Set OutputStream.delegate to nil in HTTPBodyOutputStreamBridge.deinit

[simonjbeaumont]

Motivation

When running the cancellation tests in a loop, very occasionally there would be a crash with the following backtrace:

#0	0x000000018aa008d4 in objc_opt_respondsToSelector ()
#1	0x000000018aea3410 in _outputStreamCallbackFunc ()
#2	0x000000018aea3310 in _signalEventSync ()
#3	0x000000018aeecdb0 in ___signalEventQueue_block_invoke ()
#4	0x000000018abe6cb8 in _dispatch_call_block_and_release ()
#5	0x000000018abe8910 in _dispatch_client_callout ()
#6	0x000000018abefea4 in _dispatch_lane_serial_drain ()
#7	0x000000018abf0a08 in _dispatch_lane_invoke ()
#8	0x000000018abfb61c in _dispatch_root_queue_drain_deferred_wlh ()
#9	0x000000018abfae90 in _dispatch_workloop_worker_thread ()
#10	0x000000018ad96114 in _pthread_wqthread ()

This seems to indicate that the output stream is trying to access its delegate. However, when running with debug logging enabled I can see that the delegate has already been deinitialized.

This is likely a result of the delegate itself owning the stream and setting the stream delegate to self, which IIUC is an established pattern. This presents a race in teardown.

Modifications

This patch sets the output stream delegate to nil in the delegate deinit.

Result

No attempts to call the delegate will happen after it is has been deinitailzed.

Test Plan

With this patch, the failing test passes when run an order of magnitude more times than were required to reliably reproduce the crash without the patch.