Speeds up commands like "git-annex find --in remote" by over 50%.
Profiling showed that adjustGitEnv was 21% of the time and 37% of the
allocations of that command. It copied the environment each time with
getEnvironment.
The only repeated use of adjustGitEnv is in withIndexFile, which tends to
be run at least once per file. So, it was optimised by keeping a cache of
the environment, which can be reused.
There could be other better ways to optimise this. Maybe get the while
environment once at startup. But, then it would have to be serialized back
out each time running a child process, so I doubt that would be a net win.
It might be better to cache a version of the environment that is
pre-modified to use .git-annex/index. But, profiling doesn't show that
modifying the enviroment is taking any significant time.
The queue could potentially contain changes from before withAltRepo, and
get flushed inside the call, which would apply the changes to the modified
repo.
Or, changes could be queued in withAltRepo that were intended to affect
the modified repo, but don't get flushed until later.
I don't know of any cases where either happens, but better safe than sorry.
Note that this affect withIndexFile, which is used in git-annex branch
updates. So, it potentially makes things slower. Should not be by much;
the overhead consists only of querying the current queue a couple of times,
and potentially flushing changes queued within withAltRepo earlier, that
could have maybe been bundled with other later changes.
Notice in particular that the existing queue is not flushed when calling
withAltRepo. So eg when git annex add needs to stage files in the index,
it will still bundle them together efficiently.
This is actually worse than I thought; when git is being run with a
detached work tree, GIT_INDEX_FILE is treated as a path relative to CWD,
instead of the normal behavior of relative the top of the work tree.
This seems to make it basically impossible for any program that wants to
use GIT_INDEX_FILE to use anything other than an absolute path to it; there
are too many configurations to keep straight that can change how git
interprets what should be a simple relative path to a file.
(I have complained to the git developers.)
This affected git annex view. It turns out that some other places
that use GIT_INDEX_FILE were already working around the bug. I removed the
workaround from Annex.Branch since the new workaround will do.
When git-annex is used with a git version older than 2.2.0, disable support for
adjusted branches, since GIT_COMMON_DIR is needed to update them and was first
added in that version of git.
