Ignore /COMMIT_MSG in files matchers even more

We have mostly managed to ignore the /COMMIT_MSG in files matchers
(because it is unintuitive that it would be considered), but the recent
change to allow negated regexes in irrelevant-files exposed the fact
that it can still have an effect in that case.  The new feature hasn't
been used yet, so we could silently correct this, but a very similar
construction would be possible with the deprecated style of regex with
a negative lookahead.  Just in case someone wrote that, this change
includes a release note letting them know they can drop the /COMMIT_MSG
from their regex.

Change-Id: Ide04ed01224b5c0c48ab8d3c15ea7aef324cc42d

releasenotes/notes/commit-message-files-93e0233cfbed411d.yaml

@@ -0,0 +1,10 @@
+---
+fixes:
+  - |
+    The pseudo-file "/COMMIT_MSG" is not considered at all when
+    determining whether a change matches files or irrelevant-files
+    matchers.  It was previously not considered in most cases, but in
+    the case of a negated regular expression in an irrelevant-files
+    stanza, it could still have an effect.  This has been corrected
+    and any regexes that explicitly mention "/COMMIT_MSG" may remove
+    it.

tests/fixtures/layouts/files-negate.yaml

@@ -26,5 +26,5 @@
       jobs:
         - project-test-files:
             files:
-              - regex: ^(dontrun|README|/COMMIT_MSG)$
+              - regex: ^(dontrun|README)$
                 negate: true

zuul/change_matcher.py

@@ -18,10 +18,10 @@ This module defines classes used in matching changes based on job
 configuration.
 """
 
-import re
-
 from zuul.lib.re2util import ZuulRegex
 
+COMMIT_MSG = '/COMMIT_MSG'
+
 
 class AbstractChangeMatcher(object):
     """An abstract class that matches change attributes against regular
@@ -135,8 +135,6 @@ class AbstractMatcherCollection(AbstractChangeMatcher):
 
 class AbstractMatchFiles(AbstractMatcherCollection):
 
-    commit_regex = re.compile('^/COMMIT_MSG$')
-
     @property
     def regexes(self):
         for matcher in self.matchers:
@@ -148,18 +146,17 @@ class MatchAllFiles(AbstractMatchFiles):
     def matches(self, change):
         # NOTE(yoctozepto): make irrelevant files matcher match when
         # there are no files to check - return False (NB: reversed)
-        if not (hasattr(change, 'files') and change.files):
+        if not hasattr(change, 'files'):
             return False
-        if len(change.files) == 1 and self.commit_regex.match(change.files[0]):
+        files = [c for c in change.files if c != COMMIT_MSG]
+        if not files:
             return False
-        for file_ in change.files:
+        for file_ in files:
             matched_file = False
             for regex in self.regexes:
                 if regex.match(file_):
                     matched_file = True
                     break
-            if self.commit_regex.match(file_):
-                matched_file = True
             if not matched_file:
                 return False
         return True
@@ -170,11 +167,12 @@ class MatchAnyFiles(AbstractMatchFiles):
     def matches(self, change):
         # NOTE(yoctozepto): make files matcher match when
         # there are no files to check - return True
-        if not (hasattr(change, 'files') and change.files):
+        if not hasattr(change, 'files'):
             return True
-        if len(change.files) == 1 and self.commit_regex.match(change.files[0]):
+        files = [c for c in change.files if c != COMMIT_MSG]
+        if not files:
             return True
-        for file_ in change.files:
+        for file_ in files:
             for regex in self.regexes:
                 if regex.match(file_):
                     return True