Enable progressive mode

Adds a new option --progressive that ease adoption of the linter as
it will only report failure if it detects an increase number
of violations since previous commit.

README.rst

@@ -94,6 +94,8 @@ The following is the output from ``ansible-lint --help``, providing an overview
       -q                    quieter, although not silent output
       -p                    parseable output in the format of pep8
       --parseable-severity  parseable output including severity of rule
+      --progressive         Return success if it detects a reduction in number of violations compared with
+                            previous git commit. This feature works only on git repository clones.
       -r RULESDIR           Specify custom rule directories. Add -R to keep using embedded rules from
                             /usr/local/lib/python3.8/site-packages/ansiblelint/rules
       -R                    Keep default rules when using -r
@@ -111,6 +113,23 @@ The following is the output from ``ansible-lint --help``, providing an overview
       -c CONFIG_FILE        Specify configuration file to use. Defaults to ".ansible-lint"
       --version             show program's version number and exit
 
+Progressive mode
+----------------
+
+In order to ease tool adoption, git users can enable the progressive mode using
+``--progressive`` option. This makes the linter return a success even if
+some failures are found, as long the total number of violations did not
+increase since the previous commit.
+
+As expected, this mode makes the linter run twice if it finds any violations.
+The second run is performed against a temporary git working copy that contains
+the previous commit. All the violations that were already present are removed
+from the list and the final result is displayed.
+
+The most notable benefit introduced by this mode it does not prevent merging
+new code while allowing developer to address historical violation at his own
+speed.
+
 CI/CD
 -----
 

lib/ansiblelint/__main__.py

@@ -24,7 +24,9 @@
 import logging
 import os
 import pathlib
+import subprocess
 import sys
+from contextlib import contextmanager
 from typing import TYPE_CHECKING, List, Set, Type
 
 from rich.markdown import Markdown
@@ -168,6 +170,35 @@ def main() -> int:
     # Assure we do not print duplicates and the order is consistent
     matches = sorted(set(matches))
 
+    mark_as_success = False
+    if matches and options.progressive:
+        _logger.info(
+            "Matches found, running again on previos revision in order to detect regressions")
+        with _previous_revision():
+            if not options.playbook:
+                # no args triggers auto-detection mode
+                playbooks = get_playbooks_and_roles(options=options)
+            else:
+                playbooks = sorted(set(options.playbook))
+
+            old_matches = list()
+            checked_files: Set[str] = set()
+            for playbook in playbooks:
+                runner = Runner(rules, playbook, options.tags,
+                                options.skip_list, options.exclude_paths,
+                                options.verbosity, checked_files)
+                old_matches.extend(runner.run())
+            # remove old matches from current list
+            if len(old_matches) > len(matches):
+                _logger.warning(
+                    "Total violation(s) reducted from %s to %s since previous "
+                    "commit, will mark result as success",
+                    len(old_matches), len(matches))
+                mark_as_success = True
+            current_len = len(matches)
+            matches = set(matches) - set(old_matches)
+            _logger.warning("Removed %s previously known violation(s)", current_len - len(matches))
+
     for match in matches:
         print(formatter.format(match, options.colored))
 
@@ -177,12 +208,33 @@ def main() -> int:
         for match in matches:
             print(formatter.format(match))
 
-    if matches:
+    if matches and not mark_as_success:
         return report_outcome(matches, options=options)
     else:
         return 0
 
 
+@contextmanager
+def _previous_revision():
+    """Create or updates a temporary workdir containing the previous revious."""
+    worktree_dir = ".cache/old-rev"
+    try:
+        revision = subprocess.run(
+            ["git", "rev-parse", "HEAD^1"],
+            check=True,
+            universal_newlines=True).stdout
+        p = pathlib.Path(worktree_dir)
+        p.mkdir(parents=True, exist_ok=True)
+        os.system(f"git worktree add -f {worktree_dir}")
+        os.chdir(worktree_dir)
+        os.system(f"git checkout {revision}")
+        yield
+    except Exception:
+        raise
+    else:
+        os.chdir("../..")
+
+
 if __name__ == "__main__":
     try:
         sys.exit(main())

lib/ansiblelint/cli.py

@@ -111,6 +111,12 @@ def get_cli_parser() -> argparse.ArgumentParser:
                         default=False,
                         action='store_true',
                         help="parseable output including severity of rule")
+    parser.add_argument('--progressive', dest='progressive',
+                        default=False,
+                        action='store_true',
+                        help="Return success if it detects a reduction in number"
+                        " of violations compared with previous git commit. This "
+                        "feature works only on git repository clones.")
     parser.add_argument('-r', action=AbspathArgAction, dest='rulesdir',
                         default=[], type=Path,
                         help="Specify custom rule directories. Add -R "