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
The annex object for it may have been modified due to hard link, and
that should be cleaned up when the new version is added. If another
associated file has the old key's content, that's linked into the annex
object. Otherwise, update location log to reflect that content has been
lost.
When a v6 unlocked files is removed from the work tree,
unused doesn't show it. When it gets removed from the index,
unused does show it. This is the same as a locked file.
This only adds 1 stat to each file fscked for locked files, so
added overhead is minimal.
For unlocked files it has to access the database to see if a file
is modified.
If multiple files point to the same annex object, the user may want to
modify them independently, so don't use a hard link.
Also, check diskreserve when copying.
Note that the implementation uses replaceFile, so that the actual
replacement of the work tree file is atomic. This seems a good property to
have!
It would be possible for unlock in v6 mode to be run on files that do not
have their content present. However, that would be a behavior change from
before, and I don't see any immediate need to support it, so I didn't
implement it.
This avoids querying the database when the content file doen't exist
(or otherwise fails the provided check). However, it does add overhead of
querying the database, and will certianly impact performance.
The Keys database can hold multiple inode caches for a given key. One for
the annex object, and one for each pointer file, which may not be hard
linked to it.
Inode caches for a key are recorded when its content is added to the annex,
but only if it has known pointer files. This is to avoid the overhead of
maintaining the database when not needed.
When the smudge filter outputs a file's content, the inode cache is not
updated, because git's smudge interface doesn't let us write the file. So,
dropping will fall back to doing an expensive verification then. Ideally,
git's interface would be improved, and then the inode cache could be
updated then too.
This removes ambiguity, because while someone might have "WORM--foo" in a
file that's not intended to be a git-annex pointer file,
"annex/objects/WORM--foo" is less likely.
Also, 664cc987e8 had a caveat about symlink
targets being parsed as pointer files, and now the same parser is used for
both.
I did not include any hash directories before the key in the pointer file,
as they're not needed. However, if they were included, the parser would
still work ok.
Note that this changes the default behavior of git add in a newly
initialized repository; it will add files to the annex.
Don't like that this could break workflows, but it's necessary in order for
any pointer files in the repo to be handled by git-annex.
Since all places where a repo is used in direct mode need to have git-annex
upgraded before the repo can safely be converted to v6, the upgrade needs
to be manual for now.
I suppose that at some point I'll want to drop all the direct mode support
code. At that point, will stop supporting v5, and will need to auto-upgrade
any remaining v5 repos. If possible, I'd like to carry the direct mode
support for say, a year or so, to give people plenty of time to upgrade and
avoid disruption.
