Fix issue with discarded result sets; add related test

lib/PgSqlTupleResult.php

@@ -16,6 +16,10 @@ class PgSqlTupleResult extends TupleResult {
         parent::__construct(new Producer(static function (callable $emit) use ($handle) {
             $count = \pg_num_rows($handle);
             for ($i = 0; $i < $count; ++$i) {
+                if (!\is_resource($handle)) {
+                    return; // Result object discarded, simply return.
+                }
+
                 $result = \pg_fetch_assoc($handle);
                 if ($result === false) {
                     throw new FailureException(\pg_result_error($handle));

lib/PqHandle.php

@@ -228,6 +228,10 @@ class PqHandle implements Handle {
             }
         }
 
+        if (!$result) {
+            return null; // Connection closing, end result set.
+        }
+
         switch ($result->status) {
             case pq\Result::TUPLES_OK: // End of result set.
                 return null;

test/AbstractLinkTest.php

@@ -61,6 +61,28 @@ abstract class AbstractLinkTest extends TestCase {
         });
     }
 
+    public function testQueryWithUnconsumedTupleResult() {
+        Loop::run(function () {
+            /** @var \Amp\Postgres\TupleResult $result */
+            $result = yield $this->connection->query("SELECT * FROM test");
+
+            $this->assertInstanceOf(TupleResult::class, $result);
+
+            /** @var \Amp\Postgres\TupleResult $result */
+            $result = yield $this->connection->query("SELECT * FROM test");
+
+            $this->assertInstanceOf(TupleResult::class, $result);
+
+            $data = $this->getData();
+
+            for ($i = 0; yield $result->advance(); ++$i) {
+                $row = $result->getCurrent();
+                $this->assertSame($data[$i][0], $row['domain']);
+                $this->assertSame($data[$i][1], $row['tld']);
+            }
+        });
+    }
+
     public function testQueryWithCommandResult() {
         Loop::run(function () {
             /** @var \Amp\Postgres\CommandResult $result */