Fix an unusual nonterminating task graph node

Summary:
Fixes T12114. There were a couple of bugs here:

  - We could draw too many joining lines if a node had a parent with multiple descendants.
  - We could incorrectly ignore columns because of an `unset()`.

I //think// this fixes both things without collateral damage. This whole thing is a little hard to understand/debug and has grown beyond its original scope, so I'll probably rewrite it if there are more issues.

Test Plan:
  - Unit tests.
  - My local repro is clean now:

{F2424920}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12114

Differential Revision: https://secure.phabricator.com/D17211

src/infrastructure/diff/view/PHUIDiffGraphView.php

@@ -50,7 +50,6 @@ final class PHUIDiffGraphView extends Phobject {
 
       $thread_count = $pos;
       for ($n = 0; $n < $thread_count; $n++) {
-
         if (empty($threads[$n])) {
           $line .= ' ';
           continue;
@@ -60,7 +59,7 @@ final class PHUIDiffGraphView extends Phobject {
           if ($found) {
             $line .= ' ';
             $joins[] = $n;
-            unset($threads[$n]);
+            $threads[$n] = false;
           } else {
             $line .= 'o';
             $found = true;
@@ -114,6 +113,7 @@ final class PHUIDiffGraphView extends Phobject {
           if ($thread_commit == $parent) {
             $found = true;
             $splits[] = $idx;
+            break;
           }
         }
 

src/infrastructure/diff/view/__tests__/PHUIDiffGraphViewTestCase.php

@@ -45,10 +45,10 @@ final class PHUIDiffGraphViewTestCase extends PhabricatorTestCase {
       '^',
       '|^',
       'o ',
-      '|^',
-      '||^',
-      'o  ',
-      'x',
+      '| ^',
+      '| |^',
+      'o   ',
+      'x   ',
     );
 
     $this->assertGraph($picture, $graph, pht('Reverse Tree'));
@@ -71,7 +71,30 @@ final class PHUIDiffGraphViewTestCase extends PhabricatorTestCase {
       'x ',
     );
 
-    $this->assertGraph($picture, $graph, pht('Reverse Tree'));
+    $this->assertGraph($picture, $graph, pht('Terminated Tree'));
+  }
+
+  public function testThreeWayGraphJoin() {
+    $nodes = array(
+      'A' => array('D', 'C', 'B'),
+      'B' => array('D'),
+      'C' => array('B', 'E', 'F'),
+      'D' => array(),
+      'E' => array(),
+      'F' => array(),
+    );
+
+    $graph = $this->newGraph($nodes);
+    $picture = array(
+      '^',
+      '||o',
+      '|o|',
+      'x| ||',
+      ' | x|',
+      ' |  x',
+    );
+
+    $this->assertGraph($picture, $graph, pht('Three-Way Tree'));
   }
 
   private function newGraph(array $nodes) {