Add ability to show a list of tags

Usage:

```sh
./test-run.py --tags
./test-run.py app-tap/ --tags
```

Follows up #22

README.md

@@ -365,6 +365,13 @@ Usage:
 test-run will run only those tests, which have at least one of the
 provided tags.
 
+Show a list of tags:
+
+```sh
+./test-run.py --tags
+./test-run.py app-tap/ --tags
+```
+
 The tags metainfo should be placed within a first comment of a test
 file.
 

lib/options.py

@@ -33,6 +33,9 @@ class Options(object):
     _instance = None
     _initialized = False
 
+    # Just some unique marker.
+    _show_tags = {}
+
     def __new__(cls, *args, **kwargs):
         """Make the class singleton."""
         if cls._instance:
@@ -278,19 +281,37 @@ def __init__(self):
         parser.add_argument(
                 '--tags',
                 dest='tags',
+                const=self._show_tags,
+                nargs='?',
                 default=None,
                 type=split_list,
                 help="""Comma separated list of tags.
 
                 If tags are provided, test-run will run only those
                 tests, which are marked with ANY of the provided
                 tags.
+
+                If the option is given without a parameter (at the
+                last position), test-run will show a list of tags
+                and stop.
                 """)
 
         # XXX: We can use parser.parse_intermixed_args() on
         # Python 3.7 to understand commands like
         # ./test-run.py foo --exclude bar baz
         self.args = parser.parse_args()
+
+        # If `--tags foo,bar` is passed, just keep the list in
+        # `args.tags`.
+        #
+        # If `--tags` is passed without a parameter, clean up
+        # `args.tags` and toggle `args.show_tags`.
+        if self.args.tags == self._show_tags:
+            self.args.tags = None
+            self.args.show_tags = True
+        else:
+            self.args.show_tags = False
+
         self.check()
 
         Options._initialized = True

lib/test_suite.py

@@ -164,7 +164,8 @@ def collect_tests(self):
         else:
             raise ValueError('Cannot collect tests of unknown type')
 
-        if not Options().args.reproduce:
+        # In given cases, this large output looks redundant.
+        if not Options().args.reproduce and not Options().args.show_tags:
             color_stdout("Collecting tests in ", schema='ts_text')
             color_stdout(
                 '%s (Found %s tests)' % (

test-run.py

@@ -55,6 +55,7 @@
 
 from lib import Options
 from lib.colorer import color_stdout
+from lib.utils import find_tags
 from lib.utils import print_tail_n
 from lib.utils import PY3
 from lib.worker import get_task_groups
@@ -221,6 +222,19 @@ def main_consistent():
     return (-1 if failed_test_ids else 0)
 
 
+def show_tags():
+    # Collect tests in the same way as when we run them.
+    collected_tags = set()
+    for name, task_group in get_task_groups().items():
+        for task_id in task_group['task_ids']:
+            test_name, _ = task_id
+            for tag in find_tags(test_name):
+                collected_tags.add(tag)
+
+    for tag in sorted(collected_tags):
+        color_stdout(tag + '\n')
+
+
 if __name__ == "__main__":
     # In Python 3 start method 'spawn' in multiprocessing module becomes
     # default on Mac OS.
@@ -275,6 +289,10 @@ def open_as_utf8(*args, **kwargs):
 
     status = 0
 
+    if Options().args.show_tags:
+        show_tags()
+        exit(status)
+
     force_parallel = bool(Options().args.reproduce)
     if not force_parallel and Options().args.jobs == -1:
         status = main_consistent()