[2.4.x] Test suite hangs on Linux

[zdohnal]

See the latest 2.4.x CI results - the tests on Linux hangs, which blocks the last 2.4.x release.

$ /.configure && make test

Expected behavior
Test suite passes

[michaelrsweet]

I wish I could reproduce this on a local system or VM image...

[zdohnal]

@michaelrsweet I hope I gonna get to this in next two weeks... I get it quite often.

[zdohnal]

So far I know it is stuck on lock somewhere in subscription related code IIRC.

[michaelrsweet]

Hmm, OK I just tried again on my Ubuntu laptop and was able to get the hang during the restart test. Investigating...

[michaelrsweet]

Well, the backtrace shows a single thread stuck waiting for a write lock in cupsdAddSubscription, which has been called from cupsdLoadAllSubscriptions which was called from cupsdReadConfiguration. I don't see anything obvious to go wrong there, and the only time the lock is used is in the call to cupsdAddSubscription.

cupsdDeleteAllSubscriptions appears to be double-write-locking the rw-lock, so I'll try fixing that issue... Surprised that Coverity didn't detect that issue... :/

[michaelrsweet]

[master 6d15d8e] Clean up potential subscription locking issues (Issue #1346)

[zdohnal]

The CI now passes, LGTM, thanks!