Allow arc to be run in arcanist/ or libphutil/

Summary:
Currently, if you run `arc` in arcanist/ or libphutil/ and your PATH and on-disk configuration are set up so a different version of arc or libphutil are the ones that actually load, we fail with an exception like "running arcanist in a different copy of arcanist is not supported".

This causes problems for Harbormaster, since we'd like to be able to run 'arc' in a copy of libphutil/ and have it execute unit tests for that copy rather than failing abruptly. So, if we detect that we're in arcanist/ or libphutil/, execute 'arc' again with the same arguments but force it to load the working copy in place of either the 'arcanist/' or the 'libphutil/' that it decided to load.

This is pretty much horrible black magic.

Test Plan: Ran 'arc list --trace' inside copies of libphutil/ and arcanist/ outside of the normal include chain. Saw it detect these, emit a message, and re-execute itself correctly.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D4225

scripts/__init_script__.php

@@ -40,7 +40,11 @@ function arcanist_adjust_php_include_path() {
 }
 arcanist_adjust_php_include_path();
 
-@include_once 'libphutil/scripts/__init_script__.php';
+if (getenv('ARC_PHUTIL_PATH')) {
+  @include_once getenv('ARC_PHUTIL_PATH').'/scripts/__init_script__.php';
+} else {
+  @include_once 'libphutil/scripts/__init_script__.php';
+}
 if (!@constant('__LIBPHUTIL__')) {
   echo "ERROR: Unable to load libphutil. Put libphutil/ next to arcanist/, or ".
        "update your PHP 'include_path' to include the parent directory of ".

scripts/arcanist.php

@@ -73,6 +73,11 @@ try {
   $system_config = ArcanistBaseWorkflow::readSystemArcConfig();
   $working_copy = ArcanistWorkingCopyIdentity::newFromPath($working_directory);
 
+  reenter_if_this_is_arcanist_or_libphutil(
+    $console,
+    $working_copy,
+    $original_argv);
+
   // Load additional libraries, which can provide new classes like configuration
   // overrides, linters and lint engines, unit test engines, etc.
 
@@ -535,3 +540,68 @@ function arcanist_load_libraries(
     }
   }
 }
+
+
+/**
+ * NOTE: SPOOKY BLACK MAGIC
+ *
+ * When arc is run in a copy of arcanist other than itself, or a copy of
+ * libphutil other than the one we loaded, reenter the script and force it
+ * to use the current working directory instead of the default.
+ *
+ * In the case of execution inside arcanist/, we force execution of the local
+ * arc binary.
+ *
+ * In the case of execution inside libphutil/, we force the local copy to load
+ * instead of the one selected by default rules.
+ *
+ * @param PhutilConsole                 Console.
+ * @param ArcanistWorkingCopyIdentity   The current working copy.
+ * @param array                         Original arc arguments.
+ * @return void
+ */
+function reenter_if_this_is_arcanist_or_libphutil(
+  PhutilConsole $console,
+  ArcanistWorkingCopyIdentity $working_copy,
+  array $original_argv) {
+
+  $project_id = $working_copy->getProjectID();
+  if ($project_id != 'arcanist' && $project_id != 'libphutil') {
+    // We're not in a copy of arcanist or libphutil.
+    return;
+  }
+
+  $library_names = array(
+    'arcanist'  => 'arcanist',
+    'libphutil' => 'phutil',
+  );
+
+  $library_root = phutil_get_library_root($library_names[$project_id]);
+  $project_root = $working_copy->getProjectRoot();
+  if (Filesystem::isDescendant($library_root, $project_root)) {
+    // We're in a copy of arcanist or libphutil, but already loaded the correct
+    // copy. Continue execution normally.
+    return;
+  }
+
+  if ($project_id == 'libphutil') {
+    $console->writeLog(
+      "This is libphutil! Forcing this copy to load...\n");
+    $original_argv[0] = dirname(phutil_get_library_root('arcanist')).'/bin/arc';
+    $libphutil_path = $project_root;
+  } else {
+    $console->writeLog(
+      "This is arcanist! Forcing this copy to run...\n");
+    $original_argv[0] = $project_root.'/bin/arc';
+    $libphutil_path = dirname(phutil_get_library_root('phutil'));
+  }
+
+  $err = phutil_passthru(
+    phutil_is_windows()
+      ? 'set ARC_PHUTIL_PATH=%s & %Ls'
+      : 'ARC_PHUTIL_PATH=%s %Ls',
+    $libphutil_path,
+    $original_argv);
+
+  exit($err);
+}