Relnotes python script

The old shell script no longer works in the LTS setup. This new script is written in Python so hopefully more maintainable, and reflects what we need to do in the post-LTS world.

I'll look into hooking this up into the release pipeline later.

Closes bazelbuild#16569.

PiperOrigin-RevId: 484510724
Change-Id: Id448b6325fcaffd295e8a7aae3fb6e9672a227df

scripts/release/relnotes.py

@@ -0,0 +1,105 @@
+# Copyright 2022 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Script to generate release notes."""
+
+import re
+import subprocess
+
+import requests
+
+
+def get_last_release():
+  """Discovers the last stable release name from GitHub."""
+  response = requests.get("https://github.com/bazelbuild/bazel/releases/latest")
+  return response.url.split("/")[-1]
+
+
+def git(*args):
+  """Runs git as a subprocess, and returns its stdout as a list of lines."""
+  return subprocess.check_output(["git"] +
+                                 list(args)).decode("utf-8").strip().split("\n")
+
+
+def extract_relnotes(commit_message_lines):
+  """Extracts relnotes from a commit message (passed in as a list of lines)."""
+  relnote_lines = []
+  in_relnote = False
+  for line in commit_message_lines:
+    if not line or line.startswith("PiperOrigin-RevId:"):
+      in_relnote = False
+    m = re.match(r"^RELNOTES(?:\[(INC|NEW)\])?:", line)
+    if m is not None:
+      in_relnote = True
+      line = line[len(m[0]):]
+      if m[1] == "INC":
+        line = "**[Incompatible]** " + line.strip()
+    line = line.strip()
+    if in_relnote and line:
+      relnote_lines.append(line)
+  relnote = " ".join(relnote_lines)
+  relnote_lower = relnote.strip().lower().rstrip(".")
+  if relnote_lower == "n/a" or relnote_lower == "none":
+    return None
+  return relnote
+
+
+def get_relnotes_between(base, head):
+  """Gets all relnotes for commits between `base` and `head`."""
+  commits = git("rev-list", f"{base}..{head}", "--grep=RELNOTES")
+  relnotes = []
+  rolled_back_commits = set()
+  # We go in reverse-chronological order, so that we can identify rollback
+  # commits and ignore the rolled-back commits.
+  for commit in commits:
+    if commit in rolled_back_commits:
+      continue
+    lines = git("show", "-s", commit, "--pretty=format:%B")
+    m = re.match(r"^Automated rollback of commit ([\dA-Fa-f]+)", lines[0])
+    if m is not None:
+      rolled_back_commits.add(m[1])
+      # The rollback commit itself is also skipped.
+      continue
+    relnote = extract_relnotes(lines)
+    if relnote is not None:
+      relnotes.append(relnote)
+  return relnotes
+
+
+if __name__ == "__main__":
+  # Get the last stable release.
+  last_release = get_last_release()
+  print("last_release is", last_release)
+  git("fetch", "origin", f"refs/tags/{last_release}:refs/tags/{last_release}")
+
+  # Assuming HEAD is on the current (to-be-released) release, find the merge
+  # base with the last release so that we know which commits to generate notes
+  # for.
+  merge_base = git("merge-base", "HEAD", last_release)[0]
+  print("merge base with", last_release, "is", merge_base)
+
+  # Generate notes for all commits from last branch cut to HEAD, but filter out
+  # any identical notes from the previous release branch.
+  cur_release_relnotes = get_relnotes_between(merge_base, "HEAD")
+  last_release_relnotes = set(get_relnotes_between(merge_base, last_release))
+  filtered_relnotes = [
+      note for note in cur_release_relnotes if note not in last_release_relnotes
+  ]
+
+  # Reverse so that the notes are in chronological order.
+  filtered_relnotes.reverse()
+  print()
+  print()
+  for note in filtered_relnotes:
+    print("*", note)