Allow subtypes to specify "mutations", to control the behavior of the "Change Subtype" action

Summary:
Fixes T13415. Provide a way for subtypes to customize the behavior of "Change Subtype" actions that appear above comment areas.

Subtypes may disable this action by specifying `"mutations": []`, or provide a list of subtypes.

The bulk editor and API can still perform any change.

Test Plan:
  - Tried to define an invalid "mutations" list with a bad subtype, got a sensible error.
  - Specified a limited mutations list and an empty mutations list, verified that corresponding tasks got corresponding actions.
  - Used the bulk editor to perform a freeform mutation.
  - Verified that tasks of a subtype with no "mutations" still work the same way they used to (allow mutation into any subtype).

Maniphest Tasks: T13415

Differential Revision: https://secure.phabricator.com/D20810

src/applications/maniphest/config/PhabricatorManiphestConfigOptions.php

@@ -344,6 +344,8 @@ dictionary with these keys:
   - `children` //Optional map.// Configure options shown to the user when
      they "Create Subtask". See below.
   - `fields` //Optional map.// Configure field behaviors. See below.
+  - `mutations` //Optional list.// Configure which subtypes this subtype
+    can easily be converted to by using the "Change Subtype" action. See below.
 
 Each subtype must have a unique key, and you must define a subtype with
 the key "%s", which is used as a default subtype.
@@ -421,6 +423,31 @@ Each field supports these options:
     subtypes.
   - `name` //Optional string.// Custom name of this field for the subtype.
 
+
+The `mutations` key allows you to control the behavior of the "Change Subtype"
+action above the comment area. By default, this action allows users to change
+the task subtype into any other subtype.
+
+If you'd prefer to make it more difficult to change subtypes or offer only a
+subset of subtypes, you can specify the list of subtypes that "Change Subtypes"
+offers. For example, if you have several similar subtypes and want to allow
+tasks to be converted between them but not easily converted to other types,
+you can make the "Change Subtypes" control show only these options like this:
+
+```
+  {
+    ...
+    "mutations": ["bug", "issue", "defect"]
+    ...
+  }
+```
+
+If you specify an empty list, the "Change Subtypes" action will be completely
+hidden.
+
+This mutation list is advisory and only configures the UI. Tasks may still be
+converted across subtypes freely by using the Bulk Editor or API.
+
 EOTEXT
       ,
       $subtype_default_key));

src/applications/transactions/editengine/PhabricatorEditEngineSubtype.php

@@ -14,6 +14,7 @@ final class PhabricatorEditEngineSubtype
   private $childSubtypes = array();
   private $childIdentifiers = array();
   private $fieldConfiguration = array();
+  private $mutations;
 
   public function setKey($key) {
     $this->key = $key;
@@ -78,6 +79,15 @@ final class PhabricatorEditEngineSubtype
     return $this->childIdentifiers;
   }
 
+  public function setMutations($mutations) {
+    $this->mutations = $mutations;
+    return $this;
+  }
+
+  public function getMutations() {
+    return $this->mutations;
+  }
+
   public function hasTagView() {
     return (bool)strlen($this->getTagText());
   }
@@ -152,6 +162,7 @@ final class PhabricatorEditEngineSubtype
           'icon' => 'optional string',
           'children' => 'optional map<string, wild>',
           'fields' => 'optional map<string, wild>',
+          'mutations' => 'optional list<string>',
         ));
 
       $key = $value['key'];
@@ -217,6 +228,28 @@ final class PhabricatorEditEngineSubtype
           'with key "%s". This subtype is required and must be defined.',
           self::SUBTYPE_DEFAULT));
     }
+
+    foreach ($config as $value) {
+      $key = idx($value, 'key');
+
+      $mutations = idx($value, 'mutations');
+      if (!$mutations) {
+        continue;
+      }
+
+      foreach ($mutations as $mutation) {
+        if (!isset($map[$mutation])) {
+          throw new Exception(
+            pht(
+              'Subtype configuration is invalid: subtype with key "%s" '.
+              'specifies that it can mutate into subtype "%s", but that is '.
+              'not a valid subtype.',
+              $key,
+              $mutation));
+        }
+      }
+    }
+
   }
 
   public static function newSubtypeMap(array $config) {
@@ -267,6 +300,8 @@ final class PhabricatorEditEngineSubtype
         }
       }
 
+      $subtype->setMutations(idx($entry, 'mutations'));
+
       $map[$key] = $subtype;
     }
 

src/applications/transactions/editengine/PhabricatorEditEngineSubtypeMap.php

@@ -53,6 +53,44 @@ final class PhabricatorEditEngineSubtypeMap
     return clone($this->datasource);
   }
 
+  public function getMutationMap($source_key) {
+    return mpull($this->getMutations($source_key), 'getName');
+  }
+
+  public function getMutations($source_key) {
+    $mutations = $this->subtypes;
+
+    $subtype = idx($this->subtypes, $source_key);
+    if ($subtype) {
+      $map = $subtype->getMutations();
+      if ($map !== null) {
+        $map = array_fuse($map);
+        foreach ($mutations as $key => $mutation) {
+          if ($key === $source_key) {
+            // This is the current subtype, so we always want to show it.
+            continue;
+          }
+
+          if (isset($map[$key])) {
+            // This is an allowed mutation, so keep it.
+            continue;
+          }
+
+          // Discard other subtypes as mutation options.
+          unset($mutations[$key]);
+        }
+      }
+    }
+
+    // If the only available mutation is the current subtype, treat this like
+    // no mutations are available.
+    if (array_keys($mutations) === array($source_key)) {
+      $mutations = array();
+    }
+
+    return $mutations;
+  }
+
   public function getCreateFormsForSubtype(
     PhabricatorEditEngine $edit_engine,
     PhabricatorEditEngineSubtypeInterface $object) {

src/applications/transactions/engineextension/PhabricatorSubtypeEditEngineExtension.php

@@ -29,9 +29,17 @@ final class PhabricatorSubtypeEditEngineExtension
     PhabricatorApplicationTransactionInterface $object) {
 
     $subtype_type = PhabricatorTransactions::TYPE_SUBTYPE;
+    $subtype_value = $object->getEditEngineSubtype();
 
     $map = $object->newEditEngineSubtypeMap();
+
+    if ($object->getID()) {
+      $options = $map->getMutationMap($subtype_value);
+    } else {
+      // NOTE: This is a crude proxy for "are we in the bulk edit workflow".
+      // We want to allow any mutation.
       $options = $map->getDisplayMap();
+    }
 
     $subtype_field = id(new PhabricatorSelectEditField())
       ->setKey(self::EDITKEY)
@@ -40,12 +48,12 @@ final class PhabricatorSubtypeEditEngineExtension
       ->setTransactionType($subtype_type)
       ->setConduitDescription(pht('Change the object subtype.'))
       ->setConduitTypeDescription(pht('New object subtype key.'))
-      ->setValue($object->getEditEngineSubtype())
+      ->setValue($subtype_value)
       ->setOptions($options);
 
-    // If subtypes are configured, enable changing them from the bulk editor
+    // If subtypes are configured, enable changing them from the bulk editor.
-    // and comment action stack.
+    // Bulk editor
-    if ($map->getCount() > 1) {
+    if ($options) {
       $subtype_field
         ->setBulkEditLabel(pht('Change subtype to'))
         ->setCommentActionLabel(pht('Change Subtype'))