BUG/MINOR: checks: do not trust errno in write event before any syscall

The only case where errno is supposed to be valid is when the connection
has just got the CO_FL_ERROR flag and errno is not zero, because it will
have been set by the same function that has set the flag. For all other
situations, we need to check the socket using getsockopt(), but only do
it once, since it clears the pending error code. For this reason, we
assign the error code to errno in order not to lose it. The same call
is made at the entry of event_srv_chk_r(), event_srv_chk_w(), and
wake_srv_chk() so that we get a chance to collect errors reported by
the poller or by failed syscalls.

Note that this fix relies on the 4 previous patches, so backporters
must be very careful.

src/checks.c

@@ -777,6 +777,47 @@ static int httpchk_build_status_header(struct server *s, char *buffer)
 	return hlen;
 }
 
+/* Check the connection. If an error has already been reported or the socket is
+ * closed, keep errno intact as it is supposed to contain the valid error code.
+ * If no error is reported, check the socket's error queue using getsockopt().
+ * Warning, this must be done only once when returning from poll, and never
+ * after an I/O error was attempted, otherwise the error queue might contain
+ * inconsistent errors. If an error is detected, the CO_FL_ERROR is set on the
+ * socket. Returns non-zero if an error was reported, zero if everything is
+ * clean (including a properly closed socket).
+ */
+static int retrieve_errno_from_socket(struct connection *conn)
+{
+	int skerr;
+	socklen_t lskerr = sizeof(skerr);
+
+	if (conn->flags & CO_FL_ERROR && ((errno && errno != EAGAIN) || !conn->ctrl))
+		return 1;
+
+	if (!conn->ctrl)
+		return 0;
+
+	if (getsockopt(conn->t.sock.fd, SOL_SOCKET, SO_ERROR, &skerr, &lskerr) == 0)
+		errno = skerr;
+
+	if (errno == EAGAIN)
+		errno = 0;
+
+	if (!errno) {
+		/* we could not retrieve an error, that does not mean there is
+		 * none. Just don't change anything and only report the prior
+		 * error if any.
+		 */
+		if (conn->flags & CO_FL_ERROR)
+			return 1;
+		else
+			return 0;
+	}
+
+	conn->flags |= CO_FL_ERROR | CO_FL_SOCK_WR_SH | CO_FL_SOCK_RD_SH;
+	return 1;
+}
+
 /*
  * This function is used only for server health-checks. It handles
  * the connection acknowledgement. If the proxy requires L7 health-checks,
@@ -787,21 +828,19 @@ static void event_srv_chk_w(struct connection *conn)
 {
 	struct check *check = conn->owner;
 	struct server *s = check->server;
-	int fd = conn->t.sock.fd;
 	struct task *t = check->task;
 
-	if (conn->flags & (CO_FL_SOCK_WR_SH | CO_FL_DATA_WR_SH))
+	if (retrieve_errno_from_socket(conn)) {
+		set_server_check_status(check, HCHK_STATUS_L4CON, strerror(errno));
+		__conn_data_stop_both(conn);
+		goto out_wakeup;
+	}
+
+	if (conn->flags & (CO_FL_ERROR | CO_FL_SOCK_WR_SH | CO_FL_DATA_WR_SH)) {
+		/* if the output is closed, we can't do anything */
 		conn->flags |= CO_FL_ERROR;
-
-	if (unlikely(conn->flags & CO_FL_ERROR)) {
-		int skerr, err = errno;
-		socklen_t lskerr = sizeof(skerr);
-
-		if (!getsockopt(fd, SOL_SOCKET, SO_ERROR, &skerr, &lskerr) && skerr)
-			err = skerr;
-
-		set_server_check_status(check, HCHK_STATUS_L4CON, strerror(err));
-		goto out_error;
+		set_server_check_status(check, HCHK_STATUS_L4CON, NULL);
+		goto out_wakeup;
 	}
 
 	if (conn->flags & (CO_FL_HANDSHAKE | CO_FL_WAIT_WR))
@@ -814,6 +853,7 @@ static void event_srv_chk_w(struct connection *conn)
 			conn->xprt->snd_buf(conn, check->bo, MSG_DONTWAIT | MSG_NOSIGNAL);
 			if (conn->flags & CO_FL_ERROR) {
 				set_server_check_status(check, HCHK_STATUS_L4CON, strerror(errno));
+				__conn_data_stop_both(conn);
 				goto out_wakeup;
 			}
 			if (check->bo->o) {
@@ -834,9 +874,6 @@ static void event_srv_chk_w(struct connection *conn)
 	__conn_data_stop_send(conn);   /* nothing more to write */
  out_incomplete:
 	return;
- out_error:
-	conn->flags |= CO_FL_ERROR;
-	goto out_wakeup;
 }
 
 /*
@@ -862,6 +899,12 @@ static void event_srv_chk_r(struct connection *conn)
 	int done;
 	unsigned short msglen;
 
+	if (retrieve_errno_from_socket(conn)) {
+		set_server_check_status(check, HCHK_STATUS_L4CON, strerror(errno));
+		__conn_data_stop_both(conn);
+		goto out_wakeup;
+	}
+
 	if (unlikely((check->result & SRV_CHK_FAILED) || (conn->flags & CO_FL_ERROR))) {
 		/* in case of TCP only, this tells us if the connection failed */
 		if (!(check->result & SRV_CHK_FAILED))
@@ -1271,6 +1314,10 @@ static int wake_srv_chk(struct connection *conn)
 {
 	struct check *check = conn->owner;
 
+	if (retrieve_errno_from_socket(conn)) {
+		set_server_check_status(check, HCHK_STATUS_L4CON, strerror(errno));
+	}
+
 	if (unlikely(conn->flags & CO_FL_ERROR)) {
 		/* We may get error reports bypassing the I/O handlers, typically
 		 * the case when sending a pure TCP check which fails, then the I/O