In my single-threaded code, exceptions stop execution and print a stack
backtrace. This is less informative than I would like [1], but sometimes it
provides crucial clues.
My multi-threaded code generally uses agents (just because that's the best way
I've found to get something that behaves like pmap but with better multicore
processor utilization [2]). By default, exceptions in agents are silent, which
is a default that I've always found to be perplexing. But if I pass an
:error-handler argument to agent (as in [2] below) then I can at least see the
exception... but execution doesn't halt and I don't get a backtrace. That means
that if I'm running in a multi-core mode (as I usually am) then the first
unfortunate thing is that I have to notice the printed exception (because
execution continues past it if I don't). But the even more unfortunate thing is
that I then get no backtrace. In order to get a backtrace I have to re-run my
whole system in single-threaded mode, and then hope that I hit the same error
(since my systems almost always involve a lot of randomness).
I figure it would be easy to put an exit in the :error-handler that would quit
the whole program, but I don't know how I could get the stack backtrace for the
triggering exception.
Does anyone know how to get this?
Thanks,
-Lee
[1] Oh, how I long to see the values of locals up the stack in these
backtraces. I understand there are ways to get this but that they require
particular tool chains (emacs w/ritz?); my longing is for this kind of
functionality no matter how I launch my code, or at least when I launch it via
"lein run". The lack of this feature is the #1 thing I miss in Clojure relative
to Common Lisp.
[2] Specifically, pmap won't keep all cores busy if a thread that starts later
finishes earlier than one that starts earlier. Here's the code that I use
instead of pmap, which assumes there's a boolean-valued var called
single-thread-mode:
(defn pmapall
"Like pmap but: 1) coll should be finite, 2) the returned sequence
will not be lazy, 3) calls to f may occur in any order, to maximize
multicore processor utilization, and 4) takes only one coll so far."
[f coll]
(if single-thread-mode
(doall (map f coll))
(let [agents (map #(agent % :error-handler (fn [agnt except] (println
except))) coll)]
(dorun (map #(send % f) agents))
(apply await agents)
(doall (map deref agents)))))
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