Improve sequencing of various content/header checks in abstract block diffs

Summary:
Ref T13425. Some diff checks currently sequence incorrectly:

  - When we're rendering block lists, syntax highlighting isn't relevant.
  - The "large change" guard can prevent rendering of otherwise-renderable changes.
  - Actual errors in the document engine (like bad JSON in a ".ipynb" file) aren't surfaced properly.

Improve sequencing somewhat to resolve these issues.

Test Plan:
  - Viewed a notebook, no longer saw a "highlighting disabled" warning.
  - Forced a notebook to fail, got a useful inline error instead of a popup dialog error.
  - Forced a notebook to have a large number of differences, got a rendering out of it.

Maniphest Tasks: T13425

Differential Revision: https://secure.phabricator.com/D20843

src/applications/differential/parser/DifferentialChangesetParser.php

@@ -868,8 +868,11 @@ final class DifferentialChangesetParser extends Phobject {
       ->setHighlightingDisabled($this->highlightingDisabled)
       ->setDepthOnlyLines($this->getDepthOnlyLines());
 
+    $engine_blocks = $this->newDocumentEngineBlocks();
+    $has_document_engine = ($engine_blocks !== null);
+
     $shield = null;
-    if ($this->isTopLevel && !$this->comments) {
+    if ($this->isTopLevel && !$this->comments && !$has_document_engine) {
       if ($this->isGenerated()) {
         $shield = $renderer->renderShield(
           pht(
@@ -1024,7 +1027,6 @@ final class DifferentialChangesetParser extends Phobject {
       ->setOldComments($old_comments)
       ->setNewComments($new_comments);
 
-    $engine_blocks = $this->newDocumentEngineChangesetView();
     if ($engine_blocks !== null) {
       $reference = $this->getRenderingReference();
       $parts = explode('/', $reference);
@@ -1041,6 +1043,8 @@ final class DifferentialChangesetParser extends Phobject {
         $vs = $id;
       }
 
+      $renderer->setDocumentEngineBlocks($engine_blocks);
+
       return $renderer->renderDocumentEngineBlocks(
         $engine_blocks,
         (string)$id,
@@ -1653,7 +1657,7 @@ final class DifferentialChangesetParser extends Phobject {
     return $prefix.$line;
   }
 
-  private function newDocumentEngineChangesetView() {
+  private function newDocumentEngineBlocks() {
     $changeset = $this->changeset;
     $viewer = $this->getViewer();
 
@@ -1724,7 +1728,7 @@ final class DifferentialChangesetParser extends Phobject {
     }
 
     if ($document_engine) {
-      return $document_engine->newDiffView(
+      return $document_engine->newEngineBlocks(
         $old_ref,
         $new_ref);
     }

src/applications/differential/render/DifferentialChangesetHTMLRenderer.php

@@ -270,11 +270,20 @@ abstract class DifferentialChangesetHTMLRenderer
       }
     }
 
-    if ($this->getHighlightingDisabled()) {
-      $messages[] = pht(
-        'This file is larger than %s, so syntax highlighting is '.
-        'disabled by default.',
-        phutil_format_bytes(DifferentialChangesetParser::HIGHLIGHT_BYTE_LIMIT));
+    $blocks = $this->getDocumentEngineBlocks();
+    if ($blocks) {
+      foreach ($blocks->getMessages() as $message) {
+        $messages[] = $message;
+      }
+    } else {
+      if ($this->getHighlightingDisabled()) {
+        $byte_limit = DifferentialChangesetParser::HIGHLIGHT_BYTE_LIMIT;
+        $byte_limit = phutil_format_bytes($byte_limit);
+        $messages[] = pht(
+          'This file is larger than %s, so syntax highlighting is '.
+          'disabled by default.',
+          $byte_limit);
+      }
     }
 
     return $this->formatHeaderMessages($messages);

src/applications/differential/render/DifferentialChangesetRenderer.php

@@ -35,6 +35,7 @@ abstract class DifferentialChangesetRenderer extends Phobject {
   private $highlightingDisabled;
   private $scopeEngine = false;
   private $depthOnlyLines;
+  private $documentEngineBlocks;
 
   private $oldFile = false;
   private $newFile = false;
@@ -239,6 +240,16 @@ abstract class DifferentialChangesetRenderer extends Phobject {
     return $this->oldChangesetID;
   }
 
