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authorBjörn Gustavsson <[email protected]>2017-11-10 05:09:46 +0100
committerBjörn Gustavsson <[email protected]>2017-11-14 06:35:56 +0100
commit3679444fb654e9cba1252c6df0be5170e5388639 (patch)
treec5357a66497a6af3208289b72d1ba907b395c98b /lib
parent19245b1ac7bf881319950adb973ff2ce24fea23e (diff)
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Fix broken receive optimization
When a ref is created before performing a receive that will only receive message containing that ref, there is a compiler optimization to avoid scanning messages that can't possible contain the newly created ref. Magnus Lång pointed out that the implementation of the optimization is flawed. Exceptions or recursive calls could cause the receive operation to scan the receive queue from a position beyond the expected message (that is, the message containing the ref would never be matched out). See the receive_opt_exception/1 and receive_opt_recursion/1 test cases in receive_SUITE. It turns out that we can simplify the implementation of the optimization while fixing the bug (suggested by Magnus Lång). We actually don't need the c_p->msg.mark field. It is enough to have c_p->msg.saved_pos; if it is non-zero, it is a valid position in the message qeueue. All we need to do is to ensure that we clear c_p->msg.saved_pos when a receive is exited normally or abnormally. We can clear c_p->msg.saved_pos in JOIN_MESSAGE(), since it is called both when leaving a receive because a message matched and because there was a timeout and the 'after' clause was executed. In addition, we need to clear c_p->msg.saved_pos when an exception is caught. https://bugs.erlang.org/browse/ERL-511
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