The ContentIdentifier can contain almost anything, so could have characters
that are not fit for the filesystem, or might be longer than a key usually
is, or contain a newline, or .... genKeyName deals with those problems.
This should not present a back-compat issue, because this is a temporary
key used while downloading the imported file, before the real key for it
can be generated.
Some recent changes to use mask missed that async exceptions can still
be thrown inside it. The goal is to make sure a block of cleanup code
runs entirely, w/o being interrupted by an async exception, so use
uninterruptibleMask.
Also, converted a few to bracket, which is nicer.
Tested the forcerestart code path and it works.
The hairy part is, what if an async exception is caught when it's in
restart?
If it's in the part that stops the old process, the old process
is left in the handle. The next attempt to use the CoProcessHandle
will then throw an IO exception, which will result in restart getting
run again. So I think this will work, but have not actually tested it.
The use of withMVarMasked lets it start the new process and fill the
mvar with it, even if there's an async exception at that point.
Note that exceptions are masked while running forcerestart, so
do not need to worry about an async exception being thrown while it's
recovering from an async exception.
Since an external process can be in the middle of some operation when an
async exception is received, it has to be shut down then. Using
cleanupProcess will close its IO handles and send it a SIGTERM.
If a special remote choses to catch SIGTERM, it's fine for it to do some
cleanup then, but until it finishes, git-annex will be blocked waiting
for it. If a special remote blocked SIGTERM, it would cause a hang.
Mentioned in docs.
Also, in passing, fixed a FD leak, it was not closing the error handle
when shutting down the external. In practice that didn't matter before because
it was only run when git-annex was itself shutting down, but now that it
can run on exception, it would have been a problem.
Fixes reversion in recent conversions, the old code relied on the GC
apparently, but the new code explicitly waits on the process, so must
close stdin handle first or the command will never exit.