+  public function setDocumentEngineBlocks(
+    PhabricatorDocumentEngineBlocks $blocks) {
+    $this->documentEngineBlocks = $blocks;
+    return $this;
+  }
+
+  public function getDocumentEngineBlocks() {
+    return $this->documentEngineBlocks;
+  }
+
   public function setNewComments(array $new_comments) {
     foreach ($new_comments as $line_number => $comments) {
       assert_instances_of($comments, 'PhabricatorInlineCommentInterface');
@@ -355,6 +366,16 @@ abstract class DifferentialChangesetRenderer extends Phobject {
     $notice = null;
     if ($this->getIsTopLevel()) {
       $force = (!$content && !$props);
+
+      // If we have DocumentEngine messages about the blocks, assume they
+      // explain why there's no content.
+      $blocks = $this->getDocumentEngineBlocks();
+      if ($blocks) {
+        if ($blocks->getMessages()) {
+          $force = false;
+        }
+      }
+
       $notice = $this->renderChangeTypeHeader($force);
     }
 

src/applications/files/diff/PhabricatorDocumentEngineBlocks.php

@@ -4,6 +4,16 @@ final class PhabricatorDocumentEngineBlocks
   extends Phobject {
 
   private $lists = array();
+  private $messages = array();
+
+  public function addMessage($message) {
+    $this->messages[] = $message;
+    return $this;
+  }
+
+  public function getMessages() {
+    return $this->messages;
+  }
 
   public function addBlockList(PhabricatorDocumentRef $ref, array $blocks) {
     assert_instances_of($blocks, 'PhabricatorDocumentEngineBlock');
@@ -20,6 +30,10 @@ final class PhabricatorDocumentEngineBlocks
     $rows = array();
     $lists = $this->lists;
 
+    if (count($lists) != 2) {
+      return array();
+    }
+
     $specs = array();
     foreach ($this->lists as $list) {
       $specs[] = $this->newDiffSpec($list['blocks']);

src/applications/files/document/PhabricatorDocumentEngine.php

@@ -37,7 +37,7 @@ abstract class PhabricatorDocumentEngine
     return false;
   }
 
-  public function newDiffView(
+  public function newEngineBlocks(
     PhabricatorDocumentRef $uref,
     PhabricatorDocumentRef $vref) {
     throw new PhutilMethodNotImplementedException();

src/applications/files/document/PhabricatorImageDocumentEngine.php

@@ -27,7 +27,7 @@ final class PhabricatorImageDocumentEngine
     return ($uref->getFile() && $vref->getFile());
   }
 
-  public function newDiffView(
+  public function newEngineBlocks(
     PhabricatorDocumentRef $uref,
     PhabricatorDocumentRef $vref) {
 

src/applications/files/document/PhabricatorJupyterDocumentEngine.php

@@ -41,16 +41,23 @@ final class PhabricatorJupyterDocumentEngine
     return true;
   }
 
-  public function newDiffView(
+  public function newEngineBlocks(
     PhabricatorDocumentRef $uref,
     PhabricatorDocumentRef $vref) {
 
-    $u_blocks = $this->newDiffBlocks($uref);
-    $v_blocks = $this->newDiffBlocks($vref);
+    $blocks = new PhabricatorDocumentEngineBlocks();
 
-    return id(new PhabricatorDocumentEngineBlocks())
-      ->addBlockList($uref, $u_blocks)
-      ->addBlockList($vref, $v_blocks);
+    try {
+      $u_blocks = $this->newDiffBlocks($uref);
+      $v_blocks = $this->newDiffBlocks($vref);
+
+      $blocks->addBlockList($uref, $u_blocks);
+      $blocks->addBlockList($vref, $v_blocks);
+    } catch (Exception $ex) {
+      $blocks->addMessage($ex->getMessage());
+    }
+
+    return $blocks;
   }
 
   private function newDiffBlocks(PhabricatorDocumentRef $ref) {