreon
In Annex.Content, the object file was statted after pointer files were
populated. But if annex.thin is set, once the pointer files are
populated, the object file can potentially be modified via the hard
link. So, it was possible, though seemingly very unlikely, for the inode
of the modified object file to be cached.
Command.Fix and Command.Fsck had similar problems, statting the work
tree files after they were in place. Changed them to stat the temp file
that gets moved into place. This does rely on .git/annex being on the
same filesystem. If it's not, the cached inode will not be the same as
the one that the temp file gets moved to. Result will be that git-annex
will later need to do an expensive verification of the content of the
worktree files. Note that the cross-filesystem move of the temp file
already is a larger amount of extra work, so this seems acceptable.
Sponsored-by: Luke Shumaker on Patreon
