Fix "Expression tree is too large" for strings with large number of NULL bytes (#274)

In SQLite, it is generally difficult to work with `NULL` bytes in
strings, because SQLite uses C-style string syntax, which means it
considers the first `NULL` byte as the string terminator in some
scenarios.

While it is possible to store and read `NULL` bytes within strings (with
some limitations), there is no way to express the `NULL` byte in a
string literal. It needs to be done in a different way:

1. Concatenation: `str_part1 || CHAR(0) || str_part2 || …`
2. Passing as a parameter to a prepared statement.
3. Using HEX notation and casting to string.

More details: https://sqlite.org/nulinstr.html

At the moment, the SQLite driver uses concatenation (option 1), which
seems to fail for strings that contain many `NULL` bytes:

```
SQLSTATE[HY000]: General error: 1 Expression tree is too large (maximum depth 1000)
```

This PR fixes this by using HEX encoding.

Author: JanJakes

tests/WP_SQLite_Driver_Tests.php

@@ -3882,6 +3882,22 @@ public function testNullCharactersInStrings(): void {
 		);
 	}
 
+	public function testLargeNumberOfNullCharactersInStrings(): void {
+		$this->assertQuery( 'CREATE TABLE t (value TEXT)' );
+
+		$long_string_with_null_bytes = str_repeat( "abcdef\0xyz", 1000 );
+
+		$this->assertQuery(
+			sprintf(
+				"INSERT INTO t (value) VALUES ('%s')",
+				$long_string_with_null_bytes
+			)
+		);
+
+		$result = $this->assertQuery( 'SELECT value FROM t' );
+		$this->assertSame( $long_string_with_null_bytes, $result[0]->value );
+	}
+
 	public function testColumnDefaults(): void {
 		$this->assertQuery(
 			"

wp-includes/sqlite-ast/class-wp-sqlite-driver.php

@@ -3339,7 +3339,7 @@ private function translate_string_literal( WP_Parser_Node $node ): string {
 		$value = $token->get_value();
 
 		/*
-		 * 5. Translate datetime literals.
+		 * Translate datetime literals.
 		 *
 		 * Process only strings that could possibly represent a datetime
 		 * literal ("YYYY-MM-DDTHH:MM:SS", "YYYY-MM-DDTHH:MM:SSZ", etc.).
@@ -3349,7 +3349,7 @@ private function translate_string_literal( WP_Parser_Node $node ): string {
 		}
 
 		/*
-		 * 6. Handle null characters.
+		 * Handle null characters.
 		 *
 		 * SQLite doesn't fully support null characters (\u0000) in strings.
 		 * However, it can store them and read them, with some limitations.
@@ -3358,25 +3358,26 @@ private function translate_string_literal( WP_Parser_Node $node ): string {
 		 * Removing them would damage the serialized data.
 		 *
 		 * There is no way to store null bytes using a string literal, so we
-		 * need to split the string and concatenate null bytes with its parts.
+		 * need to pass the value as a HEX string and cast it back to TEXT.
 		 * This will convert literals will null bytes to expressions.
 		 *
 		 * Alternatively, we could replace string literals with parameters and
 		 * pass them using prepared statements. However, that's not universally
 		 * applicable for all string literals (e.g., in default column values).
 		 *
+		 * We can't use the "part1 || CHAR(0) || part2 || ..." syntax, because
+		 * with a large number of null bytes, SQLite throws the following error:
+		 *
+		 *   SQLSTATE[HY000]:
+		 *   General error: 1 Expression tree is too large (maximum depth 1000)
+		 *
 		 * See:
 		 *   https://www.sqlite.org/nulinstr.html
 		 */
-		$parts = array();
-		foreach ( explode( "\0", $value ) as $segment ) {
-			// Escape and quote each segment.
-			$parts[] = "'" . str_replace( "'", "''", $segment ) . "'";
-		}
-		if ( count( $parts ) > 1 ) {
-			return '(' . implode( ' || CHAR(0) || ', $parts ) . ')';
+		if ( strpos( $value, "\0" ) !== false ) {
+			return sprintf( "CAST(x'%s' AS TEXT)", bin2hex( $value ) );
 		}
-		return $parts[0];
+		return $this->connection->quote( $value );
 	}
 
 	/**