Austin,
You might find a macro like this helpful -- just use it directly instead of
future. You can replace println with whatever sort of logging you want.
(defmacro logged-future
"Given a body, execute it in a try/catch and log any errors."
[& body]
(let [line (:line (meta &form))
file *file*]
`(future
(try
~@body
(catch Throwable t#
(println t# "Unhandled exception at:"
~file "line:" ~line
"on thread:"
(.getName (Thread/currentThread))))))))
On Sat, Jan 2, 2021 at 5:59 PM Austin Haas <aus...@pettomato.com> wrote:
> Ah, thanks for pointing that out. I must've overlooked your example,
> because I'd already written off futures.
>
> It seems like what you are suggesting, catch and print, might be about as
> good as I could hope for. If I don't want to block the main thread, then I
> don't see what else I could possibly do but print the exception. I guess I
> could store it somewhere, but in any case, I'd use this same pattern.
>
> Thanks, Justin.
> On Saturday, January 2, 2021 at 1:44:55 PM UTC-8 noise...@gmail.com wrote:
>
>> to be clear, in my second example you see the error from the future
>> without using deref
>>
>> good luck finding your solution
>>
>> On Sat, Jan 2, 2021 at 12:50 PM Austin Haas <aus...@pettomato.com> wrote:
>>
>>> Thank you very much for the explanation, Justin.
>>>
>>> I don't see how I can use futures, though, without blocking on the main
>>> thread (to get the exception when it occurs). I'm spawning a long-running
>>> process that never returns a value.
>>> On Saturday, January 2, 2021 at 12:43:14 AM UTC-8 noise...@gmail.com
>>> wrote:
>>>
>>>> By the time the exception is caught, you are already outside the
>>>> context of the Thread which the repl client is interacting with. The
>>>> default exception handler has no information tying the executing thread to
>>>> the repl process (not to mention the dynamic variables clojure is using to
>>>> associate output from code your client runs with that socket connection).
>>>>
>>>> You probably don't want to rebind the root exception handler to show
>>>> *all* exceptions to your client socket. Which means that you need to set up
>>>> some soft of infrastructure connecting the information about the failed
>>>> function to the socket you are listening to.
>>>>
>>>> I find "future" very convenient for this, it uses a pool which will
>>>> perform better than creating Threads ad-hoc, and will capture Exceptions
>>>> and re-throw when you deref (of course, it's up to you to ensure you deref,
>>>> or use try/catch and otherwise forward the failure information via the
>>>> catch block). Also, it conveys dynamic bindings for things like
>>>> clojure.core/*out* and clojure.core/*err* that java classes don't know
>>>> about.
>>>>
>>>> (ins)user=> (def fut (future (throw (Exception. "oops"))))
>>>> #'user/fut
>>>> (ins)user=> @fut ; waits until deref to raise the error
>>>> Execution error at user/fn (REPL:11).
>>>> oops
>>>> (ins)user=> (def fut2 (future (try (throw (Exception. "oops")) (catch
>>>> Exception e (println "wat\n" e))))) ; prints instead of raising
>>>> #'user/fut2
>>>> user=> wat
>>>> #error {
>>>> :cause oops
>>>> :via
>>>> [{:type java.lang.Exception
>>>> :message oops
>>>> :at [user$fn__165 invokeStatic NO_SOURCE_FILE 13]}]
>>>> :trace
>>>> [[user$fn__165 invokeStatic NO_SOURCE_FILE 13]
>>>> [user$fn__165 invoke NO_SOURCE_FILE 13]
>>>> [clojure.core$binding_conveyor_fn$fn__5754 invoke core.clj 2030]
>>>> [clojure.lang.AFn call AFn.java 18]
>>>> [java.util.concurrent.FutureTask run FutureTask.java 264]
>>>> [java.util.concurrent.ThreadPoolExecutor runWorker
>>>> ThreadPoolExecutor.java 1128]
>>>> [java.util.concurrent.ThreadPoolExecutor$Worker run
>>>> ThreadPoolExecutor.java 628]
>>>> [java.lang.Thread run Thread.java 834]]}
>>>>
>>>>
>>>>
>>>> On Thu, Dec 31, 2020 at 1:48 PM Austin Haas <aus...@pettomato.com>
>>>> wrote:
>>>>
>>>>>
>>>>> Problem: When I connect to a socket server and create a thread,
>>>>> exceptions in the thread are printed in the server's process, not the
>>>>> client's. I'd like them to appear in the client's process, where the
>>>>> thread
>>>>> was created.
>>>>>
>>>>> (I'm using the term "process" very generally here, because I don't
>>>>> understand what is going on.)
>>>>>
>>>>> From searching, I understand that there are some other things at play,
>>>>> like System/err, but I don't understand what is happening or how I can
>>>>> work
>>>>> around it. Why does an exception thrown in the client process show in the
>>>>> client process, but an exception thrown in a thread created by the client
>>>>> process shows in the server process? Why doesn't binding *err* in a thread
>>>>> seem to have any effect? Any suggestions or workarounds?
>>>>>
>>>>> I'm not using futures, because this is a long-running process that
>>>>> never returns a value.
>>>>>
>>>>> Example transcript:
>>>>>
>>>>> # Socker server
>>>>>
>>>>> (The only command entered is the first one, which begins with clj.
>>>>> Everything after "user=>" is due to the client below.)
>>>>>
>>>>> $ clj -J-Dclojure.server.myrepl='{:port
>>>>> 5555,:accept,clojure.core.server/repl}'
>>>>> Clojure 1.10.1
>>>>> user=> My second message.
>>>>> Exception in thread "Thread-0" clojure.lang.ExceptionInfo: My second
>>>>> exception {}
>>>>> at user$eval5$fn__141.invoke(NO_SOURCE_FILE:7)
>>>>> at clojure.lang.AFn.run(AFn.java:22)
>>>>> at java.lang.Thread.run(Thread.java:745)
>>>>> Exception in thread "Thread-1" clojure.lang.ExceptionInfo: My third
>>>>> exception {}
>>>>> at user$eval144$fn__145.invoke(NO_SOURCE_FILE:16)
>>>>> at clojure.lang.AFn.run(AFn.java:22)
>>>>> at java.lang.Thread.run(Thread.java:745)
>>>>>
>>>>> # Client
>>>>>
>>>>> $ nc localhost 5555
>>>>> user=> (println "My first message.")
>>>>> My first message.
>>>>> nil
>>>>> user=> (throw (ex-info "My first exception." {}))
>>>>> Execution error (ExceptionInfo) at user/eval3 (REPL:2).
>>>>> My first exception.
>>>>> user=> (.start
>>>>> (Thread.
>>>>> (fn []
>>>>> (println "My second message.")
>>>>> (throw (ex-info "My second exception" {})))))
>>>>> nil
>>>>> user=> (.start
>>>>> (Thread.
>>>>> (let [out *out*
>>>>> err *err*]
>>>>> (fn []
>>>>> (binding [*out* out
>>>>> *err* err]
>>>>> (println "My third message.")
>>>>> (throw (ex-info "My third exception" {})))))))
>>>>> nil
>>>>> My third message.
>>>>>
>>>>> Any clues would be appreciated. Thanks!
>>>>>
>>>>> --
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>>>>> .
>>>>>
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--
Sean A Corfield -- (904) 302-SEAN
An Architect's View -- https://corfield.org/
World Singles Networks, LLC. -- https://worldsinglesnetworks.com/
"Perfection is the enemy of the good."
-- Gustave Flaubert, French realist novelist (1821-1880)
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