bugs-fixed-since: support --easy-backport argument

This argument will make the tool to ignore master patches that don't
cherry-pick cleanly into each stable branch. This is sometimes useful to
distinguish between backport candidates that are easy to process, and
those that will require some more work to resolve git conflicts.

Change-Id: I9f0d1803fc753bad4e31dbd14244f18ba1672625

README.rst

@@ -955,6 +955,13 @@ Example::
 
   ./bugs-fixed-since.py -B -r ../neutron --start=8.0.0
 
+Use ``-e`` option to ignore patches that don't apply cleanly to one of stable
+branches.
+
+Example::
+
+  ./bugs-fixed-since.py -e -r ../neutron --start=8.0.0
+
 
 lp-filter-bugs-by-importance.py
 -------------------------------

bugs-fixed-since.py

@@ -19,6 +19,7 @@ This tool will list bugs that were fixed in project master.
 import argparse
 import re
 
+from git import cmd
 from git import Repo
 
 
@@ -42,6 +43,12 @@ def _parse_args():
         action='store_true',
         help='whether to skip patches backported to all stable branches',
     )
+    parser.add_argument(
+        '--easy-backport', '-e',
+        action='store_true',
+        default=False,
+        help='whether to include easy (no git conflicts) backports only',
+    )
     return parser.parse_args()
 
 
@@ -62,6 +69,28 @@ def _extract_changeid(commit):
         return id_
 
 
+def _is_easy_backport(repo, commit):
+    g_cmd = cmd.Git(working_dir=repo.working_tree_dir)
+    for ref in repo.refs:
+        # consider a patch easy to backport if only it cleanly applies to all
+        # stable branches; otherwise it will potentially require more work to
+        # resolve git conflicts
+        if ref.name.startswith('origin/stable/'):
+            # before applying any patches, make sure the tree is clean and
+            # fully reflects remote head
+            g_cmd.clean(force=True, d=True, x=True)
+            g_cmd.reset(hard=True)
+            g_cmd.checkout(ref.name)
+            try:
+                g_cmd.cherry_pick(commit.hexsha)
+            except cmd.GitCommandError:
+                # cherry-pick does not have a 'dry run' mode, so we need to
+                # actually clean up after a failure
+                g_cmd.cherry_pick(abort=True)
+                return False
+    return True
+
+
 def main():
     args = _parse_args()
 
@@ -87,6 +116,11 @@ def main():
                 _backported_to_all_stable_branches(repo, id_)):
             continue
 
+        # skip patches that result in git conflicts in any of stable branches
+        if (args.easy_backport and
+                not _is_easy_backport(repo, commit)):
+            continue
+
         # collect every bug number mentioned in the message
         for match in re.finditer(BUG_PATTERN, commit.message):
             bugs.add(match.group('bugnum'))