Decrease lock id collisions

Author: antonkomarev

src/LockId/PostgresLockId.php

@@ -17,8 +17,9 @@
 
 final class PostgresLockId
 {
-    private const DB_INT64_VALUE_MIN = 0;
-    private const DB_INT64_VALUE_MAX = 9223372036854775807;
+    private const DB_INT64_VALUE_MIN = -9_223_372_036_854_775_808;
+    private const DB_INT64_VALUE_MAX = 9_223_372_036_854_775_807;
+    private const DB_INT32_VALUE_MAX = 2_147_483_647;
 
     public function __construct(
         public readonly int $id,
@@ -43,9 +44,22 @@ public static function fromLockId(
         );
     }
 
+    /**
+     * Generates a deterministic signed 32-bit integer ID
+     * from a string for use as a Postgres advisory lock key.
+     *
+     * The crc32 function returns an unsigned 32-bit integer (0 to 4_294_967_295).
+     * Postgres advisory locks require a signed 32-bit integer (-2_147_483_648 to 2_147_483_647).
+     *
+     * This method shifts the crc32 output into the signed int32 range by subtracting (2^31) + 1,
+     * ensuring the result fits within Postgres's required range and preserves uniqueness.
+     *
+     * @param string $string The input string to hash into an int32 lock ID.
+     * @return int The signed 32-bit integer suitable for Postgres advisory locks.
+     */
     private static function generateIdFromString(
         string $string,
     ): int {
-        return crc32($string);
+        return crc32($string) - self::DB_INT32_VALUE_MAX - 1;
     }
 }

test/Integration/AbstractIntegrationTestCase.php

@@ -91,26 +91,36 @@ private function findPostgresAdvisoryLockInConnection(
         PDO $dbConnection,
         PostgresLockId $postgresLockId,
     ): object | null {
-        $id = $postgresLockId->id;
-
-        $lockObjectId = $id % self::POSTGRES_BLOCK_SIZE;
-        $lockCatalogId = ($id - $lockObjectId) / self::POSTGRES_BLOCK_SIZE;
+        // For one-argument advisory locks, Postgres stores the signed 64-bit key as two 32-bit integers:
+        // classid = high 32 bits, objid = low 32 bits.
+        $lockClassId = ($postgresLockId->id >> 32) & 0xFFFFFFFF;
+        $lockObjectId = $postgresLockId->id & 0xFFFFFFFF;
+
+        // Convert to signed 32-bit if necessary (Postgres stores as signed)
+        if ($lockClassId > 0x7FFFFFFF) {
+            $lockClassId -= 0x100000000;
+        }
+        if ($lockObjectId > 0x7FFFFFFF) {
+            $lockObjectId -= 0x100000000;
+        }
 
         $statement = $dbConnection->prepare(
             <<<'SQL'
                 SELECT *
                 FROM pg_locks
                 WHERE locktype = 'advisory'
-                AND classid = :lock_catalog_id
+                AND classid = :lock_class_id
                 AND objid = :lock_object_id
+                AND objsubid = :lock_object_subid
                 AND pid = :connection_pid
                 AND mode = :mode
                 SQL,
         );
         $statement->execute(
             [
-                'lock_catalog_id' => $lockCatalogId,
+                'lock_class_id' => $lockClassId,
                 'lock_object_id' => $lockObjectId,
+                'lock_object_subid' => 1, // For one keyed value
                 'connection_pid' => $dbConnection->pgsqlGetPid(),
                 'mode' => self::MODE_EXCLUSIVE,
             ],
@@ -150,7 +160,7 @@ private function closeAllPostgresPdoConnections(): void
     {
         $this->initPostgresPdoConnection()->query(
             <<<'SQL'
-            SELECT PG_TERMINATE_BACKEND(pid) 
+            SELECT PG_TERMINATE_BACKEND(pid)
             FROM pg_stat_activity
             WHERE pid <> PG_BACKEND_PID()
             SQL,

test/Unit/LockId/PostgresLockIdTest.php

@@ -42,7 +42,7 @@ public function test_it_can_create_postgres_lock_id_from_lock_id(): void
 
         $postgresLockId = PostgresLockId::fromLockId($lockId);
 
-        $this->assertSame(3632233996, $postgresLockId->id);
+        $this->assertSame(1484750348, $postgresLockId->id);
     }
 
     public function test_it_can_create_postgres_lock_id_from_lock_id_with_value(): void
@@ -51,6 +51,6 @@ public function test_it_can_create_postgres_lock_id_from_lock_id_with_value(): v
 
         $postgresLockId = PostgresLockId::fromLockId($lockId);
 
-        $this->assertSame(782632948, $postgresLockId->id);
+        $this->assertSame(-1364850700, $postgresLockId->id);
     }
 }