git-annex: thread blocked indefinitely in an STM transaction
failed
git-annex: sqlite query crashed
CallStack (from HasCallStack):
error, called at ./Database/Handle.hs:98:42 in main:Database.Handle
failed
This needs further investigation.
Use same, simpler method to make only one thread open the export db as
is used for the ContentIdentifier db.
And, always update the export db once before using.
Had to add two more API calls to override export APIs that are not safe
for use in combination with import.
It's unfortunate that removeExportDirectory is documented to be allowed
to remove non-empty directories. I'm not entirely sure why it's that
way, my best guess is it was intended to make it easy to implement with
just rm -rf.
For now, it's only allowed when exporttree=yes is also set.
That simplified the implementation, but could later be changed if
there's a remote that makes sense to be an import but not an export.
However, it may work just as well to make a remote be readonly to
prevent export to it while still allowing import.
Not sure if my reasoning about the races really holds.
It would certianly be possible to better guard against races by using
Linux-specific renameat2 with RENAME_EXCHANGE or RENAME_NOREPLACE.
Or by using link and relying on it not overwriting existing files -- but
that would need a filesystem that supports hard links and directory can
be used in filesystems that don't.
This does not avoid all possible races, but it does avoid all likely
ones, and is demonstratably better than git's own handling of races
where files get modified at the same time as it's updating the working
tree.
The main thing this won't detect are not unlikely races where part
of a file gets changed while it's being copied and then the file is
restored to its original condition before the modification check.
No, it's more likely that the limitations of checking inode, size,
and mtime won't detect certian modifications, involving eg mmapped
files.
The branch is only updated once the export is 100% complete. This way,
if an export is started but interrupted and so the remote does not yet
contain some of the files, an import will make a commit on the old
branch, and so won't delete the missing files.
