Resolving a racing condition while adding ports on the app's side.

An earlier attempt (ad6265786871) to resolve this condition on the
router's side added a new issue: the app could get a request before
acquiring a port.

go/port.go

@@ -50,7 +50,11 @@ func add_port(p *port) {
 		port_registry_.m = make(map[port_key]*port)
 	}
 
+	old := port_registry_.m[p.key]
+
+	if old == nil {
 		port_registry_.m[p.key] = p
+	}
 
 	port_registry_.Unlock()
 }

src/nxt_process.c

@@ -590,6 +590,17 @@ nxt_process_close_ports(nxt_task_t *task, nxt_process_t *process)
 }
 
 
+void
+nxt_process_connected_port_add(nxt_process_t *process, nxt_port_t *port)
+{
+    nxt_thread_mutex_lock(&process->cp_mutex);
+
+    nxt_port_hash_add(&process->connected_ports, port);
+
+    nxt_thread_mutex_unlock(&process->cp_mutex);
+}
+
+
 void
 nxt_process_connected_port_remove(nxt_process_t *process, nxt_port_t *port)
 {
@@ -602,7 +613,7 @@ nxt_process_connected_port_remove(nxt_process_t *process, nxt_port_t *port)
 
 
 nxt_port_t *
-nxt_process_connected_port_find_add(nxt_process_t *process, nxt_port_t *port)
+nxt_process_connected_port_find(nxt_process_t *process, nxt_port_t *port)
 {
     nxt_port_t  *res;
 
@@ -610,10 +621,6 @@ nxt_process_connected_port_find_add(nxt_process_t *process, nxt_port_t *port)
 
     res = nxt_port_hash_find(&process->connected_ports, port->pid, port->id);
 
-    if (nxt_slow_path(res == NULL)) {
-        nxt_port_hash_add(&process->connected_ports, port);
-    }
-
     nxt_thread_mutex_unlock(&process->cp_mutex);
 
     return res;

src/nxt_process.h

@@ -105,10 +105,12 @@ void nxt_process_use(nxt_task_t *task, nxt_process_t *process, int i);
 
 void nxt_process_close_ports(nxt_task_t *task, nxt_process_t *process);
 
+void nxt_process_connected_port_add(nxt_process_t *process, nxt_port_t *port);
+
 void nxt_process_connected_port_remove(nxt_process_t *process,
     nxt_port_t *port);
 
-nxt_port_t *nxt_process_connected_port_find_add(nxt_process_t *process,
+nxt_port_t *nxt_process_connected_port_find(nxt_process_t *process,
     nxt_port_t *port);
 
 void nxt_worker_process_quit_handler(nxt_task_t *task,

src/nxt_router.c

@@ -4811,7 +4811,7 @@ nxt_router_app_prepare_request(nxt_task_t *task,
 
     apr_action = NXT_APR_REQUEST_FAILED;
 
-    c_port = nxt_process_connected_port_find_add(port->process, reply_port);
+    c_port = nxt_process_connected_port_find(port->process, reply_port);
 
     if (nxt_slow_path(c_port != reply_port)) {
         res = nxt_port_send_port(task, port, reply_port, 0);
@@ -4820,10 +4820,10 @@ nxt_router_app_prepare_request(nxt_task_t *task,
             nxt_request_app_link_error(task, port->app, req_app_link,
                                 "Failed to send reply port to application");
 
-            nxt_process_connected_port_remove(port->process, reply_port);
-
             goto release_port;
         }
+
+        nxt_process_connected_port_add(port->process, reply_port);
     }
 
     buf = nxt_router_prepare_msg(task, req_app_link->request, port,

src/nxt_unit.c

@@ -4282,16 +4282,38 @@ nxt_unit_add_port(nxt_unit_ctx_t *ctx, nxt_unit_port_t *port)
     int                   rc;
     nxt_unit_impl_t       *lib;
     nxt_unit_process_t    *process;
-    nxt_unit_port_impl_t  *new_port;
+    nxt_unit_port_impl_t  *new_port, *old_port;
 
     lib = nxt_container_of(ctx->unit, nxt_unit_impl_t, unit);
 
+    pthread_mutex_lock(&lib->mutex);
+
+    old_port = nxt_unit_port_hash_find(&lib->ports, &port->id, 0);
+
+    if (nxt_slow_path(old_port != NULL)) {
+        nxt_unit_debug(ctx, "add_port: duplicate %d,%d in_fd %d out_fd %d",
+                       port->id.pid, port->id.id,
+                       port->in_fd, port->out_fd);
+
+        if (port->in_fd != -1) {
+            close(port->in_fd);
+            port->in_fd = -1;
+        }
+
+        if (port->out_fd != -1) {
+            close(port->out_fd);
+            port->out_fd = -1;
+        }
+
+        pthread_mutex_unlock(&lib->mutex);
+
+        return NXT_UNIT_OK;
+    }
+
     nxt_unit_debug(ctx, "add_port: %d,%d in_fd %d out_fd %d",
                    port->id.pid, port->id.id,
                    port->in_fd, port->out_fd);
 
-    pthread_mutex_lock(&lib->mutex);
-
     process = nxt_unit_process_get(ctx, port->id.pid);
     if (nxt_slow_path(process == NULL)) {
         rc = NXT_UNIT_ERROR;