Make it easier to use print_r() debugging

Summary:
The fixed-position side nav background thing tends to make looking at print_r() output hard. Also, it breaks Ajax, etc.

  - Loudly call out unexpected output on normal pages, to catch extra spaces before `<?php`, etc.
  - Display unexpected output in an attractive panel on normal pages.
  - Log unexpected output instead of breaking Ajax.

Test Plan:
{F32267}

Also triggered various fatals and verified they still show the right messages (no blank pages).

Reviewers: vrana, btrahan, chad

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D4892

support/PhabricatorStartup.php

@@ -17,6 +17,7 @@ final class PhabricatorStartup {
 
   private static $startTime;
   private static $globals = array();
+  private static $capturingOutput;
 
 
 /* -(  Accessing Request Information  )-------------------------------------- */
@@ -78,6 +79,8 @@ final class PhabricatorStartup {
     self::verifyRewriteRules();
 
     self::detectPostMaxSizeTriggered();
+
+    self::beginOutputCapture();
   }
 
 
@@ -149,6 +152,26 @@ final class PhabricatorStartup {
     phutil_load_library($phabricator_root.'/src');
   }
 
+/* -(  Output Capture  )----------------------------------------------------- */
+
+
+  public static function beginOutputCapture() {
+    if (self::$capturingOutput) {
+      self::didFatal("Already capturing output!");
+    }
+    self::$capturingOutput = true;
+    ob_start();
+  }
+
+
+  public static function endOutputCapture() {
+    if (!self::$capturingOutput) {
+      return null;
+    }
+    self::$capturingOutput = false;
+    return ob_get_clean();
+  }
+
 
 /* -(  In Case of Apocalypse  )---------------------------------------------- */
 
@@ -157,6 +180,7 @@ final class PhabricatorStartup {
    * @task apocalypse
    */
   public static function didFatal($message) {
+    self::endOutputCapture();
     $access_log = self::getGlobal('log.access');
 
     if ($access_log) {

webroot/index.php

@@ -30,6 +30,7 @@ try {
 
   $response = PhabricatorSetupCheck::willProcessRequest();
   if ($response) {
+    PhabricatorStartup::endOutputCapture();
     $sink->writeResponse($response);
     return;
   }
@@ -102,8 +103,21 @@ try {
     $response = $application->willSendResponse($response, $controller);
     $response->setRequest($request);
 
-    $sink->writeResponse($response);
+    $unexpected_output = PhabricatorStartup::endOutputCapture();
+    if ($unexpected_output) {
+      $unexpected_output = "Unexpected output:\n\n{$unexpected_output}";
+      phlog($unexpected_output);
 
+      if ($response instanceof AphrontWebpageResponse) {
+        echo hsprintf(
+          '<div style="background: #eeddff; white-space: pre-wrap;
+                       z-index: 200000; position: relative; padding: 8px;
+                       font-family: monospace;">%s</div>',
+          $unexpected_output);
+      }
+    }
+
+    $sink->writeResponse($response);
   } catch (Exception $ex) {
     $write_guard->dispose();
     if ($access_log) {