Detect developer error when constructing forms with absolute URIs

Summary: Ref T1921. Ref T4339. If you `phabricator_form()` with an absolute URI, we silently drop the CSRF tokens. This can be confusing if you meant to specify `"/some/path"` but ended up specifying `"http://this.install.com/some/path"`. In all current cases that I can think of / am aware of, this indicates an error in the code. Make it more obvious what's happening and how to fix it. The error only fires in developer mode.

Test Plan: Hit this case, also rendered normal forms.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4339, T1921

Differential Revision: https://secure.phabricator.com/D8044

src/infrastructure/javelin/markup.php

@@ -38,8 +38,32 @@ function javelin_tag(
 function phabricator_form(PhabricatorUser $user, $attributes, $content) {
   $body = array();
 
-  if (strcasecmp(idx($attributes, 'method'), 'POST') == 0 &&
-      !preg_match('#^(https?:|//)#', idx($attributes, 'action'))) {
+  $http_method = idx($attributes, 'method');
+  $is_post = (strcasecmp($http_method, 'POST') === 0);
+
+  $http_action = idx($attributes, 'action');
+  $is_absolute_uri = preg_match('#^(https?:|//)#', $http_action);
+
+  if ($is_post) {
+    if ($is_absolute_uri) {
+      $is_dev = PhabricatorEnv::getEnvConfig('phabricator.developer-mode');
+      if ($is_dev) {
+        $form_domain = id(new PhutilURI($http_action))
+          ->getDomain();
+        $host_domain = id(new PhutilURI(PhabricatorEnv::getURI('/')))
+          ->getDomain();
+
+        if (strtolower($form_domain) == strtolower($host_domain)) {
+          throw new Exception(
+            pht(
+              "You are building a <form /> that submits to Phabricator, but ".
+              "has an absolute URI in its 'action' attribute ('%s'). To avoid ".
+              "leaking CSRF tokens, Phabricator does not add CSRF information ".
+              "to forms with absolute URIs. Instead, use a relative URI.",
+              $http_action));
+        }
+      }
+    } else {
       $body[] = phutil_tag(
         'input',
         array(
@@ -56,6 +80,7 @@ function phabricator_form(PhabricatorUser $user, $attributes, $content) {
           'value' => true,
         ));
     }
+  }
 
   if (is_array($content)) {
     $body = array_merge($body, $content);