CLEANUP: task: make sure tasklet handlers always indicate their statuses

When tasklets were derived from tasks, there was no immediate need for
the scheduler to know their status after execution, and in a spirit of
simplicity they just started to always return NULL. The problem is that
it simply prevents the scheduler from 1) accounting their execution time,
and 2) keeping track of their current execution status. Indeed, a remote
wake-up could very well end up manipulating a tasklet that's currently
being executed. And this is the reason why those handlers have to take
the idle lock before checking their context.

In 2.5 we'll take care of making tasklets and tasks work more similarly,
but trouble is to be expected if we continue to propagate the trend of
returning NULL everywhere, especially if some fixes relying on a stricter
model later need to be backported. For this reason this patch updates all
known tasklet handlers to make them return NULL only when the tasklet was
freed. It has no effect for now and isn't even guaranteed to always be
100% safe but it puts the code into the right direction for this.

include/haproxy/task-t.h

@@ -114,6 +114,11 @@ struct task_per_thread {
 
 /* This part is common between struct task and struct tasklet so that tasks
  * can be used as-is as tasklets.
+ *
+ * Note that the process() function must ALWAYS return the task/tasklet's
+ * pointer if the task/tasklet remains valid, and return NULL if it has been
+ * deleted. The scheduler relies on this to know if it should update its state
+ * on return.
  */
 #define TASK_COMMON							\
 	struct {							\

src/mux_fcgi.c

@@ -3024,6 +3024,9 @@ struct task *fcgi_io_cb(struct task *t, void *ctx, unsigned int state)
 	 * the connection (testing !ret is enough, if fcgi_process() wasn't
 	 * called then ret will be 0 anyway.
 	 */
+	if (ret < 0)
+		t = NULL;
+
 	if (!ret && conn_in_list) {
 		struct server *srv = objt_server(conn->target);
 
@@ -3034,7 +3037,7 @@ struct task *fcgi_io_cb(struct task *t, void *ctx, unsigned int state)
 			ebmb_insert(&srv->per_thr[tid].idle_conns, &conn->hash_node->node, sizeof(conn->hash_node->hash));
 		HA_SPIN_UNLOCK(IDLE_CONNS_LOCK, &idle_conns[tid].idle_conns_lock);
 	}
-	return NULL;
+	return t;
 }
 
 /* callback called on any event by the connection handler.

src/mux_h1.c

@@ -2845,11 +2845,15 @@ struct task *h1_io_cb(struct task *t, void *ctx, unsigned int state)
 		ret |= h1_recv(h1c);
 	if (ret || b_data(&h1c->ibuf))
 		ret = h1_process(h1c);
+
 	/* If we were in an idle list, we want to add it back into it,
 	 * unless h1_process() returned -1, which mean it has destroyed
 	 * the connection (testing !ret is enough, if h1_process() wasn't
 	 * called then ret will be 0 anyway.
 	 */
+	if (ret < 0)
+		t = NULL;
+
 	if (!ret && conn_in_list) {
 		struct server *srv = objt_server(conn->target);
 
@@ -2860,7 +2864,7 @@ struct task *h1_io_cb(struct task *t, void *ctx, unsigned int state)
 			ebmb_insert(&srv->per_thr[tid].idle_conns, &conn->hash_node->node, sizeof(conn->hash_node->hash));
 		HA_SPIN_UNLOCK(IDLE_CONNS_LOCK, &idle_conns[tid].idle_conns_lock);
 	}
-	return NULL;
+	return t;
 }
 
 static void h1_reset(struct connection *conn)

src/mux_h2.c

@@ -3793,6 +3793,7 @@ struct task *h2_io_cb(struct task *t, void *ctx, unsigned int state)
 			 */
 			HA_SPIN_UNLOCK(IDLE_CONNS_LOCK, &idle_conns[tid].idle_conns_lock);
 			tasklet_free(tl);
+			t = NULL;
 			goto leave;
 		}
 		conn = h2c->conn;
@@ -3826,6 +3827,9 @@ struct task *h2_io_cb(struct task *t, void *ctx, unsigned int state)
 	 * the connection (testing !ret is enough, if h2_process() wasn't
 	 * called then ret will be 0 anyway.
 	 */
+	if (ret < 0)
+		t = NULL;
+
 	if (!ret && conn_in_list) {
 		struct server *srv = objt_server(conn->target);
 
