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

From: Junio C Hamano <gitster@pobox.com>
Date: May 6, 2022, 21:17 UTC
Message-ID: <xmqqczgqjr8y.fsf_-_@gitster.g>
In-Reply-To: <xmqqtua2jtr0.fsf@gitster.g>

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, but make sure to limit it to the case where the pointer still points at the on-stack variable of ours (the pointer may be set to point at the on-stack variable of somebody else after the slot gets reused, in which case we do not want to touch it).

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

 * So, this has been sitting in my pile of random patches for a few
   weeks.

 http.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/http.c b/http.c
index 229da4d148..85437b1980 100644
--- a/http.c
+++ b/http.c
@@ -1367,6 +1367,9 @@ void run_active_slot(struct active_request_slot *slot)
 			select(max_fd+1, &readfds, &writefds, &excfds, &select_timeout);
 		}
 	}
+
+	if (slot->finished == &finished)
+		slot->finished = NULL;
 }
 
 static void release_active_slot(struct active_request_slot *slot)

-- 
2.36.1-200-gf89ea983ca