Tolerate deallocation of statements the server has already dropped

src/Internal/PgSqlHandle.php

@@ -413,7 +413,17 @@ public function statementDeallocate(string $name): void
                 return; // Statement already deallocated.
             }
 
-            $this->query(\sprintf("DEALLOCATE %s", $name));
+            try {
+                // Send the DEALLOCATE without building a result object: an error result
+                // must not be processed through createResult(), whose fatal-error drain
+                // would consume results belonging to an operation dispatched after this
+                // one and leave that operation awaiting a response forever.
+                $this->send(\pg_send_query(...), \sprintf("DEALLOCATE %s", $name));
+            } catch (\Throwable) {
+                // Ignore failures: the server may have already dropped the statement,
+                // e.g. via the DISCARD ALL issued when the pool resets connections.
+            }
+
             unset($this->statements[$name]);
         });
         $storage->future->ignore();

src/Internal/PqHandle.php

@@ -338,9 +338,16 @@ public function statementDeallocate(string $name): void
                 return; // Statement already deallocated.
             }
 
-            $this->send(null, $statement->deallocateAsync(...));
+            try {
+                $this->send(null, $statement->deallocateAsync(...));
+            } catch (\Throwable) {
+                // Ignore failures: the server may have already dropped the statement,
+                // e.g. via the DISCARD ALL issued when the pool resets connections.
+            }
+
             unset($this->statements[$name]);
         });
+        $storage->future->ignore();
     }
 
     #[\Override]

test/StatementDeallocateTest.php

@@ -0,0 +1,51 @@
+<?php declare(strict_types=1);
+
+namespace Amp\Postgres\Test;
+
+use Amp\PHPUnit\AsyncTestCase;
+use Amp\Postgres\PostgresConfig;
+use Amp\Postgres\PostgresConnectionPool;
+use Revolt\EventLoop;
+
+/**
+ * @requires extension pgsql
+ */
+class StatementDeallocateTest extends AsyncTestCase
+{
+    public function testDeallocateRacingConnectionResetDoesNotBreakSubsequentOperations(): void
+    {
+        if (EventLoop::getDriver()->getHandle() instanceof \EvLoop) {
+            $this->markTestSkipped("ext-pgsql is not compatible with pecl-ev");
+        }
+
+        $this->setTimeout(5);
+
+        $pool = new PostgresConnectionPool(
+            PostgresConfig::fromString('host=localhost user=postgres password=postgres'),
+            1,
+        );
+
+        try {
+            $statement = $pool->prepare('SELECT 1 AS value');
+            $result = $statement->execute();
+            \iterator_to_array($result);
+
+            // Dropping the statement and result queues a DEALLOCATE for the prepared
+            // statement, while the pool's DISCARD ALL reset on the next checkout drops
+            // it server-side. The failing DEALLOCATE must not consume responses that
+            // belong to operations dispatched after it.
+            unset($result, $statement);
+
+            for ($i = 1; $i <= 3; ++$i) {
+                $rows = \iterator_to_array($pool->query(\sprintf('SELECT %d AS value', $i)));
+                self::assertSame($i, $rows[0]['value']);
+            }
+
+            $statement = $pool->prepare('SELECT 2 AS value');
+            $rows = \iterator_to_array($statement->execute());
+            self::assertSame(2, $rows[0]['value']);
+        } finally {
+            $pool->close();
+        }
+    }
+}