Avoid calling versions.webengine_versions() without QtWebEngine

Alternatively, it would have been possible to move the backend library
checking from backendproblem.py to earlyinit.py entirely. However, this
would lead to less user-friendly dialogs, as we can't e.g. offer a
button to switch the backend setting.

Fixes #6161

3 files changed, 73 insertions, 0 deletions

diff --git a/qutebrowser/config/qtargs.py b/qutebrowser/config/qtargs.py
index 7f971e2a0..66fa10339 100644
--- a/qutebrowser/config/qtargs.py
+++ b/qutebrowser/config/qtargs.py
@@ -58,6 +58,19 @@ def qt_args(namespace: argparse.Namespace) -> List[str]:
         assert objects.backend == usertypes.Backend.QtWebKit, objects.backend
         return argv
 
+    try:
+        from qutebrowser.browser.webengine import webenginesettings
+    except ImportError:
+        # This code runs before a QApplication is available, so before
+        # backendproblem.py is run to actually inform the user of the missing
+        # backend. Thus, we could end up in a situation where we're here, but
+        # QtWebEngine isn't actually available.
+        # We shouldn't call _qtwebengine_args() in this case as it relies on
+        # QtWebEngine actually being importable, e.g. in
+        # version.qtwebengine_versions().
+        log.init.debug("QtWebEngine requested, but unavailable...")
+        return argv
+
     special_prefixes = (_ENABLE_FEATURES, _DISABLE_FEATURES, _BLINK_SETTINGS)
     special_flags = [flag for flag in argv if flag.startswith(special_prefixes)]
     argv = [flag for flag in argv if not flag.startswith(special_prefixes)]
diff --git a/tests/end2end/test_invocations.py b/tests/end2end/test_invocations.py
index 30b998c02..6bf9fe623 100644
--- a/tests/end2end/test_invocations.py
+++ b/tests/end2end/test_invocations.py
@@ -23,6 +23,7 @@ import configparser
 import subprocess
 import sys
 import logging
+import importlib
 import re
 import json
 
@@ -660,3 +661,45 @@ def test_dark_mode(webengine_versions, quteproc_new, request,
     color = testutils.Color(img.pixelColor(pos))
     # For pytest debug output
     assert color == expected
+
+
+def test_unavailable_backend(request, quteproc_new):
+    """Test starting with a backend which isn't available.
+
+    If we use --qute-bdd-webengine, we test with QtWebKit here; otherwise we test with
+    QtWebEngine. If both are available, the test is skipped.
+
+    This ensures that we don't accidentally use backend-specific code before checking
+    that the chosen backend is actually available - i.e., that the error message is
+    properly printed, rather than an unhandled exception.
+    """
+    qtwe_module = "PyQt5.QtWebEngineWidgets"
+    qtwk_module = "PyQt5.QtWebKitWidgets"
+    # Note we want to try the *opposite* backend here.
+    if request.config.webengine:
+        pytest.importorskip(qtwe_module)
+        module = qtwk_module
+        backend = 'webkit'
+    else:
+        pytest.importorskip(qtwk_module)
+        module = qtwe_module
+        backend = 'webengine'
+
+    try:
+        importlib.import_module(module)
+    except ImportError:
+        pass
+    else:
+        pytest.skip(f"{module} is available")
+
+    args = [
+        '--debug', '--json-logging', '--no-err-windows',
+        '--backend', backend,
+        '--temp-basedir'
+    ]
+    quteproc_new.exit_expected = True
+    quteproc_new.start(args)
+    line = quteproc_new.wait_for(
+        message=('*qutebrowser tried to start with the Qt* backend but failed '
+                 'because * could not be imported.*'))
+    line.expected = True
diff --git a/tests/unit/config/test_qtargs.py b/tests/unit/config/test_qtargs.py
index 6bc1b2873..55acb4bb1 100644
--- a/tests/unit/config/test_qtargs.py
+++ b/tests/unit/config/test_qtargs.py
@@ -105,6 +105,23 @@ class TestQtArgs:
             assert arg in args
 
 
+def test_no_webengine_available(monkeypatch, config_stub, parser, stubs):
+    """Test that we don't fail if QtWebEngine is requested but unavailable.
+
+    Note this is not inside TestQtArgs because we don't want the reduce_args patching
+    here.
+    """
+    monkeypatch.setattr(qtargs.objects, 'backend', usertypes.Backend.QtWebEngine)
+    monkeypatch.setattr(qtargs.version, 'webenginesettings', None)
+
+    fake = stubs.ImportFake({'qutebrowser.browser.webengine': False}, monkeypatch)
+    fake.patch()
+
+    parsed = parser.parse_args([])
+    args = qtargs.qt_args(parsed)
+    assert args == [sys.argv[0]]
+
+
 @pytest.mark.usefixtures('reduce_args')
 class TestWebEngineArgs: