OpenSSL: multi interface handshake could hang

John-Mark Bell filed bug #3000052 that identified a problem (with
an associated patch) with the OpenSSL handshake state machine
when the multi interface is used:

Performing an https request using a curl multi handle and using
select or epoll to wait for events results in a hang. It appears
that the cause is the fix for bug #2958179, which makes
ossl_connect_common unconditionally return from the step 2 loop
when fetching from a multi handle.

When ossl_connect_step2 has completed, it updates
connssl->connecting_state to ssl_connect_3. ossl_connect_common
will then return to the caller, as a multi handle is in
use. Eventually, the client code will call curl_multi_fdset to
obtain an updated fdset to select or epoll on. For https
requests, curl_multi_fdset will cause https_getsock to be called.
https_getsock will only return a socket handle if the
connecting_state is ssl_connect_2_reading or
ssl_connect_2_writing.  Therefore, the client will never obtain a
valid fdset, and thus not drive the multi handle, resulting in a
hang.

(http://curl.haxx.se/bug/view.cgi?id=3000052)

lib/ssluse.c

@@ -2425,8 +2425,18 @@ ossl_connect_common(struct connectdata *conn,
       /* socket is readable or writable */
     }
 
+    /* Run transaction, and return to the caller if it failed or if
+     * this connection is part of a multi handle and this loop would
+     * execute again. This permits the owner of a multi handle to
+     * abort a connection attempt before step2 has completed while
+     * ensuring that a client using select() or epoll() will always
+     * have a valid fdset to wait on.
+     */
     retcode = ossl_connect_step2(conn, sockindex);
-    if(retcode || (data->state.used_interface == Curl_if_multi))
+    if(retcode || (data->state.used_interface == Curl_if_multi &&
+                   (ssl_connect_2 == connssl->connecting_state ||
+                    ssl_connect_2_reading == connssl->connecting_state ||
+                    ssl_connect_2_writing == connssl->connecting_state)))
       return retcode;
 
   } /* repeat step2 until all transactions are done. */