Add Pdo\Sqlite::ATTR_TRANSACTION_MODE

This commit implements GH-8967.

SQLite supports multiple transaction modes. These include:
- DEFERRED (default) only acquires a lock when you start a read/write
- IMMEDIATE acquires a reserved lock
- EXCLUSIVE acquires an exclusive lock (stricter than immediate)

In WAL mode IMMEDIATE and EXCLUSIVE are identical.

One reason for wanting to specify a transaction mode is that SQLite
doesn't respect busy_timeout when a DEFERRED transaction tries to
upgrade a read lock to a write lock. Normally if you try to acquire a
lock and have busy_timeout configured, SQLite will wait for that period
until giving up and erroring out (SQLITE_BUSY). With DEFERRED, if you
have a transaction that first reads and there's a concurrent writer
while it's trying to upgrade to a write lock, you will immediately get
SQLITE_BUSY regardless of your busy_timeout.

Prior to this commit, the only available workarounds were:
- Using $pdo->exec("BEGIN IMMEDIATE TRANSACTION") instead of
  $pdo->beginTransaction()
- Doing a dummy write at the start of each transaction so you don't get
  stuck with a read lock

Both of those aren't very usable, especially in a framework context
where the user doesn't have complete control over how transactions are
started.

To address that, this commit adds four class constants to Pdo\Sqlite:
- ATTR_TRANSACTION_MODE -- a new attribute
- TRANSACTION_MODE_DEFERRED = 0
- TRANSACTION_MODE_IMMEDIATE = 1
- TRANSACTION_MODE_EXCLUSIVE = 2

These can be used as:
  $pdo->setAttribute(
    $pdo::ATTR_TRANSACTION_MODE,
    $pdo::TRANSACTION_MODE_IMMEDIATE
  );

Author: stancl

ext/pdo_sqlite/pdo_sqlite.stub.php

@@ -39,6 +39,13 @@ class Sqlite extends \PDO
     /** @cvalue PDO_SQLITE_ATTR_EXPLAIN_STATEMENT */
     public const int ATTR_EXPLAIN_STATEMENT = UNKNOWN;
 
+    /** @cvalue PDO_SQLITE_ATTR_TRANSACTION_MODE */
+    public const int ATTR_TRANSACTION_MODE = UNKNOWN;
+
+    public const int TRANSACTION_MODE_DEFERRED = 0;
+    public const int TRANSACTION_MODE_IMMEDIATE = 1;
+    public const int TRANSACTION_MODE_EXCLUSIVE = 2;
+
 #if SQLITE_VERSION_NUMBER >= 3043000
     public const int EXPLAIN_MODE_PREPARED = 0;
     public const int EXPLAIN_MODE_EXPLAIN = 1;

ext/pdo_sqlite/pdo_sqlite_arginfo.h

@@ -27,6 +27,12 @@ extern zend_module_entry pdo_sqlite_module_entry;
 #include "TSRM.h"
 #endif
 
+enum pdo_sqlite_transaction_mode {
+	PDO_SQLITE_TRANSACTION_MODE_DEFERRED = 0,
+	PDO_SQLITE_TRANSACTION_MODE_IMMEDIATE = 1,
+	PDO_SQLITE_TRANSACTION_MODE_EXCLUSIVE = 2
+};
+
 PHP_MINIT_FUNCTION(pdo_sqlite);
 PHP_MSHUTDOWN_FUNCTION(pdo_sqlite);
 PHP_RINIT_FUNCTION(pdo_sqlite);

ext/pdo_sqlite/php_pdo_sqlite.h

@@ -51,6 +51,7 @@ typedef struct {
 	struct pdo_sqlite_func *funcs;
 	struct pdo_sqlite_collation *collations;
 	zend_fcall_info_cache authorizer_fcc;
+	enum pdo_sqlite_transaction_mode transaction_mode;
 } pdo_sqlite_db_handle;
 
