Linter: Avoid processing empty streams

When a linter returns an empty stream, it should not be
processed.  In the case of process_output_corrected,
processing it is extremely destructive, zero-ing the
file.  In the other modes, there is no output to be
processed, so it is merely wasted processing.

Also emit warnings for any messages found on a
stream which is not being processed, or when
there was a non-zero exit code.

Followup enhancements might add ways for linter bears
to skip known messages where a linter is noisy, or
ignore known non-zero codes.

Fixes coala#2711
Fixes coala#5415

coalib/bearlib/abstractions/Linter.py

@@ -2,7 +2,7 @@
 from functools import partial, partialmethod
 import logging
 import inspect
-from itertools import chain, compress
+from itertools import chain
 import re
 import shutil
 from subprocess import check_call, CalledProcessError, DEVNULL
@@ -676,14 +676,42 @@ def run(self, filename=None, file=None, **kwargs):
                 self.debug("Running '{}'".format(
                     ' '.join(str(arg) for arg in arguments)))
 
-                output = run_shell_command(
+                result = run_shell_command(
                     arguments,
                     stdin=''.join(file) if options['use_stdin'] else None,
                     cwd=self.get_config_dir())
 
-                output = tuple(compress(
-                    output,
-                    (options['use_stdout'], options['use_stderr'])))
+                stdout, stderr = result
+                output = None
+                if options['use_stdout'] and options['use_stderr']:
+                    output = stdout, stderr
+                elif options['use_stdout']:
+                    if stdout:
+                        output = stdout,
+                    if stderr:
+                        logging.warning(
+                            '{}: Discarded stderr: {}'.format(
+                                self.__class__.__name__, stderr))
+
+                elif options['use_stderr']:
+                    if stderr:
+                        output = stderr,
+                    if stdout:
+                        logging.warning(
+                            '{}: Discarded stdout: {}'.format(
+                                self.__class__.__name__, stdout))
+
+                if result.code:
+                    logging.warning(
+                        '{}: Exit code {}'.format(
+                            self.__class__.__name__, result.code))
+
+                if not output:
+                    logging.warning(
+                        '{}: No output; skipping processing'.format(
+                            self.__class__.__name__))
+                    return []
+
                 if options['strip_ansi']:
                     output = tuple(map(strip_ansi, output))
                 if len(output) == 1:

tests/bearlib/abstractions/LinterTest.py

@@ -320,6 +320,36 @@ def create_arguments(filename, file, config_file):
         process_output_mock.assert_called_once_with(('hello stdout\n',
                                                      'hello stderr\n'), '', [])
 
+    def test_discarded_stream(self):
+        process_output_mock = Mock()
+        logging.getLogger().setLevel(logging.DEBUG)
+
+        class TestLinter:
+
+            @staticmethod
+            def process_output(output, filename, file):
+                process_output_mock(output, filename, file)
+
+            @staticmethod
+            def create_arguments(filename, file, config_file):
+                code = '\n'.join(['import sys',
+                                  "print('hello stderr', file=sys.stderr)"])
+                return '-c', code
+
+        uut = (linter(sys.executable, use_stdout=True)
+               (TestLinter)
+               (self.section, None))
+
+        with self.assertLogs('', level='DEBUG') as cm:
+            uut.run('', [])
+
+        process_output_mock.assert_not_called()
+
+        self.assertEqual(cm.output, [
+            'WARNING:root:TestLinter: Discarded stderr: hello stderr\n',
+            'WARNING:root:TestLinter: No output; skipping processing',
+            ])
+
     def test_strip_ansi(self):
         process_output_mock = Mock()