Run emacs in gdb, preferably from the src directory where you compiled emacs:

   $ cd ~/emacs-build/src
   $ gdb ./emacs

[There’s a special .gdbinit file in that directory that defines handy emacs
debugging commands.]

When it hangs, hit C-z or something to exit to the debugger (you can’t use
C-c like usual when debugging emacs, because emacs’ standard .gdbinit file
changes gdb’s SIGINT handling to make C-g work properly on ttys).

Then look at a C backtrace (“backtrace”) and lisp backtrace (“xbacktrace”)
to see what’s up.

For more fun, do the above from another emacs session using emacs’ `M-x gdb’
command.

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