bug#33145: 27.0.50; rcirc race receives duplicate messages

[Basil L. Contovounesios]

Currently, rcirc-filter clears the accumulator variable
rcirc-process-output only after each of its constituent lines has been
fed to rcirc-process-server-response, which in turn runs the abnormal
hook rcirc-receive-message-functions.

If this hook contains a function which allows process filters to be run,
e.g. by way of accept-process-output, and if process output arrives soon
enough, then rcirc-filter might be called more than once before its
previous invocation has managed to clear rcirc-process-output.

In other words, subsequent invocations may see process output that is
already being handled by previous invocations, thus potentially printing
duplicate messages in the process buffer.

Patch addressing this to follow.

Thanks,

-- 
Basil

In GNU Emacs 27.0.50 (build 31, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll 
bars)
 of 2018-10-24 built on thunk
Repository revision: 00027ff9d0f646662458bdb47cc7e2214f439698
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