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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