minor #17524 Simplify migration LegacyBridge example (jzohrab)

This PR was squashed before being merged into the 6.2 branch.

Discussion
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Simplify migration LegacyBridge example

Rather than return null and then return the prior request, just send the request, and fall back to the LegacyBridge if needed.

I implemented my LegacyBridge following the example given in the docs (thank you for the starting point!) and then realized that the code flow could be simplified.  Cheers and regards! jz

Commits
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99eecb6 Simplify migration LegacyBridge example

Author: OskarStark

migration.rst

@@ -275,16 +275,13 @@ could look something like this::
     $request = Request::createFromGlobals();
     $response = $kernel->handle($request);
 
-    /*
-     * LegacyBridge will take care of figuring out whether to boot up the
-     * existing application or to send the Symfony response back to the client.
-     */
-    $scriptFile = LegacyBridge::prepareLegacyScript($request, $response, __DIR__);
-    if ($scriptFile !== null) {
-        require $scriptFile;
-    } else {
+    if (false === $response->isNotFound()) {
+        // Symfony successfully handled the route.
         $response->send();
+    } else {
+        LegacyBridge::handleRequest($request, $response, __DIR__);
     }
+
     $kernel->terminate($request, $response);
 
 There are 2 major deviations from the original file:
@@ -297,10 +294,9 @@ Line 18
   it over. For instance, by replacing outdated or redundant libraries with
   Symfony components.
 
-Line 41 - 50
-  Instead of sending the Symfony response directly, a ``LegacyBridge`` is
-  called to decide whether the legacy application should be booted and used to
-  create the response instead.
+Line 41 - 46
+  If Symfony handled the response, it is sent; otherwise, the ``LegacyBridge``
+  handles the request.
 
 This legacy bridge is responsible for figuring out which file should be loaded
 in order to process the old application logic. This can either be a front
@@ -316,19 +312,50 @@ somewhat like this::
 
     class LegacyBridge
     {
-        public static function prepareLegacyScript(Request $request, Response $response, string $publicDirectory): ?string
+
+        /**
+         * Map the incoming request to the right file. This is the
+         * key function of the LegacyBridge.
+         *
+         * Sample code only. Your implementation will vary, depending on the
+         * architecture of the legacy code and how it's executed.
+         *
+         * If your mapping is complicated, you may want to write unit tests
+         * to verify your logic, hence this is public static.
+         */
+        public static function getLegacyScript(Request $request): string
         {
-            // If Symfony successfully handled the route, you do not have to do anything.
-            if (false === $response->isNotFound()) {
-                return null;
+            $requestPathInfo = $request->getPathInfo();
+            $legacyRoot = __DIR__ . '/../';
+
+            // Map a route to a legacy script:
+            if ($requestPathInfo == '/customer/') {
+                return "{$legacyRoot}src/customers/list.php";
             }
 
-            // Figure out how to map to the needed script file
-            // from the existing application and possibly (re-)set
-            // some env vars.
-            $legacyScriptFilename = ...;
+            // Map a direct file call, e.g. an ajax call:
+            if ($requestPathInfo == 'inc/ajax_cust_details.php') {
+                return "{$legacyRoot}inc/ajax_cust_details.php";
+            }
+
+            // ... etc.
+
+            throw new \Exception("Unhandled legacy mapping for $requestPathInfo");
+        }
+
+
+        public static function handleRequest(Request $request, Response $response, string $publicDirectory): ?string
+        {
+            $legacyScriptFilename = LegacyBridge::getLegacyScript($request);
+
+            // Possibly (re-)set some env vars (e.g. to handle forms
+            // posting to PHP_SELF):
+            $p = $request->getPathInfo();
+            $_SERVER['PHP_SELF'] = $p;
+            $_SERVER['SCRIPT_NAME'] = $p;
+            $_SERVER['SCRIPT_FILENAME'] = $legacyScriptFilename;
 
-            return $legacyScriptFilename;
+            require $legacyScriptFilename;
         }
     }