bug#24365: 24.4; [PATCH] url-http-options fails when connection refused

[Mark Schreiber <mark7@alumni.cmu.edu>]

url-http-options fails when a web server refuses its connection.

Evaluating (url-http-options "http://127.0.0.1:33333/") results in the
following error:

Debugger entered--Lisp error: (wrong-type-argument number-or-marker-p nil)
  url-http-options("http://127.0.0.1:33333/")
  eval((url-http-options "http://127.0.0.1:33333/") nil)
  elisp--eval-last-sexp(nil)
  eval-last-sexp(nil)
  funcall-interactively(eval-last-sexp nil)
  call-interactively(eval-last-sexp nil nil)
  command-execute(eval-last-sexp)

In my case, this caused the third-party package org-caldav to fail with
this error when M-x org-caldav-sync was invoked and given an incorrect
org-caldav-url.

I believe that the correct behavior would be to have url-http-options
return nil, which is the behavior of that function when it receives an
HTTP error response.

The issue is still present in master.  The following patch against
master causes url-http-options to return nil in the above case.
---
url-http-options would fail internally when passed an URL to a
web server refusing the connection.  Change its behavior to return nil,
as would occur if an HTTP error code were received.

* lisp/url/url-http.el (url-http-options)

Copyright-paperwork-exempt: yes
---
 lisp/url/url-http.el | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/lisp/url/url-http.el b/lisp/url/url-http.el
index 927d0bb..e55b645 100644
--- a/lisp/url/url-http.el
+++ b/lisp/url/url-http.el
@@ -1512,10 +1512,11 @@ url-http-options
   (let* ((url-request-method "OPTIONS")
 	 (url-request-data nil)
 	 (buffer (url-retrieve-synchronously url))
+         (response-status (when buffer (url-http-symbol-value-in-buffer
+                                        'url-http-response-status buffer 0)))
 	 (header nil)
 	 (options nil))
-    (when (and buffer (= 2 (/ (url-http-symbol-value-in-buffer
-			       'url-http-response-status buffer 0) 100)))
+    (when (and response-status (= 2 (/ response-status 100)))
       ;; Only parse the options if we got a 2xx response code!
       (with-current-buffer buffer
 	(save-restriction
-- 
2.1.4