Revert "MEDIUM: queue: simplify again the process_srv_queue() API"

This reverts commit c83e45e9b0.

The recent changes since 5304669e1 MEDIUM: queue: make
pendconn_process_next_strm() only return the pendconn opened a tiny race
condition between stream_free() and process_srv_queue(), as the pendconn
is accessed outside of the lock, possibly while it's being freed. A
different approach is required.

include/haproxy/queue.h

@@ -34,7 +34,7 @@ extern struct pool_head *pool_head_pendconn;
 
 struct pendconn *pendconn_add(struct stream *strm);
 int pendconn_dequeue(struct stream *strm);
-void process_srv_queue(struct server *s);
+void process_srv_queue(struct server *s, int server_locked);
 unsigned int srv_dynamic_maxconn(const struct server *s);
 int pendconn_redistribute(struct server *s);
 int pendconn_grab_from_px(struct server *s);

include/haproxy/stream.h

@@ -339,7 +339,7 @@ static inline void stream_choose_redispatch(struct stream *s)
 	      ((s->be->lbprm.algo & BE_LB_KIND) == BE_LB_KIND_RR)))) {
 		sess_change_server(s, NULL);
 		if (may_dequeue_tasks(objt_server(s->target), s->be))
-			process_srv_queue(objt_server(s->target));
+			process_srv_queue(objt_server(s->target), 0);
 
 		s->flags &= ~(SF_DIRECT | SF_ASSIGNED | SF_ADDR_SET);
 		si->state = SI_ST_REQ;

src/backend.c

@@ -818,7 +818,7 @@ out_ok:
 			sess_change_server(s, srv);
 		} else {
 			if (may_dequeue_tasks(conn_slot, s->be))
-				process_srv_queue(conn_slot);
+				process_srv_queue(conn_slot, 0);
 		}
 	}
 
@@ -1830,7 +1830,7 @@ int srv_redispatch_connect(struct stream *s)
 
 		/* release other streams waiting for this server */
 		if (may_dequeue_tasks(srv, s->be))
-			process_srv_queue(srv);
+			process_srv_queue(srv, 0);
 		return 1;
 	}
 	/* if we get here, it's because we got SRV_STATUS_OK, which also
@@ -1906,7 +1906,7 @@ void back_try_conn_req(struct stream *s)
 			/* release other streams waiting for this server */
 			sess_change_server(s, NULL);
 			if (may_dequeue_tasks(srv, s->be))
-				process_srv_queue(srv);
+				process_srv_queue(srv, 0);
 
 			/* Failed and not retryable. */
 			si_shutr(si);
@@ -2234,7 +2234,7 @@ void back_handle_st_cer(struct stream *s)
 		_HA_ATOMIC_INC(&s->be->be_counters.failed_conns);
 		sess_change_server(s, NULL);
 		if (may_dequeue_tasks(objt_server(s->target), s->be))
-			process_srv_queue(objt_server(s->target));
+			process_srv_queue(objt_server(s->target), 0);
 
 		/* shutw is enough so stop a connecting socket */
 		si_shutw(si);

src/cli.c

@@ -2565,7 +2565,7 @@ int pcli_wait_for_response(struct stream *s, struct channel *rep, int an_bit)
 				HA_ATOMIC_DEC(&__objt_server(s->target)->cur_sess);
 			}
 			if (may_dequeue_tasks(__objt_server(s->target), be))
-				process_srv_queue(__objt_server(s->target));
+				process_srv_queue(__objt_server(s->target), 0);
 		}
 
 		s->target = NULL;

src/queue.c

@@ -331,9 +331,10 @@ static struct pendconn *pendconn_process_next_strm(struct server *srv, struct pr
 }
 
 /* Manages a server's connection queue. This function will try to dequeue as
- * many pending streams as possible, and wake them up.
+ * many pending streams as possible, and wake them up. <server_locked> must
+ * only be set if the caller already hold the server lock.
  */
-void process_srv_queue(struct server *s)
+void process_srv_queue(struct server *s, int server_locked)
 {
 	struct proxy  *p = s->proxy;
 	int done = 0;

src/server.c

@@ -1818,8 +1818,11 @@ const char *server_parse_maxconn_change_request(struct server *sv,
 		sv->maxconn = v;
 	}
 
+	/* server_parse_maxconn_change_request requires the server lock held.
+	 * Specify it to process_srv_queue to prevent a deadlock.
+	 */
 	if (may_dequeue_tasks(sv, sv->proxy))
-		process_srv_queue(sv);
+		process_srv_queue(sv, 1);
 
 	return NULL;
 }

src/stream.c

@@ -623,7 +623,7 @@ static void stream_free(struct stream *s)
 			_HA_ATOMIC_DEC(&__objt_server(s->target)->cur_sess);
 		}
 		if (may_dequeue_tasks(objt_server(s->target), s->be))
-			process_srv_queue(objt_server(s->target));
+			process_srv_queue(objt_server(s->target), 0);
 	}
 
 	if (unlikely(s->srv_conn)) {
@@ -1815,7 +1815,7 @@ struct task *process_stream(struct task *t, void *context, unsigned int state)
 			}
 			sess_change_server(s, NULL);
 			if (may_dequeue_tasks(srv, s->be))
-				process_srv_queue(srv);
+				process_srv_queue(srv, 0);
 		}
 	}