to track down a blocked indefinitely on MVar that seems to occur after
sqlite throws ErrorBusy but that I have not been able to reproduce when
I made commits synthetically throw ErrorBusy.
Sponsored-by: Dartmouth College's Datalad project
After commit f4bdecc4ec, there is no
longer any distinction between SingleWriter and MultiWriter's handling
of read after write.
Databases that were SingleWriter still have lock files that are used to
prevent multiple writers.
This does make writing to such databases a bit more expensive,
because the MultiWriter code path that is now used opens a second db
connection in order to write to them.
This removes a messy caveat that was easy to forget and caused at least one
bug. The price paid is that, after a write to a MultiWriter db, it has to
close the db connection that it had been using to read, and open a new
connection. So it might be a little bit slower. But, writes are usually
batched together, so there's often only a single write, and so there should
not be much of a slowdown. Notice that SingleWriter already closed the db
connection after a write, so paid the same overhead.
This is the second try at fixing a bug: git-annex get when run as the first
git-annex command in a new repo did not populate all unlocked files.
(Reversion in version 8.20210621)
Sponsored-by: Boyd Stephen Smith Jr. on Patreon
This does not change the overall license of the git-annex program, which
was already AGPL due to a number of sources files being AGPL already.
Legally speaking, I'm adding a new license under which these files are
now available; I already released their current contents under the GPL
license. Now they're dual licensed GPL and AGPL. However, I intend
for all my future changes to these files to only be released under the
AGPL license, and I won't be tracking the dual licensing status, so I'm
simply changing the license statement to say it's AGPL.
(In some cases, others wrote parts of the code of a file and released it
under the GPL; but in all cases I have contributed a significant portion
of the code in each file and it's that code that is getting the AGPL
license; the GPL license of other contributors allows combining with
AGPL code.)
The export database has writes made to it and then expects to read back
the same data immediately. But, the way that Database.Handle does
writes, in order to support multiple writers, makes that not work, due
to caching issues. This resulted in export re-uploading files it had
already successfully renamed into place.
Fixed by allowing databases to be opened in MultiWriter or SingleWriter
mode. The export database only needs to support a single writer; it does
not make sense for multiple exports to run at the same time to the same
special remote.
All other databases still use MultiWriter mode. And by inspection,
nothing else in git-annex seems to be relying on being able to
immediately query for changes that were just written to the database.
This commit was supported by the NSF-funded DataLad project.
Refactored some common code into initDb.
This only deals with the problem when creating new databases. If a repo
got bad permissions into it, it's up to the user to deal with it.
This commit was sponsored by Ole-Morten Duesund on Patreon.
Writes are optimised by queueing up multiple writes when possible.
The queue is flushed after the Annex monad action finishes. That makes it
happen on program termination, and also whenever a nested Annex monad action
finishes.
Reads are optimised by checking once (per AnnexState) if the database
exists. If the database doesn't exist yet, all reads return mempty.
Reads also cause queued writes to be flushed, so reads will always be
consistent with writes (as long as they're made inside the same Annex monad).
A future optimisation path would be to determine when that's not necessary,
which is probably most of the time, and avoid flushing unncessarily.
Design notes for this commit:
- separate reads from writes
- reuse a handle which is left open until program
exit or until the MVar goes out of scope (and autoclosed then)
- writes are queued
- queue is flushed periodically
- immediate queue flush before any read
- auto-flush queue when database handle is garbage collected
- flush queue on exit from Annex monad
(Note that this may happen repeatedly for a single database connection;
or a connection may be reused for multiple Annex monad actions,
possibly even concurrent ones.)
- if database does not exist (or is empty) the handle
is not opened by reads; reads instead return empty results
- writes open the handle if it was not open previously
Fsck can use the queue for efficiency since it is write-heavy, and only
reads a value before writing it. But, the queue is not suited to the Keys
database.