 typedef struct {
@@ -75,7 +76,8 @@ enum {
 	PDO_SQLITE_ATTR_READONLY_STATEMENT,
 	PDO_SQLITE_ATTR_EXTENDED_RESULT_CODES,
 	PDO_SQLITE_ATTR_BUSY_STATEMENT,
-	PDO_SQLITE_ATTR_EXPLAIN_STATEMENT
+	PDO_SQLITE_ATTR_EXPLAIN_STATEMENT,
+	PDO_SQLITE_ATTR_TRANSACTION_MODE
 };
 
 typedef int pdo_sqlite_create_collation_callback(void*, int, const void*, int, const void*);

ext/pdo_sqlite/php_pdo_sqlite_int.h

@@ -255,7 +255,20 @@ static bool sqlite_handle_begin(pdo_dbh_t *dbh)
 {
 	pdo_sqlite_db_handle *H = (pdo_sqlite_db_handle *)dbh->driver_data;
 
-	if (sqlite3_exec(H->db, "BEGIN", NULL, NULL, NULL) != SQLITE_OK) {
+	const char *begin_statement = "BEGIN";
+	switch (H->transaction_mode) {
+		case PDO_SQLITE_TRANSACTION_MODE_DEFERRED:
+			begin_statement = "BEGIN DEFERRED TRANSACTION";
+			break;
+		case PDO_SQLITE_TRANSACTION_MODE_IMMEDIATE:
+			begin_statement = "BEGIN IMMEDIATE TRANSACTION";
+			break;
+		case PDO_SQLITE_TRANSACTION_MODE_EXCLUSIVE:
+			begin_statement = "BEGIN EXCLUSIVE TRANSACTION";
+			break;
+	}
+
+	if (sqlite3_exec(H->db, begin_statement, NULL, NULL, NULL) != SQLITE_OK) {
 		pdo_sqlite_error(dbh);
 		return false;
 	}
@@ -286,11 +299,16 @@ static bool sqlite_handle_rollback(pdo_dbh_t *dbh)
 
 static int pdo_sqlite_get_attribute(pdo_dbh_t *dbh, zend_long attr, zval *return_value)
 {
+	pdo_sqlite_db_handle *H = (pdo_sqlite_db_handle *)dbh->driver_data;
+
 	switch (attr) {
 		case PDO_ATTR_CLIENT_VERSION:
 		case PDO_ATTR_SERVER_VERSION:
 			ZVAL_STRING(return_value, (char *)sqlite3_libversion());
 			break;
+		case PDO_SQLITE_ATTR_TRANSACTION_MODE:
+			ZVAL_LONG(return_value, H->transaction_mode);
+			break;
 
 		default:
 			return 0;
@@ -326,6 +344,19 @@ static bool pdo_sqlite_set_attr(pdo_dbh_t *dbh, zend_long attr, zval *val)
 			}
 			sqlite3_extended_result_codes(H->db, lval);
 			return true;
+		case PDO_SQLITE_ATTR_TRANSACTION_MODE:
+			if (!pdo_get_long_param(&lval, val)) {
+				return false;
+			}
+			switch (lval) {
+				case PDO_SQLITE_TRANSACTION_MODE_DEFERRED:
+				case PDO_SQLITE_TRANSACTION_MODE_IMMEDIATE:
+				case PDO_SQLITE_TRANSACTION_MODE_EXCLUSIVE:
+					H->transaction_mode = lval;
+					return true;
+				default:
+					return false;
+			}
 	}
 	return false;
 }