@@ -3839,7 +3843,7 @@ struct task *h2_io_cb(struct task *t, void *ctx, unsigned int state)
 
 leave:
 	TRACE_LEAVE(H2_EV_H2C_WAKE);
-	return NULL;
+	return t;
 }
 
 /* callback called on any event by the connection handler.
@@ -4473,13 +4477,15 @@ struct task *h2_deferred_shut(struct task *t, void *ctx, unsigned int state)
 
 		if (!h2s->cs) {
 			h2s_destroy(h2s);
-			if (h2c_is_dead(h2c))
+			if (h2c_is_dead(h2c)) {
 				h2_release(h2c);
+				t = NULL;
+			}
 		}
 	}
  end:
 	TRACE_LEAVE(H2_EV_STRM_SHUT);
-	return NULL;
+	return t;
 }
 
 /* shutr() called by the conn_stream (mux_ops.shutr) */

src/mux_pt.c

@@ -75,16 +75,18 @@ struct task *mux_pt_io_cb(struct task *t, void *tctx, unsigned int status)
 			ctx->conn->subs = NULL;
 		} else if (ctx->cs->data_cb->wake)
 			ctx->cs->data_cb->wake(ctx->cs);
-		return NULL;
+		return t;
 	}
 	conn_ctrl_drain(ctx->conn);
-	if (ctx->conn->flags & (CO_FL_ERROR | CO_FL_SOCK_RD_SH | CO_FL_SOCK_WR_SH))
+	if (ctx->conn->flags & (CO_FL_ERROR | CO_FL_SOCK_RD_SH | CO_FL_SOCK_WR_SH)) {
 		mux_pt_destroy(ctx);
+		t = NULL;
+	}
 	else
 		ctx->conn->xprt->subscribe(ctx->conn, ctx->conn->xprt_ctx, SUB_RETRY_RECV,
 		    &ctx->wait_event);
 
-	return NULL;
+	return t;
 }
 
 /* Initialize the mux once it's attached. It is expected that conn->ctx

src/ssl_sock.c

@@ -5915,7 +5915,7 @@ leave:
 			ebmb_insert(&srv->per_thr[tid].idle_conns, &conn->hash_node->node, sizeof(conn->hash_node->hash));
 		HA_SPIN_UNLOCK(IDLE_CONNS_LOCK, &idle_conns[tid].idle_conns_lock);
 	}
-	return NULL;
+	return t;
 }
 
 /* Receive up to <count> bytes from connection <conn>'s socket and store them

src/stream_interface.c

@@ -774,7 +774,7 @@ struct task *si_cs_io_cb(struct task *t, void *ctx, unsigned int state)
 	int ret = 0;
 
 	if (!cs)
-		return NULL;
+		return t;
 
 	if (!(si->wait_event.events & SUB_RETRY_SEND) && !channel_is_empty(si_oc(si)))
 		ret = si_cs_send(cs);
@@ -784,7 +784,7 @@ struct task *si_cs_io_cb(struct task *t, void *ctx, unsigned int state)
 		si_cs_process(cs);
 
 	stream_release_buffers(si_strm(si));
-	return (NULL);
+	return t;
 }
 
 /* This function is designed to be called from within the stream handler to

src/xprt_handshake.c

@@ -122,8 +122,9 @@ out:
 		}
 		tasklet_free(ctx->wait_event.tasklet);
 		pool_free(xprt_handshake_ctx_pool, ctx);
+		t = NULL;
 	}
-	return NULL;
+	return t;
 }
 
 static int xprt_handshake_init(struct connection *conn, void **xprt_ctx)

src/xprt_quic.c

@@ -3864,7 +3864,7 @@ struct task *quic_conn_io_cb(struct task *t, void *context, unsigned int state)
 		qc_send_ppkts(ctx);
 	}
 
-	return NULL;
+	return t;
 }
 
 /* Receive up to <count> bytes from connection <conn>'s socket and store them