Handle an invalid $XDG_RUNTIME_DIR gracefully

This is probably against the XDG basedir spec, but some people have a
Linux without anything setting $XDG_RUNTIME_DIR correctly.

Fixes #971.

CHANGELOG.asciidoc

@@ -187,6 +187,8 @@ Fixed
 - `:tab-detach` now fails correctly when there's only one tab open.
 - Various small issues with the command completion
 - Fixed hang when using multiple spaces in a row with the URL completion
+- qutebrowser now still starts with an incorrectly configured
+  `$XDG_RUNTIME_DIR`.
 
 v0.8.3
 ------

qutebrowser/utils/standarddir.py

@@ -33,6 +33,11 @@ from qutebrowser.utils import log, qtutils, debug
 _args = None
 
 
+class EmptyValueError(Exception):
+
+    """Error raised when QStandardPaths returns an empty value."""
+
+
 def config():
     """Get a location for configs."""
     typ = QStandardPaths.ConfigLocation
@@ -104,9 +109,18 @@ def runtime():
     else:  # pragma: no cover
         # RuntimeLocation is a weird path on OS X and Windows.
         typ = QStandardPaths.TempLocation
+
     overridden, path = _from_args(typ, _args)
+
     if not overridden:
-        path = _writable_location(typ)
+        try:
+            path = _writable_location(typ)
+        except EmptyValueError:
+            # Fall back to TempLocation when RuntimeLocation is misconfigured
+            if typ == QStandardPaths.TempLocation:
+                raise
+            path = _writable_location(QStandardPaths.TempLocation)
+
         # This is generic, but per-user.
         #
         # For TempLocation:
@@ -128,7 +142,7 @@ def _writable_location(typ):
     typ_str = debug.qenum_key(QStandardPaths, typ)
     log.misc.debug("writable location for {}: {}".format(typ_str, path))
     if not path:
-        raise ValueError("QStandardPaths returned an empty value!")
+        raise EmptyValueError("QStandardPaths returned an empty value!")
     # Qt seems to use '/' as path separator even on Windows...
     path = path.replace('/', os.sep)
     return path

tests/unit/utils/test_standarddir.py

@@ -90,7 +90,7 @@ class TestWritableLocation:
         monkeypatch.setattr(
             'qutebrowser.utils.standarddir.QStandardPaths.writableLocation',
             lambda typ: '')
-        with pytest.raises(ValueError):
+        with pytest.raises(standarddir.EmptyValueError):
             standarddir._writable_location(QStandardPaths.DataLocation)
 
     def test_sep(self, monkeypatch):
@@ -137,6 +137,23 @@ class TestStandardDir:
             monkeypatch.delenv('XDG_{}_HOME'.format(var), raising=False)
         assert func() == str(tmpdir.join(*subdirs))
 
+    @pytest.mark.linux
+    @pytest.mark.qt_log_ignore(r'^QStandardPaths: XDG_RUNTIME_DIR points to '
+                               r'non-existing path')
+    def test_linux_invalid_runtimedir(self, monkeypatch, tmpdir):
+        """With invalid XDG_RUNTIME_DIR, fall back to TempLocation."""
+        monkeypatch.setenv('XDG_RUNTIME_DIR', str(tmpdir / 'does-not-exist'))
+        monkeypatch.setenv('TMPDIR', str(tmpdir / 'temp'))
+        assert standarddir.runtime() == str(tmpdir / 'temp' / 'qute_test')
+
+    def test_runtimedir_empty_tempdir(self, monkeypatch, tmpdir):
+        """With an empty tempdir on non-Linux, we should raise."""
+        monkeypatch.setattr(standarddir.sys, 'platform', 'nt')
+        monkeypatch.setattr(standarddir.QStandardPaths, 'writableLocation',
+                            lambda typ: '')
+        with pytest.raises(standarddir.EmptyValueError):
+            standarddir.runtime()
+
     @pytest.mark.parametrize('func, elems, expected', [
         (standarddir.data, 2, ['qute_test', 'data']),
         (standarddir.config, 1, ['qute_test']),