ext/pdo_sqlite/sqlite_driver.c

@@ -0,0 +1,88 @@
+--TEST--
+PDO_sqlite: Testing ATTR_TRANSACTION_MODE
+--EXTENSIONS--
+pdo_sqlite
+--FILE--
+<?php
+
+$dsn = 'sqlite:file:foo?mode=memory&cache=shared';
+$pdo = PDO::connect($dsn);
+$pdo2 = PDO::connect($dsn);
+
+// Deferred by default before any transaction mode is set
+var_dump($pdo->getAttribute(PDO\Sqlite::ATTR_TRANSACTION_MODE) === PDO\Sqlite::TRANSACTION_MODE_DEFERRED);
+
+// Both should return true - setting DEFERRED
+var_dump($pdo->setAttribute(PDO\Sqlite::ATTR_TRANSACTION_MODE, PDO\Sqlite::TRANSACTION_MODE_DEFERRED));
+var_dump($pdo->getAttribute(PDO\Sqlite::ATTR_TRANSACTION_MODE) === PDO\Sqlite::TRANSACTION_MODE_DEFERRED);
+
+// Both should return true - setting IMMEDIATE
+var_dump($pdo->setAttribute(PDO\Sqlite::ATTR_TRANSACTION_MODE, PDO\Sqlite::TRANSACTION_MODE_IMMEDIATE));
+var_dump($pdo->getAttribute(PDO\Sqlite::ATTR_TRANSACTION_MODE) === PDO\Sqlite::TRANSACTION_MODE_IMMEDIATE);
+
+// Both should return true - setting EXCLUSIVE
+var_dump($pdo->setAttribute(PDO\Sqlite::ATTR_TRANSACTION_MODE, PDO\Sqlite::TRANSACTION_MODE_EXCLUSIVE));
+var_dump($pdo->getAttribute(PDO\Sqlite::ATTR_TRANSACTION_MODE) === PDO\Sqlite::TRANSACTION_MODE_EXCLUSIVE);
+
+// Setting the numeric equivalents of the above. All should return true
+var_dump($pdo->setAttribute(PDO\Sqlite::ATTR_TRANSACTION_MODE, 0));
+var_dump($pdo->getAttribute(PDO\Sqlite::ATTR_TRANSACTION_MODE) === PDO\Sqlite::TRANSACTION_MODE_DEFERRED);
+var_dump($pdo->setAttribute(PDO\Sqlite::ATTR_TRANSACTION_MODE, 1));
+var_dump($pdo->getAttribute(PDO\Sqlite::ATTR_TRANSACTION_MODE) === PDO\Sqlite::TRANSACTION_MODE_IMMEDIATE);
+var_dump($pdo->setAttribute(PDO\Sqlite::ATTR_TRANSACTION_MODE, 2));
+var_dump($pdo->getAttribute(PDO\Sqlite::ATTR_TRANSACTION_MODE) === PDO\Sqlite::TRANSACTION_MODE_EXCLUSIVE);
+
+// Cannot set a random numeric value
+var_dump($pdo->setAttribute(PDO\Sqlite::ATTR_TRANSACTION_MODE, 4));
+
+// Set $pdo to deferred, try to get immediate transaction in $pdo2. There should be no lock contention
+$pdo->setAttribute(PDO\Sqlite::ATTR_TRANSACTION_MODE, PDO\Sqlite::TRANSACTION_MODE_DEFERRED);
+$pdo->beginTransaction();
+try {
+    $pdo2->exec('begin immediate transaction');
+    $pdo2->rollBack();
+    printf("Database is not locked\n");
+} catch (PDOException $e) {
+    printf("Database is locked: %s\n", $e->getMessage());
+}
+$pdo->rollBack();
+
+// Set $pdo to immediate, try to get immediate transaction in $pdo2. There SHOULD be lock contention
+$pdo->setAttribute(PDO\Sqlite::ATTR_TRANSACTION_MODE, PDO\Sqlite::TRANSACTION_MODE_IMMEDIATE);
+$pdo->beginTransaction();
+try {
+    $pdo2->exec('begin immediate transaction');
+    printf("Database is not locked\n");
+} catch (PDOException $e) {
+    printf("Database is locked: %s\n", $e->getMessage());
+}
+$pdo->rollBack();
+
+// Set $pdo to exclusive, try to get immediate transaction in $pdo2. There SHOULD be lock contention
+$pdo->setAttribute(PDO\Sqlite::ATTR_TRANSACTION_MODE, PDO\Sqlite::TRANSACTION_MODE_EXCLUSIVE);
+$pdo->beginTransaction();
+try {
+    $pdo2->exec('begin immediate transaction');
+    printf("Database is not locked\n");
+} catch (PDOException $e) {
+    printf("Database is locked: %s\n", $e->getMessage());
+}
+?>
+--EXPECT--
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(true)
+bool(false)
+Database is not locked
+Database is locked: SQLSTATE[HY000]: General error: 6 database table is locked
+Database is locked: SQLSTATE[HY000]: General error: 6 database schema is locked: main