Use i- as prefix for the IPC path on macOS

With Qt 5.12, standarddir.runtime() gives us a path in /private/var/folders/...
instead of /var/folders/... like before. Due to that change, the path length is
105 chars, which is too long for a named socket (104 seems to be okay).

The complete name is just slightly too long, so using i- instead of ipc- fixes
things...

Fixes #4471
See #888

doc/changelog.asciidoc

@@ -62,6 +62,9 @@ Changed
   `org.qutebrowser.qutebrowser.appdata.xml`.
 - The `qute-pass` userscript now understands domains in gpg filenames
   in addition to directory names.
+- macOS: The IPC socket path used to communicate with existing instances
+  changed due to changes in Qt 5.12. Please make sure to quit qutebrowser
+  before upgrading.
 
 Fixed
 ~~~~~

qutebrowser/misc/ipc.py

@@ -65,11 +65,9 @@ def _get_socketname(basedir):
     data_to_hash = '-'.join(parts_to_hash).encode('utf-8')
     md5 = hashlib.md5(data_to_hash).hexdigest()
 
-    target_dir = standarddir.runtime()
-
-    parts = ['ipc']
-    parts.append(md5)
-    return os.path.join(target_dir, '-'.join(parts))
+    prefix = 'i-' if utils.is_mac else 'ipc-'
+    filename = '{}{}'.format(prefix, md5)
+    return os.path.join(standarddir.runtime(), filename)
 
 
 class Error(Exception):

tests/unit/misc/test_ipc.py

@@ -177,11 +177,6 @@ def md5(inp):
 
 class TestSocketName:
 
-    POSIX_TESTS = [
-        (None, 'ipc-{}'.format(md5('testusername'))),
-        ('/x', 'ipc-{}'.format(md5('testusername-/x'))),
-    ]
-
     WINDOWS_TESTS = [
         (None, 'qutebrowser-testusername'),
         ('/x', 'qutebrowser-testusername-{}'.format(md5('/x'))),
@@ -203,7 +198,10 @@ class TestSocketName:
         assert socketname == expected
 
     @pytest.mark.mac
-    @pytest.mark.parametrize('basedir, expected', POSIX_TESTS)
+    @pytest.mark.parametrize('basedir, expected', [
+        (None, 'i-{}'.format(md5('testusername'))),
+        ('/x', 'i-{}'.format(md5('testusername-/x'))),
+    ])
     def test_mac(self, basedir, expected):
         socketname = ipc._get_socketname(basedir)
         parts = socketname.split(os.sep)
@@ -211,7 +209,10 @@ class TestSocketName:
         assert parts[-1] == expected
 
     @pytest.mark.linux
-    @pytest.mark.parametrize('basedir, expected', POSIX_TESTS)
+    @pytest.mark.parametrize('basedir, expected', [
+        (None, 'ipc-{}'.format(md5('testusername'))),
+        ('/x', 'ipc-{}'.format(md5('testusername-/x'))),
+    ])
     def test_linux(self, basedir, fake_runtime_dir, expected):
         socketname = ipc._get_socketname(basedir)
         expected_path = str(fake_runtime_dir / 'qutebrowser' / expected)
@@ -630,8 +631,6 @@ class TestSendOrListen:
         assert ret_client is None
 
     @pytest.mark.posix(reason="Unneeded on Windows")
-    @pytest.mark.xfail(qtutils.version_check('5.12', compiled=False) and
-                       utils.is_mac, reason="Broken, see #4471")
     def test_correct_socket_name(self, args):
         server = ipc.send_or_listen(args)
         expected_dir = ipc._get_socketname(args.basedir)