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+---
+postid: 089
+title: A few changes to Feedparse's HTTP requester (prelude to Feedbot)
+date: March 19, 2019
+author: Lucian Mogoșanu
+tags: tech, tmsr
+---
+
+This post introduces s-xml-feedparse-http-fixes:
+
+* the [V patch][s-xml-feedparse-http-fixes.vpatch]; and
+* my [seal][s-xml-feedparse-http-fixes.vpatch.spyked.sig].
+
+So what do these changes slash "HTTP fixes" consist of, more precisely?
+Two pieces of code are being repaired -- one of the repairs being
+required by Feedbot, while the other is more or less an opportunity for
+making the code a tad clea[nr]er. The latter (simpler) one goes as
+follows:
+
+~~~~ {.diff}
++(defvar *http-request-timeout* 60)
+
+...
+
+-  (parse-feed-string (http-request-with-timeout url 60)))
++  (parse-feed-string (http-request-with-timeout url *http-request-timeout*)))
+~~~~
+
+In other words, we're calling `http-request-with-timeout` with an
+operator-configurable timeout setting -- as it should have been in the
+first place, but let's not go into too many details, lest I end up
+burning myself and applying ice on the wound.
+
+The former (more important) change is along the lines of:
+
+~~~~ {.diff}
++(defun http-request-thread (url timeout)
++  (handler-case
++      (drakma:http-request url :connection-timeout timeout)
++    ;; Note: this will have to be revisited once we have a sane view of
++    ;; all the conditions returned by drakma.
++    (t (c)
++      (return-from http-request-thread (values nil c)))))
+
+... and a bit later:
+
+   (let ((req-thread (sb-thread:make-thread
+-                   #'(lambda (url timeout)
+-                (drakma:http-request url :connection-timeout timeout))
++             #'http-request-thread
+~~~~
+
+This means that the thread we're creating now runs the code in
+`http-request-thread` instead of an anonymous function; but more
+importantly, that the call to [Drakma][drakma]'s `http-request` is now
+guarded by a [handler-case][handler-case] that, long story short, acts
+as a wildcard exception catcher.
+
+Feedbot benefits from this change by making sure that all HTTP requests
+-- that, as we know from above, occur on a separate thread -- are
+deterministic, i.e. the CL run-time doesn't end up throwing exceptions
+at us if somehow a request times out or who knows what other condition
+out of the operator's control. I know, this isn't the most fortunate of
+implementation choices, but well... if I do the right thing(tm) and fix
+Drakma at this very moment rather than later, and then that other thing,
+and then the other one, then I might get to publish Feedbot in a decade
+or so. Let's say that the debt is manageable -- for now, but not for
+long.
+
+Up next: Feedbot, part the first.
+
+[s-xml-feedparse-http-fixes.vpatch]: http://lucian.mogosanu.ro/src/s-xml/v/patches/s-xml-feedparse-http-fixes.vpatch
+[s-xml-feedparse-http-fixes.vpatch.spyked.sig]: http://lucian.mogosanu.ro/src/s-xml/v/seals/s-xml-feedparse-http-fixes.vpatch.spyked.sig
+[drakma]: /posts/y05/087-feedparse.html#selection-300.0-300.1
+[handler-case]: https://archive.is/x7yfi