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Debugging Emacs with threads

From: Eli Zaretskii
Subject: Debugging Emacs with threads
Date: Sun, 11 Dec 2016 18:26:12 +0200

There's a nasty issue that people who debug Emacs with threads need to
be aware of.

When Emacs stops due to a breakpoint, the thread that is the current
one is in sync between GDB and Emacs.  IOW, the current_thread
variable describes the same thread on which GDB commands will act.
But as soon as you say something like "thread 1" at the GDB prompt,
this synchronization is lost: GDB acts on the thread you specified,
while current_thread is still pointing at the thread which was the
current one when Emacs stopped.

The result of this is that invoking functions in Emacs will likely
crash, because thread-specific variables used by Emacs are incorrect
for the thread whose stack GDB will use to create the call-stack frame
for the function it calls.  In particular, xbacktrace will definitely
crash.  And since src/.gdbinit automatically invokes xbacktrace when
you type "bt", any "bt" issued when the threads mismatch will ruin the
debugging session.

It would be good to protect xbacktrace from this, and avoid calling
functions in Emacs in this situation.  Unfortunately, I couldn't find
any way of gleaning the OS thread ID from the GDB thread number.
Maybe Tom, who knows a lot about GDB, could help, or someone else
might have an idea that we then could implement in .gdbinit?


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