Regenerate native PHP mu-plugins when an existing file has stale content

packages/common/lib/mu-plugins.ts

@@ -112,18 +112,25 @@ async function getExistingNativePhpMuPluginsDir(
 		return null;
 	}
 
-	const expectedFiles = getStandardMuPlugins( options )
-		.map( ( plugin ) => plugin.filename )
-		.sort();
-	const actualFiles = existingFiles.filter( ( file ) => file.endsWith( '.php' ) ).sort();
-
-	if (
-		expectedFiles.length !== actualFiles.length ||
-		expectedFiles.some( ( filename, index ) => filename !== actualFiles[ index ] )
-	) {
+	const expectedPlugins = getStandardMuPlugins( options );
+	const actualFiles = existingFiles.filter( ( file ) => file.endsWith( '.php' ) );
+
+	if ( expectedPlugins.length !== actualFiles.length ) {
 		return null;
 	}
 
+	for ( const plugin of expectedPlugins ) {
+		let content: string;
+		try {
+			content = await readFile( path.join( muPluginsDir, plugin.filename ), 'utf8' );
+		} catch {
+			return null;
+		}
+		if ( content !== plugin.content ) {
+			return null;
+		}
+	}
+
 	return muPluginsDir;
 }
 

packages/common/lib/tests/mu-plugins.test.ts

@@ -116,6 +116,27 @@ describe( 'writeStudioMuPluginsForNativePhpRuntime', () => {
 		expect( generatedPlugins ).not.toContain( '0-disable-auto-updates.php' );
 	} );
 
+	it( 'should reuse the existing mu-plugins directory when contents are up to date', async () => {
+		const firstDir = await writeStudioMuPluginsForNativePhpRuntime( sitePath, false );
+		const secondDir = await writeStudioMuPluginsForNativePhpRuntime( sitePath, false );
+
+		expect( secondDir ).toBe( firstDir );
+	} );
+
+	it( 'should regenerate mu-plugins when an existing file has stale content', async () => {
+		const firstDir = await writeStudioMuPluginsForNativePhpRuntime( sitePath, false );
+
+		const pluginFilename = '0-deactivate-jetpack-modules.php';
+		const expectedContent = await readFile( join( firstDir, pluginFilename ), 'utf8' );
+		writeFileSync( join( firstDir, pluginFilename ), '<?php // stale content from older Studio' );
+
+		const secondDir = await writeStudioMuPluginsForNativePhpRuntime( sitePath, false );
+		const regeneratedContent = await readFile( join( secondDir, pluginFilename ), 'utf8' );
+
+		expect( secondDir ).not.toBe( firstDir );
+		expect( regeneratedContent ).toBe( expectedContent );
+	} );
+
 	it( 'should disable Jetpack modules that affect local development', async () => {
 		await writeStudioMuPluginsForNativePhpRuntime( sitePath, false );