() Felix Dietrich <felix.dietrich@sperrhaken.name>
() Thu, 24 Mar 2022 03:33:50 +0100

   > would like to use Emacs' threads and ‘url-http’ funcs.

   You are much braver than I ;).  I am not sure if the url
   library is robust enough for that task, but you can certainly
   try and play around with it.

'Tis a fine line between brave and foolish, i'm told.  :-D

   > (a) Can anyone else reproduce this on their Emacs?

   Yes (also Debianʼs 27.1), but it does not appear to have
   anything to do with threads: it happens just as well when you
   call ‘func’ directly.

I think this tells us that error handing wrt threads are as if
they were invoked as a (top-level) command.

   [analysis: chain of failures] I have not found a good way to
   handle these errors with the url library.

Thanks for digging into the details.  After some poking around,
i realize that the url*.el facility has quite a lot of baggage
to deal w/, as well.  It's amazing how much cruft RFCs output!

   > (c) Is there a recommended way to trap process filter
   > errors for a noninteractive (perhaps batch) session?

   Fix the process filter ;).

Haha!

   I found the variable ‘command-error-function’ in the Emacs
   Lisp manual [...].

Good eye!  I have managed to adapt your code fragment to produce
a bad.el (attached here):