[gcc r12-1601] mklog: add subject line skeleton

[Jason Merrill <jason@gcc.gnu.org>]

https://gcc.gnu.org/g:58e3b17f4c42d050a1768b025712e6d18bcb76ae

commit r12-1601-g58e3b17f4c42d050a1768b025712e6d18bcb76ae
Author: Jason Merrill <jason@redhat.com>
Date:   Wed Jun 16 16:46:38 2021 -0400

    mklog: add subject line skeleton
    
    In the recent gcc-commit-mklog thread on gcc@ it occurred to me that the
    command could also fill in part of the subject line.  If the first PR is
    foo/1234, and the commit does not yet have a subject line, this will add
    
    foo: [PR1234]
    
    contrib/ChangeLog:
    
            * mklog.py: Add an initial component: [PRnnnnn] line when
            we have a PR.

Diff:
---
 contrib/mklog.py | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/contrib/mklog.py b/contrib/mklog.py
index 5c93c707128..1f59055e723 100755
--- a/contrib/mklog.py
+++ b/contrib/mklog.py
@@ -39,6 +39,7 @@ import requests
 from unidiff import PatchSet
 
 pr_regex = re.compile(r'(\/(\/|\*)|[Cc*!])\s+(?P<pr>PR [a-z+-]+\/[0-9]+)')
+prnum_regex = re.compile(r'PR (?P<comp>[a-z+-]+)/(?P<num>[0-9]+)')
 dr_regex = re.compile(r'(\/(\/|\*)|[Cc*!])\s+(?P<dr>DR [0-9]+)')
 dg_regex = re.compile(r'{\s+dg-(error|warning)')
 identifier_regex = re.compile(r'^([a-zA-Z0-9_#].*)')
@@ -67,6 +68,8 @@ PATCH must be generated using diff(1)'s -up or -cp options
 script_folder = os.path.realpath(__file__)
 root = os.path.dirname(os.path.dirname(script_folder))
 
+firstpr = ''
+
 
 def find_changelog(path):
     folder = os.path.split(path)[0]
@@ -134,6 +137,7 @@ def generate_changelog(data, no_functions=False, fill_pr_titles=False):
     prs = []
     out = ''
     diff = PatchSet(data)
+    global firstpr
 
     for file in diff:
         # skip files that can't be parsed
@@ -166,6 +170,9 @@ def generate_changelog(data, no_functions=False, fill_pr_titles=False):
                         # Found dg-warning/dg-error line
                         break
 
+    if prs:
+        firstpr = prs[0]
+
     if fill_pr_titles:
         out += get_pr_titles(prs)
 
@@ -308,8 +315,14 @@ if __name__ == '__main__':
             start = list(takewhile(lambda l: not l.startswith('#'), lines))
             end = lines[len(start):]
             with open(args.changelog, 'w') as f:
+                if not start or not start[0]:
+                    # initial commit subject line 'component: [PRnnnnn]'
+                    m = prnum_regex.match(firstpr)
+                    if m:
+                        title = f'{m.group("comp")}: [PR{m.group("num")}]'
+                        start.insert(0, title)
                 if start:
-                    # appent empty line
+                    # append empty line
                     if start[-1] != '':
                         start.append('')
                 else: