Report stdout/stderr of failed subprocesses.

CHANGELOG.asciidoc

@@ -84,6 +84,8 @@ Changed
 - `general -> log-javascript-console` got changed from a boolean to an option
   taking a loglevel (`none`, `info`, `debug`).
 - `:tab-move +/-` now wraps around if `tabs -> wrap` is `true`.
+- When a subprocess (like launched by `:spawn`) fails, its stdout/stderr is now
+  logged to the console.
 
 Deprecated
 ~~~~~~~~~~

qutebrowser/misc/guiprocess.py

@@ -109,9 +109,18 @@ class GUIProcess(QObject):
                     self._what.capitalize()))
         else:
             assert status == QProcess.NormalExit
+            # We call this 'status' here as it makes more sense to the user -
+            # it's actually 'code'.
             message.error(self._win_id, "{} exited with status {}.".format(
                 self._what.capitalize(), code))
 
+            stderr = bytes(self._proc.readAllStandardError()).decode('utf-8')
+            stdout = bytes(self._proc.readAllStandardOutput()).decode('utf-8')
+            if stdout:
+                log.procs.error("Process stdout:\n" + stdout.strip())
+            if stderr:
+                log.procs.error("Process stderr:\n" + stderr.strip())
+
     @pyqtSlot()
     def on_started(self):
         """Called when the process started successfully."""

tests/unit/misc/test_guiprocess.py

@@ -93,6 +93,8 @@ def test_start_env(monkeypatch, qtbot, py_proc):
     argv = py_proc("""
         import os
         import json
+        import sys
+
         env = dict(os.environ)
         print(json.dumps(env))
         sys.exit(0)
@@ -201,3 +203,33 @@ def test_exit_unsuccessful(qtbot, proc, guiprocess_message_mock, py_proc):
 
     msg = guiprocess_message_mock.getmsg(guiprocess_message_mock.Level.error)
     assert msg.text == "Testprocess exited with status 1."
+
+
+@pytest.mark.not_frozen
+@pytest.mark.parametrize('stream', ['stdout', 'stderr'])
+def test_exit_unsuccessful_output(qtbot, proc, caplog, py_proc, stream):
+    """When a process fails, its output should be logged."""
+    with caplog.atLevel(logging.ERROR):
+        with qtbot.waitSignal(proc.finished, raising=True, timeout=10000):
+            proc.start(*py_proc("""
+                import sys
+                print("test", file=sys.{})
+                sys.exit(1)
+            """.format(stream)))
+    assert len(caplog.records) == 2
+    assert caplog.records[1].msg == 'Process {}:\ntest'.format(stream)
+
+
+@pytest.mark.parametrize('stream', ['stdout', 'stderr'])
+def test_exit_successful_output(qtbot, proc, py_proc, stream):
+    """When a process suceeds, no output should be logged.
+
+    The test doesn't actually check the log as it'd fail because of the error
+    logging.
+    """
+    with qtbot.waitSignal(proc.finished, raising=True, timeout=10000):
+        proc.start(*py_proc("""
+            import sys
+            print("test", file=sys.{})
+            sys.exit(0)
+        """.format(stream)))