http.c: clear the 'finished' member once we are done with it

In http.c, the run_active_slot() function allows the given "slot" to
make progress by calling step_active_slots() in a loop repeatedly,
and the loop is not left until the request held in the slot
completes.

Ages ago, we used to use the slot->in_use member to get out of the
loop, which misbehaved when the request in "slot" completes (at
which time, the result of the request is copied away from the slot,
and the in_use member is cleared, making the slot ready to be
reused), and the "slot" gets reused to service a different request
(at which time, the "slot" becomes in_use again, even though it is
for a different request).  The loop terminating condition mistakenly
thought that the original request has yet to be completed.

Today's code, after baa7b67d (HTTP slot reuse fixes, 2006-03-10)
fixed this issue, uses a separate "slot->finished" member that is
set in run_active_slot() to point to an on-stack variable, and the
code that completes the request in finish_active_slot() clears the
on-stack variable via the pointer to signal that the particular
request held by the slot has completed.  It also clears the in_use
member (as before that fix), so that the slot itself can safely be
reused for an unrelated request.

One thing that is not quite clean in this arrangement is that,
unless the slot gets reused, at which point the finished member is
reset to NULL, the member keeps the value of &finished, which
becomes a dangling pointer into the stack when run_active_slot()
returns.  Clear the finished member before the control leaves the
function, which has a side effect of unconfusing compilers like
recent GCC 12 that is over-eager to warn against such an assignment.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

http.c

@@ -1367,6 +1367,32 @@ void run_active_slot(struct active_request_slot *slot)
 			select(max_fd+1, &readfds, &writefds, &excfds, &select_timeout);
 		}
 	}
+
+	/*
+	 * The value of slot->finished we set before the loop was used
+	 * to set our "finished" variable when our request completed.
+	 *
+	 * 1. The slot may not have been reused for another requst
+	 *    yet, in which case it still has &finished.
+	 *
+	 * 2. The slot may already be in-use to serve another request,
+	 *    which can further be divided into two cases:
+	 *
+	 * (a) If call run_active_slot() hasn't been called for that
+	 *     other request, slot->finished would have been cleared
+	 *     by get_active_slot() and has NULL.
+	 *
+	 * (b) If the request did call run_active_slot(), then the
+	 *     call would have updated slot->finished at the beginning
+	 *     of this function, and with the clearing of the member
+	 *     below, we would find that slot->finished is now NULL.
+	 *
+	 * In all cases, slot->finished has no useful information to
+	 * anybody at this point.  Some compilers warn us for
+	 * attempting to smuggle a pointer that is about to become
+	 * invalid, i.e. &finished.  We clear it here to assure them.
+	 */
+	slot->finished = NULL;
 }
 
 static void release_active_slot(struct active_request_slot *slot)