Add flatpak hint to :spawn message

See https://www.reddit.com/r/qutebrowser/comments/qirb5k/editor_gvim_failed_to_start_execpv_no_such_file/

doc/changelog.asciidoc

@@ -15,6 +15,16 @@ breaking changes (such as renamed commands) can happen in minor releases.
 // `Fixed` for any bug fixes.
 // `Security` to invite users to upgrade in case of vulnerabilities.
 
+[[v2.5.0]]
+v2.5.0 (unreleased)
+-------------------
+
+Changed
+~~~~~~~
+
+- Improved message if a spawned process wasn't found and a Flatpak container is
+  in use.
+
 [[v2.4.1]]
 v2.4.1 (unreleased)
 -------------------

qutebrowser/misc/guiprocess.py

@@ -27,7 +27,7 @@
 from PyQt5.QtCore import (pyqtSlot, pyqtSignal, QObject, QProcess,
                           QProcessEnvironment, QByteArray, QUrl, Qt)
 
-from qutebrowser.utils import message, log, utils, usertypes
+from qutebrowser.utils import message, log, utils, usertypes, version
 from qutebrowser.api import cmdutils, apitypes
 from qutebrowser.completion.models import miscmodels
 
@@ -273,7 +273,9 @@ def _on_error(self, error: QProcess.ProcessError) -> None:
         known_errors = ['No such file or directory', 'Permission denied']
         if (': ' in error_string and  # pragma: no branch
                 error_string.split(': ', maxsplit=1)[1] in known_errors):
-            msg += f'\n(Hint: Make sure {self.cmd!r} exists and is executable)'
+            msg += f'\nHint: Make sure {self.cmd!r} exists and is executable'
+            if version.is_flatpak():
+                msg += ' inside the Flatpak container'
 
         message.error(msg)
 

tests/unit/misc/test_guiprocess.py

@@ -26,7 +26,7 @@
 from PyQt5.QtCore import QProcess, QUrl
 
 from qutebrowser.misc import guiprocess
-from qutebrowser.utils import usertypes, utils
+from qutebrowser.utils import usertypes, utils, version
 from qutebrowser.api import cmdutils
 from qutebrowser.qt import sip
 
@@ -394,8 +394,11 @@ def test_running(qtbot, proc, py_proc):
         proc.outcome.was_successful()
 
 
-def test_failing_to_start(qtbot, proc, caplog, message_mock):
+@pytest.mark.parametrize('is_flatpak', [True, False])
+def test_failing_to_start(qtbot, proc, caplog, message_mock, monkeypatch, is_flatpak):
     """Test the process failing to start."""
+    monkeypatch.setattr(version, 'is_flatpak', lambda: is_flatpak)
+
     with caplog.at_level(logging.ERROR, 'message'):
         with qtbot.wait_signal(proc.error, timeout=5000):
             proc.start('this_does_not_exist_either', [])
@@ -405,8 +408,11 @@ def test_failing_to_start(qtbot, proc, caplog, message_mock):
         "Testprocess 'this_does_not_exist_either' failed to start:")
 
     if not utils.is_windows:
-        assert msg.text.endswith(
-            "(Hint: Make sure 'this_does_not_exist_either' exists and is executable)")
+        expected_msg = (
+            "Hint: Make sure 'this_does_not_exist_either' exists and is executable")
+        if is_flatpak:
+            expected_msg += ' inside the Flatpak container'
+        assert msg.text.endswith(expected_msg)
 
     assert not proc.outcome.running
     assert proc.outcome.status is None