Error on multiple calls to listen on same channel

lib/PgSqlExecutor.php

@@ -211,7 +211,7 @@ class PgSqlExecutor implements Executor {
      */
     public function notify(string $channel, string $payload = ""): Promise {
         if ($payload === "") {
-            return $this->query(\sprintf("NOTIFY %s"));
+            return $this->query(\sprintf("NOTIFY %s", $channel));
         }
 
         return $this->query(\sprintf("NOTIFY %s, '%s'", $channel, $payload));
@@ -222,10 +222,19 @@ class PgSqlExecutor implements Executor {
      */
     public function listen(string $channel): Promise {
         return call(function () use ($channel) {
-            yield $this->query(\sprintf("LISTEN %s"));
+            if (isset($this->listeners[$channel])) {
+                throw new QueryError(\sprintf("Already listening on channel '%s'", $channel));
+            }
+
+            $this->listeners[$channel] = $emitter =  new Emitter;
+
+            try {
+                yield $this->query(\sprintf("LISTEN %s", $channel));
+            } catch (\Throwable $exception) {
+                unset($this->listeners[$channel]);
+                throw $exception;
+            }
 
-            $emitter = new Emitter;
-            $this->listeners[$channel] = $emitter;
             Loop::enable($this->poll);
             return new Listener($emitter->iterate(), $channel, $this->unlisten);
         });

lib/PqExecutor.php

@@ -230,7 +230,13 @@ class PqExecutor implements Executor {
      */
     public function listen(string $channel): Promise {
         return call(function () use ($channel) {
-            $emitter = new Emitter;
+            if (isset($this->listeners[$channel])) {
+                throw new QueryError(\sprintf("Already listening on channel '%s'", $channel));
+            }
+
+            $this->listeners[$channel] = $emitter = new Emitter;
+
+            try {
                 yield from $this->send(
                     [$this->handle, "listenAsync"],
                     $channel,
@@ -241,8 +247,11 @@ class PqExecutor implements Executor {
                         $notification->payload = $message;
                         $emitter->emit($notification);
                     });
+            } catch (\Throwable $exception) {
+                unset($this->listeners[$channel]);
+                throw $exception;
+            }
 
-            $this->listeners[$channel] = $emitter;
             Loop::enable($this->poll);
             return new Listener($emitter->iterate(), $channel, $this->unlisten);
         });

test/AbstractConnectionTest.php

@@ -4,7 +4,9 @@ namespace Amp\Postgres\Test;
 
 use Amp\CancellationTokenSource;
 use Amp\Loop;
-use Amp\Postgres\{ CommandResult, Connection, Transaction, TransactionError, TupleResult };
+use Amp\Postgres\{
+    CommandResult, Connection, Listener, Transaction, TransactionError, TupleResult
+};
 use PHPUnit\Framework\TestCase;
 
 abstract class AbstractConnectionTest extends TestCase {
@@ -176,6 +178,66 @@ abstract class AbstractConnectionTest extends TestCase {
         });
     }
 
+    public function testListen() {
+        Loop::run(function () {
+            $channel = "test";
+            /** @var \Amp\Postgres\Listener $listener */
+            $listener = yield $this->connection->listen($channel);
+
+            $this->assertInstanceOf(Listener::class, $listener);
+
+            yield $this->connection->query(\sprintf("NOTIFY %s, '%s'", $channel, '0'));
+            yield $this->connection->query(\sprintf("NOTIFY %s, '%s'", $channel, '1'));
+
+            $count = 0;
+            Loop::defer(function () use (&$count, $listener) {
+                $listener->unlisten();
+                $this->assertSame(2, $count);
+            });
+
+            while (yield $listener->advance()) {
+                $this->assertSame($listener->getCurrent()->payload, (string) $count++);
+            }
+        });
+    }
+
+    /**
+     * @depends testListen
+     */
+    public function testNotify() {
+        Loop::run(function () {
+            $channel = "test";
+            /** @var \Amp\Postgres\Listener $listener */
+            $listener = yield $this->connection->listen($channel);
+
+            yield $this->connection->notify($channel, '0');
+            yield $this->connection->notify($channel, '1');
+
+            $count = 0;
+            Loop::defer(function () use (&$count, $listener) {
+                $listener->unlisten();
+                $this->assertSame(2, $count);
+            });
+
+            while (yield $listener->advance()) {
+                $this->assertSame($listener->getCurrent()->payload, (string) $count++);
+            }
+        });
+    }
+
+    /**
+     * @depends testListen
+     * @expectedException \Amp\Postgres\QueryError
+     * @expectedExceptionMessage Already listening on channel
+     */
+    public function testListenOnSameChannel() {
+        Loop::run(function () {
+            $channel = "test";
+            $listener = yield $this->connection->listen($channel);
+            $listener = yield $this->connection->listen($channel);
+        });
+    }
+
     public function testConnect() {
         Loop::run(function () {
             $connect = $this->getConnectCallable();