tools/validate-commit.py: enforce short summary followed by blank line

To keep git logs readable, enforce a short summary line. Strive for 80
characters, reject everything past 120. Descriptions should also stay
under 80 characters, but as error messages and links to protocol
specifications can be quite long, do not check the description.

Change-Id: Ife46b9ef6330aab015fc4ee5cc774a8ef6b9ad4e
Reviewed-on: https://code.wireshark.org/review/30667
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot
Reviewed-by: Anders Broman <a.broman58@gmail.com>

tools/validate-commit.py

@@ -96,6 +96,29 @@ def tools_dir():
 
 
 def verify_body(body):
+    old_lines = body.splitlines(True)
+    is_good = True
+    if len(old_lines) >= 2 and old_lines[1].strip():
+        print('ERROR: missing blank line after the first subject line.')
+        is_good = False
+    if len(old_lines[0]) > 80:
+        # Note that this is currently also checked by the commit-msg hook.
+        print('Warning: keep lines in the commit message under 80 characters.')
+        is_good = False
+    if not is_good:
+        print('''
+Please rewrite your commit message to our standards, matching this format:
+
+    component: a very brief summary of the change
+
+    A commit message should start with a brief summary, followed by a single
+    blank line and an optional longer description. If the change is specific to
+    a single protocol, start the summary line with the abbreviated name of the
+    protocol and a colon.
+
+    Use paragraphs to improve readability. Limit each line to 80 characters.
+
+''')
     fd, filename = tempfile.mkstemp()
     try:
         os.close(fd)
@@ -110,7 +133,7 @@ def verify_body(body):
             newbody = f.read()
     except OSError as ex:
         print('Warning: unable to invoke commit-msg hook: %s' % (ex,))
-        return True
+        return is_good
     except subprocess.CalledProcessError as ex:
         print('Bad commit message (reported by tools/commit-msg):')
         print(ex.output.strip())
@@ -118,7 +141,6 @@ def verify_body(body):
     finally:
         os.unlink(filename)
     if newbody != body:
-        old_lines = body.splitlines(True)
         new_lines = newbody.splitlines(True)
         diff = difflib.unified_diff(old_lines, new_lines,
                                     fromfile='OLD/.git/COMMIT_EDITMSG',
@@ -134,7 +156,7 @@ def verify_body(body):
         print('')
         print(''.join(diff))
         return False
-    return True
+    return is_good
 
 
 def main():