This makes sure that two threads don't check balanced preferred content at the
same time, so each thread always sees a consistent picture of what is
happening.
This does add a fairly expensive file level lock to every check of
preferred content, in commands that use prepareLiveUpdate. It would
be good to only do that when live updates are actually needed, eg when
the preferred content expression uses balanced preferred content.
Fixed successfullyFinishedLiveSizeChange to not update the rolling total
when a redundant change is in RecentChanges.
Made setRepoSizes clear RecentChanges that are no longer needed.
It might be possible to clear those earlier, this is only a convenient
point to do it.
The reason it's safe to clear RecentChanges here is that, in order for a
live update to call successfullyFinishedLiveSizeChange, a change must be
made to a location log. If a RecentChange gets cleared, and just after
that a new live update is started, making the same change, the location
log has already been changed (since the RecentChange exists), and
so when the live update succeeds, it won't call
successfullyFinishedLiveSizeChange. The reason it doesn't
clear RecentChanges when there is a reduntant live update is because
I didn't want to think through whether or not all races are avoided in
that case.
The rolling total in SizeChanges is never cleared. Instead,
calcJournalledRepoSizes gets the initial value of it, and then
getLiveRepoSizes subtracts that initial value from the current value.
Since the rolling total can only be updated by updateRepoSize,
which is called with the journal locked, locking the journal in
calcJournalledRepoSizes ensures that the database does not change while
reading the journal.
When finishedLiveUpdate was run on a different key than expected, it
blocked forever waiting for an indication the database had been updated.
Since the journal is locked when finishedLiveUpdate runs, this could
also have caused other git-annex commands to hang.
It's possible for two processes or threads to both be doing the same
operation at the same time. Eg, both dropping the same key. If one
finishes and updates the rollingtotal, then the other one needs to be
prevented from later updating the rollingtotal as well. And they could
finish at the same time, or with some time in between.
Addressed this by making updateRepoSize be called with the journal
locked, and only once it's been determined that there is an actual
location change to record in the log. updateRepoSize waits for the
database to be updated.
When there is a redundant operation, updateRepoSize won't be called,
and the redundant LiveUpdate will be removed from the database on
garbage collection.
But: There will be a window where the redundant LiveUpdate is still
visible in the db, and processes can see it, combine it with the
rollingtotal, and arrive at the wrong size. This is a small window, but
it still ought to be addressed. Unsure if it would always be safe to
remove the redundant LiveUpdate? Consider the case where two drops and a
get are all running concurrently somehow, and the order they finish is
[drop, get, drop]. The second drop seems redundant to the first, but
it would not be safe to remove it. While this seems unlikely, it's hard
to rule out that a get and drop at different stages can both be running
at the same time.
When a live size change completes successfully, the same transaction
that removes it from the database updates the rolling total for its
repository.
The idea is that when RepoSizes is read, SizeChanges will be as
well, and cached locally. Any time a change is made, the local cache
will be updated. So by comparing the local cache with the current
SizeChanges, it can learn about size changes that were made by other
processes. Then read the LiveSizeChanges, and add that in to get a live
picture of the current sizes.
Also added a SizeChangeId. This allows 2 different threads, or
processes, to both record a live size change for the same repo and key,
and update their own information without stepping on one-another's toes.
I've tested the behavior of the thread that waits for the LiveUpdate to
be finished, and it does get signaled and exit cleanly when the
LiveUpdate is GCed instead.
Made finishedLiveUpdate wait for the thread to finish updating the
database.
There is a case where GC doesn't happen in time and the database is left
with a live update recorded in it. This should not be a problem as such
stale data can also happen when interrupted and will need to be detected
when loading the database.
Balanced preferred content expressions now call startLiveUpdate.
Each command that first checks preferred content (and/or required
content) and then does something that can change the sizes of
repositories needs to call prepareLiveUpdate, and plumb it through the
preferred content check and the location log update.
So far, only Command.Drop is done. Many other commands that don't need
to do this have been updated to keep working.
There may be some calls to NoLiveUpdate in places where that should be
done. All will need to be double checked.
Not currently in a compilable state.
The use of catObjectStream is optimally fast. Although it might be
possible to combine this with git-annex branch merge to avoid some
redundant work.
Benchmarking, a git-annex branch that had 100000 files changed
took less than 1.88 seconds to run through this.
