Themes: Improve the performance of _get_block_templates_paths.

This avoids redundant recursive lookups for block template paths in the same base directory by implementing a static cache. It also replaces an potentially expensive `file_exists` call in favor of doing recursive iteration of files inside a try/catch block. 

Props thekt12, spacedmonkey, flixos90, mukesh27, joemcgill.
Fixes #58196.


git-svn-id: https://develop.svn.wordpress.org/trunk@57215 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/block-template.php

@@ -387,6 +387,49 @@ class Tests_Block_Template extends WP_UnitTestCase {
 		);
 	}
 
+	/**
+	 * Tests `_get_block_templates_paths()` for an invalid directory.
+	 *
+	 * @ticket 58196
+	 *
+	 * @covers ::_get_block_templates_paths
+	 */
+	public function test_get_block_templates_paths_dir_exists() {
+		$theme_dir = get_template_directory();
+		// Templates in the current theme.
+		$templates = array(
+			'parts/small-header.html',
+			'templates/custom-single-post-template.html',
+			'templates/index.html',
+			'templates/page-home.html',
+			'templates/page.html',
+			'templates/single.html',
+		);
+
+		$expected_template_paths = array_map(
+			static function ( $template ) use ( $theme_dir ) {
+				return $theme_dir . '/' . $template;
+			},
+			$templates
+		);
+
+		$template_paths = _get_block_templates_paths( $theme_dir );
+		$this->assertSameSets( $expected_template_paths, $template_paths );
+	}
+
+	/**
+	 * Test _get_block_templates_paths() for a invalid dir.
+	 *
+	 * @ticket 58196
+	 *
+	 * @covers ::_get_block_templates_paths
+	 */
+	public function test_get_block_templates_paths_dir_doesnt_exists() {
+		// Should return empty array for invalid path.
+		$template_paths = _get_block_templates_paths( '/tmp/random-invalid-theme-path' );
+		$this->assertSame( array(), $template_paths );
+	}
+
 	/**
 	 * Registers a test block to log `in_the_loop()` results.
 	 *