commit ab8bbc722de26bb2078942ce861989f76f28a055
Author: Greg Anderson <greg.1.anderson@greeknowe.org>
Date:   Thu May 30 08:30:19 2019 -0700

    Add marker interface ConjoinableInterface. Add a test of appending to a scaffold file that has already been appended to.

diff --git a/core/lib/Drupal/Component/Scaffold/Operations/AppendOp.php b/core/lib/Drupal/Component/Scaffold/Operations/AppendOp.php
index d4d58b1b33..d8ca757a4d 100644
--- a/core/lib/Drupal/Component/Scaffold/Operations/AppendOp.php
+++ b/core/lib/Drupal/Component/Scaffold/Operations/AppendOp.php
@@ -9,7 +9,7 @@
 /**
  * Scaffold operation to add to the beginning and/or end of a scaffold file.
  */
-class AppendOp implements OperationInterface {
+class AppendOp implements OperationInterface, ConjoinableInterface {
 
   /**
    * Path to the source file to prepend, if any.
diff --git a/core/lib/Drupal/Component/Scaffold/Operations/ConjoinableInterface.php b/core/lib/Drupal/Component/Scaffold/Operations/ConjoinableInterface.php
new file mode 100644
index 0000000000..4f494173fa
--- /dev/null
+++ b/core/lib/Drupal/Component/Scaffold/Operations/ConjoinableInterface.php
@@ -0,0 +1,15 @@
+<?php
+
+namespace Drupal\Component\Scaffold\Operations;
+
+/**
+ * Marker interface indicating that operation is conjoinable.
+ *
+ * A conjoinable operation is one that runs in addition to any previous
+ * operation defined at the same destination path. Operations that are
+ * not conjoinable simply replace anything at the same destination path.
+ *
+ * The only conjoinable operation at the moment is the "append" operation.
+ */
+interface ConjoinableInterface {
+}
diff --git a/core/lib/Drupal/Component/Scaffold/Operations/OperationCollection.php b/core/lib/Drupal/Component/Scaffold/Operations/OperationCollection.php
index 48ffdf8ee8..ce2c00ddfd 100644
--- a/core/lib/Drupal/Component/Scaffold/Operations/OperationCollection.php
+++ b/core/lib/Drupal/Component/Scaffold/Operations/OperationCollection.php
@@ -100,9 +100,11 @@ protected function collateScaffoldFiles(array $file_mappings, Interpolator $loca
       foreach ($package_file_mappings as $destination_rel_path => $op) {
         $destination = ScaffoldFilePath::destinationPath($package_name, $destination_rel_path, $location_replacements);
         // If there was already a scaffolding operation happening at this path,
-        // AppendOp operations can be joined to the previous operation using a
-        // ConjunctionOp.
-        if (isset($list_of_scaffold_files[$destination_rel_path]) && $op instanceof AppendOp) {
+        // and the new operation is Conjoinable, then use a ConjunctionOp to
+        // join together both operations. This will cause both operations to
+        // run, one after the other. At the moment, only AppendOp is conjoinable;
+        // all other operations simply replace anything at the same path.
+        if (isset($list_of_scaffold_files[$destination_rel_path]) && $op instanceof ConjoinableInterface) {
           $op = new ConjunctionOp($list_of_scaffold_files[$destination_rel_path]->op(), $op);
         }
 
diff --git a/core/tests/Drupal/Tests/Component/Scaffold/Functional/ScaffoldTest.php b/core/tests/Drupal/Tests/Component/Scaffold/Functional/ScaffoldTest.php
index 3e733c2ad2..0c18b2b5ff 100644
--- a/core/tests/Drupal/Tests/Component/Scaffold/Functional/ScaffoldTest.php
+++ b/core/tests/Drupal/Tests/Component/Scaffold/Functional/ScaffoldTest.php
@@ -138,7 +138,8 @@ public function scaffoldTestValues() {
     return [
       [
         'drupal-composer-drupal-project',
-        'assertDrupalProjectSutWasScaffolded', TRUE,
+        'assertDrupalProjectSutWasScaffolded',
+        TRUE,
       ],
       [
         'drupal-drupal',
@@ -155,6 +156,11 @@ public function scaffoldTestValues() {
         'assertDrupalDrupalFileWasAppended',
         FALSE,
       ],
+      [
+        'drupal-drupal-append-to-append',
+        'assertFileWasAppendedToAppendedFile',
+        FALSE,
+      ],
       [
         'drupal-drupal-test-append',
         'assertDrupalDrupalFileWasAppended',
@@ -338,6 +344,12 @@ protected function assertDrupalDrupalFileWasAppended($sut, $is_link, $project_na
     $this->assertCommonDrupalAssetsWereScaffolded($docroot, $is_link);
   }
 
