Implemented alternateJournal, which git-remote-annex
uses to avoid any writes to the git-annex branch while setting up
a special remote from an annex:: url.
That prevents the remote.log from being overwritten with the special
remote configuration from the url, which might not be 100% the same as
the existing special remote configuration.
And it prevents an overwrite deleting of other stuff that was
already in the remote.log.
Also, when the branch was created by git-remote-annex, only delete it
at the end if nothing else has been written to it by another command.
This fixes the race condition described in
797f27ab05, where git-remote-annex
set up the branch and git-annex init and other commands were
run at the same time and their writes to the branch were lost.
Capstone to this feature. Any transitions that have been performed on an
unmerged remote ref but not on the local git-annex branch, or vice-versa
have to be applied on the fly when reading files.
Sponsored-by: Dartmouth College's Datalad project
Improved support for using git-annex in a read-only repository, git-annex
branch information from remotes that cannot be merged into the git-annex
branch will now not crash it, but will be merged in memory.
To avoid this making git-annex behave one way in a read-only repository,
and another way when it can write, it's important that Annex.Branch.get
return the same thing (modulo log file compaction) in both cases.
This manages that mostly. There are some exceptions:
- When there is a transition in one of the remote git-annex branches
that has not yet been applied to the local or other git-annex branches.
Transitions are not handled.
- `git-annex log` runs git log on the git-annex branch, and so
it will not be able to show information coming from the other, not yet
merged branches.
- Annex.Branch.files only looks at files in the git-annex branch and not
unmerged branches. This affects git-annex info output.
- Annex.Branch.hs.overBranchFileContents ditto. Affects --all and
also importfeed (but importfeed cannot work in a read-only repo
anyway).
- CmdLine.Seek.seekFilteredKeys when precaching location logs.
Note use of Annex.Branch.fullname
- Database.ContentIdentifier.needsUpdateFromLog and updateFromLog
These warts make this not suitable to be merged yet.
This readonly code path is more expensive, since it has to query several
branches. The value does get cached, but still large queries will be
slower in a read-only repository when there are unmerged git-annex
branches.
When annex.merge-annex-branches=false, updateTo skips doing anything,
and so the read-only repository code does not get triggered. So a user who
is bothered by the extra work can set that.
Other writes to the repository can still result in permissions errors.
This includes the initial creation of the git-annex branch, and of course
any writes to the git-annex branch.
Sponsored-by: Dartmouth College's Datalad project
My worry was that a preferred content expression that matches on metadata
would have removed the location log from cache, causing an expensive
re-read when a Seek action later checked the location log.
Especially when the --all optimisation in the previous commit
pre-cached the location log.
This also means that the --all optimisation could cache the metadata log
too, if it wanted too, but not currently done.
The cache is a list, with the most recently accessed file first. That
optimises it for the common case of reading the same file twice, eg a
get, examine, followed by set reads it twice. And sync --content reads the
location log 3 times in a row commonly.
But, as a list, it should not be made to be too long. I thought about
expanding it to 5 items, but that seemed unlikely to be a win commonly
enough to outweigh the extra time spent checking the cache.
Clearly there could be some further benchmarking and tuning here.
The cache was removed way back in 2012,
commit 3417c55189
Then I forgot I had removed it! I remember clearly multiple times when I
thought, "this reads the same data twice, but the cache will avoid that
being very expensive".
The reason it was removed was it messed up the assistant noticing when
other processes made changes. That same kind of problem has recently
been addressed when adding the optimisation to avoid reading the journal
unnecessarily.
Indeed, enableInteractiveJournalAccess is run in just the
right places, so can just piggyback on it to know when it's not safe
to use the cache.
The only price paid is one additional MVar read per write to the journal.
Presumably writing a journal file dominiates over a MVar read time by
several orders of magnitude.
--batch does not get the speedup because then it needs to notice when
another process has made a change. Also made the assistant and other damon
modes bypass the optimisation, which would not help them anyway.
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.)
All commands that often have to read a lot of information from
the git-annex branch should now be nearly as fast as before
the branch was introduced.
Before fsck was taking approximatly 3 hours, now it's running in 8 minutes.
The code is very nasty. It should be rewritten to read the header line
from git cat-file, and then read the specified number of bytes of content.
git is slow when the index file is large and has to be rewritten each time
a file is changed. To speed this up, added a journal where changes are
recorded before being fed into the index file and committed to the
git-annex branch. The entire journal can be fed into git with just 2
commands, and only one write of the index file.
