Once we've signaled a cond var, it may take some small amount of time for
the goroutines released to hit the spot we're wanting them to be at. Give
them an appropriate amount of time.
Locking/unlocking the queueMutex is allowed, but not required, for
Cond.Signal() and Cond.Broadcast(). The magic at play here is just a little
too much for golangci-lint, as we take the address of queueMutex and this is
mostly used in archiver.go; the variable still gets flagged as unused.
This introduces two sync.Cond pointers to the archiver package. If they're
non-nil when we go to process a request, we'll wait until signalled (at all)
to proceed. The tests will then create the sync.Cond so that it can signal
at-will and sanity-check the state of the queue at different phases.
The author believes that nil-checking these two sync.Cond pointers on every
archive processing will introduce minimal overhead with no impact on
maintainability.
The prime benefit being sought here is for large archives to not
clog up the rendering process and cause unsightly proxy timeouts.
As a secondary benefit, archive-in-progress is moved out of the
way into a /tmp file so that new archival requests for the same
commit will not get fulfilled based on an archive that isn't yet
finished.
This asynchronous system is fairly primitive; request comes in, we'll
spawn off a new goroutine to handle it, then we'll mark it as done.
Status requests will see if the file exists in the final location,
and report the archival as done when it exists.
Fixes#11265
