phabricator: do not look up .arcconfig across home directory

Summary:
Defend against random files outside `HOME` (`TESTTMP`) like `/tmp/.arcconfig`
affecting behavior. The idea is similar to `GIT_CEILING_DIRECTORIES`.

Reviewed By: lmvasquezg

Differential Revision: D61672165

fbshipit-source-id: 1a7b40aa786f696d2241488eb9ac6ee74bd542ea

eden/scm/sapling/ext/extlib/phabricator/arcconfig.py

@@ -69,8 +69,11 @@ def loadforpath(path):
             userconfig["_arcconfig_path"] = path
             return userconfig
         parent = os.path.dirname(path)
-        if parent == path:
-            # We have reached a root (e.g. "C:\\" on Windows).
+        if parent == path or path == homedir:
+            # We have reached a root (e.g. "C:\\" on Windows),
+            # or reached at homedir (set to TESTTMP in tests),
+            # do not look up further. This avoids random files
+            # in TMP affects test behavior unexpectedly.
             break
         path = parent
 

eden/scm/tests/test-fb-ext-arcconfig.t

@@ -32,3 +32,16 @@
   $ echo '{"user": true}' > $HOME/.arcrc
   $ hg debugarcconfig
   {"_arcconfig_path": "$TESTTMP/repo", "hello": "world", "user": true}
+
+# .arcconfig lookup is scoped at $HOME
+
+  $ cd
+  $ mkdir -p x/y
+  $ echo '{"foo": "bar"}' > x/.arcconfig
+  $ cd x/y
+  $ hg init
+  $ hg debugarcconfig
+  {"_arcconfig_path": "$TESTTMP/x", "foo": "bar", "user": true}
+  $ HOME=$PWD hg debugarcconfig
+  abort: no .arcconfig found
+  [255]