[drakma-devel] [usocket-devel] [patch] Resubmit drakma timeout for sbcl.

[Nikodemus Siivola]

2011/3/29 Chun Tian (binghe) <binghe.lisp at gmail.com>:

> I know SBCL's WITH-TIMEOUT cannot nest, I learn this from GBBopen's portable-threads.lisp [1], and
> it also give a nested version SBCL's WITH-TIMEOUT, much shorter than yours:

Amusingly, neither SBCL's own, nor GBBopen's WITH-TIMEOUT is asynch
unwind safe. The one I posted is -- that's what the WITHOUT-INTERRUPTS
and WITH-LOCAL-INTERRUPTS were for. :) But yeah, it's miles saner than
the SB-EXT:WITH-TIMEOUT.

The GBBopen's WITH-TIMEOUT also suffers from the minor defect that the
dynamic context (special variables, handlers, restarts, etc) the
timeout-body runs in is unpredictable -- for most cases this does not
matter, but it can lead to hard to reproduce bugs.

> I didn't use this version simply because I think the WITH-TIMEOUT form in usocket's SOCKET-CONNECT
> has no chance to be nested.

It has. A function in a library cannot know if it will be nested in
some dynamic context or not. The corrollary is that library code
should never use SB-EXT;WITH-TIMEOUT.

 (defun foo ()
    ...stuff that uses socket-connect with a local timeout > 1.0
somewhere in its guts...)

 (with-timeout 1.0
   (handler-case (foo)
    (error () :bad-foo)))

If the fetch takes longer than intended, the outer timeout can get
converted into an USOCKET-TIMEOUT-ERROR (whatever the exact name),
which in turn gets swallowed by the HANDLER-CASE.

Cheers,

 -- Nikodemus