+  protected function assertFileWasAppendedToAppendedFile($sut, $is_link, $project_name) {
+    $docroot = $sut;
+    $this->assertScaffoldedFile($docroot . '/robots.txt', FALSE, '#in drupal-drupal-append-to-append composer.json fixture.*This content is prepended to the top of the existing robots.txt fixture.*Test version of robots.txt from drupal/core.*This content is appended to the bottom of the existing robots.txt fixture.*in profile-with-append composer.json fixture.*This content is appended to the bottom of the existing robots.txt fixture.*in drupal-drupal-append-to-append composer.json fixture#ms');
+    $this->assertCommonDrupalAssetsWereScaffolded($docroot, $is_link, $project_name);
+  }
+
   /**
    * Asserts that the scaffold files from drupal/assets are placed as expected.
    *
diff --git a/core/tests/Drupal/Tests/Component/Scaffold/fixtures/drupal-drupal-append-to-append/assets/append-to-robots.txt b/core/tests/Drupal/Tests/Component/Scaffold/fixtures/drupal-drupal-append-to-append/assets/append-to-robots.txt
new file mode 100644
index 0000000000..5290d32590
--- /dev/null
+++ b/core/tests/Drupal/Tests/Component/Scaffold/fixtures/drupal-drupal-append-to-append/assets/append-to-robots.txt
@@ -0,0 +1,3 @@
+# ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
+# This content is appended to the bottom of the existing robots.txt fixture.
+# robots.txt fixture scaffolded from "file-mappings" in drupal-drupal-append-to-append composer.json fixture.
diff --git a/core/tests/Drupal/Tests/Component/Scaffold/fixtures/drupal-drupal-append-to-append/assets/prepend-to-robots.txt b/core/tests/Drupal/Tests/Component/Scaffold/fixtures/drupal-drupal-append-to-append/assets/prepend-to-robots.txt
new file mode 100644
index 0000000000..8f7550ff5e
--- /dev/null
+++ b/core/tests/Drupal/Tests/Component/Scaffold/fixtures/drupal-drupal-append-to-append/assets/prepend-to-robots.txt
@@ -0,0 +1,3 @@
+# robots.txt fixture scaffolded from "file-mappings" in drupal-drupal-append-to-append composer.json fixture.
+# This content is prepended to the top of the existing robots.txt fixture.
+# ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
diff --git a/core/tests/Drupal/Tests/Component/Scaffold/fixtures/drupal-drupal-append-to-append/composer.json.tmpl b/core/tests/Drupal/Tests/Component/Scaffold/fixtures/drupal-drupal-append-to-append/composer.json.tmpl
new file mode 100644
index 0000000000..1919d878c6
--- /dev/null
+++ b/core/tests/Drupal/Tests/Component/Scaffold/fixtures/drupal-drupal-append-to-append/composer.json.tmpl
@@ -0,0 +1,71 @@
+{
+  "name": "fixtures/drupal-drupal-test-append",
+  "type": "project",
+  "minimum-stability": "dev",
+  "prefer-stable": true,
+  "repositories": {
+    "composer-scaffold": {
+      "type": "path",
+      "url": "__PROJECT_ROOT__",
+      "options": {
+        "symlink": true
+      }
+    },
+    "drupal-core-fixture": {
+      "type": "path",
+      "url": "../drupal-core-fixture",
+      "options": {
+        "symlink": true
+      }
+    },
+    "profile-with-append": {
+      "type": "path",
+      "url": "../profile-with-append",
+      "options": {
+        "symlink": true
+      }
+    },
+    "drupal-assets-fixture": {
+      "type": "path",
+      "url": "../drupal-assets-fixture",
+      "options": {
+        "symlink": true
+      }
+    }
+  },
+  "require": {
+    "drupal/core-composer-scaffold": "*",
+    "fixtures/profile-with-append": "*",
+    "fixtures/drupal-core-fixture": "*"
+  },
+  "extra": {
+    "composer-scaffold": {
+      "allowed-packages": [
+        "fixtures/drupal-core-fixture",
+        "fixtures/profile-with-append"
+      ],
+      "locations": {
+        "web-root": "./"
+      },
+      "symlink": __SYMLINK__,
+      "file-mapping": {
+        "[web-root]/.htaccess": false,
+        "[web-root]/robots.txt": {
+          "prepend": "assets/prepend-to-robots.txt",
+          "append": "assets/append-to-robots.txt"
+        }
+      }
+    },
+    "installer-paths": {
+      "core": ["type:drupal-core"],
+      "modules/contrib/{$name}": ["type:drupal-module"],
+      "modules/custom/{$name}": ["type:drupal-custom-module"],
+      "profiles/contrib/{$name}": ["type:drupal-profile"],
+      "profiles/custom/{$name}": ["type:drupal-custom-profile"],
+      "themes/contrib/{$name}": ["type:drupal-theme"],
+      "themes/custom/{$name}": ["type:drupal-custom-theme"],
+      "libraries/{$name}": ["type:drupal-library"],
+      "drush/Commands/contrib/{$name}": ["type:drupal-drush"]
+    }
+  }
+}
diff --git a/core/tests/Drupal/Tests/Component/Scaffold/fixtures/profile-with-append/assets/append-to-robots.txt b/core/tests/Drupal/Tests/Component/Scaffold/fixtures/profile-with-append/assets/append-to-robots.txt
new file mode 100644
index 0000000000..ab2eb43a74
--- /dev/null
+++ b/core/tests/Drupal/Tests/Component/Scaffold/fixtures/profile-with-append/assets/append-to-robots.txt
@@ -0,0 +1,3 @@
+# ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
+# This content is appended to the bottom of the existing robots.txt fixture.
+# robots.txt fixture scaffolded from "file-mappings" in profile-with-append composer.json fixture.
diff --git a/core/tests/Drupal/Tests/Component/Scaffold/fixtures/profile-with-append/composer.json.tmpl b/core/tests/Drupal/Tests/Component/Scaffold/fixtures/profile-with-append/composer.json.tmpl
new file mode 100644
index 0000000000..d67135fcac
--- /dev/null
+++ b/core/tests/Drupal/Tests/Component/Scaffold/fixtures/profile-with-append/composer.json.tmpl
@@ -0,0 +1,12 @@
+{
+  "name": "fixtures/profile-with-append",
+  "extra": {
+    "composer-scaffold": {
+      "file-mapping": {
+        "[web-root]/robots.txt": {
+          "append": "assets/append-to-robots.txt"
+        }
+      }
+    }
+  }
+}