Fix potential hang in https handshake

It has been observed that curl_multi_timeout may return a very long
timeout value (e.g., 294 seconds and some usec) just before
curl_multi_fdset returns no file descriptors for reading.  The
upshot is that select() will hang for a long time -- long enough for
an https handshake to be dropped.  The observed behavior is that
the git command will hang at the terminal and never transfer any
data.

This patch is a workaround for a probable bug in libcurl.  The bug
only seems to manifest around a very specific set of circumstances:

- curl version (from curl/curlver.h):

 #define LIBCURL_VERSION_NUM 0x071307

- git-remote-https running on an ubuntu-lucid VM.
- Connecting through squid proxy running on another VM.

Interestingly, the problem doesn't manifest if a host connects
through squid proxy running on localhost; only if the proxy is on
a separate VM (not sure if the squid host needs to be on a separate
physical machine).  That would seem to suggest that this issue
is timing-sensitive.

This patch is more or less in line with a recommendation in the
curl docs about how to behave when curl_multi_fdset doesn't return
and file descriptors:

http://curl.haxx.se/libcurl/c/curl_multi_fdset.html

Signed-off-by: Stefan Zager <szager@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

http.c

@@ -631,6 +631,18 @@ void run_active_slot(struct active_request_slot *slot)
 			FD_ZERO(&excfds);
 			curl_multi_fdset(curlm, &readfds, &writefds, &excfds, &max_fd);
 
+			/*
+			 * It can happen that curl_multi_timeout returns a pathologically
+			 * long timeout when curl_multi_fdset returns no file descriptors
+			 * to read.  See commit message for more details.
+			 */
+			if (max_fd < 0 &&
+			    (select_timeout.tv_sec > 0 ||
+			     select_timeout.tv_usec > 50000)) {
+				select_timeout.tv_sec  = 0;
+				select_timeout.tv_usec = 50000;
+			}
+
 			select(max_fd+1, &readfds, &writefds, &excfds, &select_timeout);
 		}
 	}