[bug#47288] [PATCH] guix: http-client: Tweak http-multiple-get error handling.

[Maxime Devos <maximedevos@telenet.be>]

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On Sun, 2021-03-21 at 00:43 +0000, Christopher Baines wrote:
> This isn't meant to change the way errors are handled, and arguably makes the
> code harder to read, but it's a uninformed attempt to improve the
> performance (following on from a performance regression in
> 205833b72c5517915a47a50dbe28e7024dc74e57).
> 
> I'm guessing something about Guile internals makes calling (loop ...) within
> the catch bit less performant than avoiding this and calling (loop ...) after
> the catch bit has finished. Since this happens lots, this seems to be
> sufficient to make guix weather a lot slower than it was before.

I took a look at the code, and it seems we did somthing like:

(let loop VARS
  (match sent
    NON-LOOPING-CASES
    (STUFF
     (catch #t
       THUNK-THAT-MIGHT-CALL-LOOP-IN-TAIL-POSITION
       SOME-HANDLER-THAT-DOES-NOT-CALL-LOOP)

A small demonstration of what could go wrong with such a construction
(run this in the Guile REPL):

(let loop ((attempts-todo 20))
  (catch 'oops
    (lambda ()
      (if (<= 0 attempts-todo)
          (loop (- attempts-todo 1))
          (throw 'oops)))
    (lambda _ (display 'too-bad!) (newline) (backtrace))))

Output:
too-bad!

Backtrace:
In ice-9/boot-9.scm:
  1731:15 19 (with-exception-handler #<procedure 7f73c7e615a0 at ice-9/boot-9.scm:1815:7 (exn)> _ # _ …)
  [The previous line repeated 17 times]
  1731:15  1 (with-exception-handler #<procedure 7f73c7e61240 at ice-9/boot-9.scm:1815:7 (exn)> _ # _ …)
In unknown file:
           0 (backtrace #<undefined>)

With this construction, we were nesting exception handlers within exception handlers
... So in hindsight it doesn't seem surprising this isn't very performant.
(THUNK-THAT-MIGHT-CALL-LOOP-IN-TAIL-POSITION itself is not called in the tail-position
of the '(let loop ...)' form.)

Hope that sheds some light on the matter,
Maxime

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