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Fix client concurrency - #163
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Send, sendFrame, Subscribe and Disconnect hold closeMutex while pushing to writeCh. If processLoop exited with writeCh full, the writer stayed blocked (for msgSendTimeout, or forever when it is 0) while processLoop's deferred MustDisconnect, or its ERROR handler, waited for the same mutex. Close a processLoopDone channel before processLoop takes closeMutex and have writers give up with ErrClosedUnexpectedly once it is closed. Disconnect keeps returning nil when the connection is already going away.
Unsubscribe is commonly called from the goroutine that reads C. With more messages in flight than C holds, readLoop blocked on the full C, processLoop blocked handing it the next frame, and the UNSUBSCRIBE never got written, so Unsubscribe waited forever (or until its receipt timeout, after which it blocked sending to C itself). Once Unsubscribe or abandon starts, readLoop still delivers to C while there is room and drops what doesn't fit. This also makes abandon's draining goroutine unnecessary.
Unsubscribe closed C itself when the connection was already closed, and sent an error message to it on receipt timeout, while readLoop could still be sending to C. After a Disconnect that timed out waiting for its receipt, processLoop keeps delivering already buffered MESSAGEs, so a following Unsubscribe crashed readLoop with "send on closed channel". Unsubscribe now only reports the error; readLoop, the only sender, closes C when the connection is torn down.
processLoop handed MESSAGEs to subscriptions with a blocking send, so a subscriber that didn't keep up with C stopped everything else on the connection: frames queued by Send were not written, heart-beats were not sent, and the read timeout couldn't fire. If the connection failed in the meantime, sendError blocked the same way and processLoop never exited, so the loss went unnoticed. Queue frames for subscriptions instead and stop reading while any are pending, which keeps them in order and still pushes back on the server, but keep serving writes and heart-beats. The read timer is paused while processLoop isn't reading by choice. processLoop also exits once Disconnect closes the connection. On the way out, close the subscription channels and record the reason in closeFrame instead of sending an ERROR frame to each subscription, so a subscriber that stopped reading can't keep processLoop from exiting. Reply-to UNSUBSCRIBE now ends the subscription with a made-up RECEIPT, queued behind any pending deliveries, instead of closing the channel directly, which no longer reports a spurious "channel read failed" error on C.
readLoop's only way out was a read error. If processLoop exited (read timeout, write error, ERROR frame) while readLoop was blocked handing it a frame with readCh full, readLoop blocked forever, leaking the goroutine and the frames it held.
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A few concurrency fixes