Add script to generate draft changelog entries (#16430)

The script format changelog entries based on commit history and has some
rules to filter out some changes, such as typeshed sync and changes
cherry-picked to the previous release branch.

Example of how to run it:
```
$ python misc/generate_changelog.py 1.7
Generating changelog for 1.7
Previous release was     1.6
Merge base: d7b2451
NOTE: Drop "Fix crash on ParamSpec unification (for real)", since it was in previous release branch
NOTE: Drop "Fix crash on ParamSpec unification", since it was in previous release branch
NOTE: Drop "Fix mypyc regression with pretty", since it was in previous release branch
NOTE: Drop "Clear cache when adding --new-type-inference", since it was in previous release branch
NOTE: Drop "Match note error codes to import error codes", since it was in previous release branch
NOTE: Drop "Make PEP 695 constructs give a reasonable error message", since it was in previous release branch
NOTE: Drop "Fix ParamSpec inference for callback protocols", since it was in previous release branch
NOTE: Drop "Try upgrading tox", since it was in previous release branch
NOTE: Drop "Optimize Unpack for failures", since it was in previous release branch

 * Fix crash on unpack call special-casing (Ivan Levkivskyi, PR [16381](#16381))
 * Fix file reloading in dmypy with --export-types (Ivan Levkivskyi, PR [16359](#16359))
 * Fix daemon crash caused by deleted submodule (Jukka Lehtosalo, PR [16370](#16370))
...
```

misc/generate_changelog.py

@@ -0,0 +1,201 @@
+"""Generate the changelog for a mypy release."""
+
+from __future__ import annotations
+
+import argparse
+import re
+import subprocess
+import sys
+from dataclasses import dataclass
+
+
+def find_all_release_branches() -> list[tuple[int, int]]:
+    result = subprocess.run(["git", "branch", "-r"], text=True, capture_output=True, check=True)
+    versions = []
+    for line in result.stdout.splitlines():
+        line = line.strip()
+        if m := re.match(r"origin/release-([0-9]+)\.([0-9]+)$", line):
+            major = int(m.group(1))
+            minor = int(m.group(2))
+            versions.append((major, minor))
+    return versions
+
+
+def git_merge_base(rev1: str, rev2: str) -> str:
+    result = subprocess.run(
+        ["git", "merge-base", rev1, rev2], text=True, capture_output=True, check=True
+    )
+    return result.stdout.strip()
+
+
+@dataclass
+class CommitInfo:
+    commit: str
+    author: str
+    title: str
+    pr_number: int | None
+
+
+def normalize_author(author: str) -> str:
+    # Some ad-hoc rules to get more consistent author names.
+    if author == "AlexWaygood":
+        return "Alex Waygood"
+    elif author == "jhance":
+        return "Jared Hance"
+    return author
+
+
+def git_commit_log(rev1: str, rev2: str) -> list[CommitInfo]:
+    result = subprocess.run(
+        ["git", "log", "--pretty=%H\t%an\t%s", f"{rev1}..{rev2}"],
+        text=True,
+        capture_output=True,
+        check=True,
+    )
+    commits = []
+    for line in result.stdout.splitlines():
+        commit, author, title = line.strip().split("\t", 2)
+        pr_number = None
+        if m := re.match(r".*\(#([0-9]+)\) *$", title):
+            pr_number = int(m.group(1))
+            title = re.sub(r" *\(#[0-9]+\) *$", "", title)
+
+        author = normalize_author(author)
+        entry = CommitInfo(commit, author, title, pr_number)
+        commits.append(entry)
+    return commits
+
+
+def filter_omitted_commits(commits: list[CommitInfo]) -> list[CommitInfo]:
+    result = []
+    for c in commits:
+        title = c.title
+        keep = True
+        if title.startswith("Sync typeshed"):
+            # Typeshed syncs aren't mentioned in release notes
+            keep = False
+        if title.startswith(
+            (
+                "Revert sum literal integer change",
+                "Remove use of LiteralString in builtins",
+                "Revert typeshed ctypes change",
+                "Revert use of `ParamSpec` for `functools.wraps`",
+            )
+        ):
+            # These are generated by a typeshed sync.
+            keep = False
+        if re.search(r"(bump|update).*version.*\+dev", title.lower()):
+            # Version number updates aren't mentioned
+            keep = False
+        if "pre-commit autoupdate" in title:
+            keep = False
+        if title.startswith(("Update commit hashes", "Update hashes")):
+            # Internal tool change
+            keep = False
+        if keep:
+            result.append(c)
+    return result
+
+
+def normalize_title(title: str) -> str:
+    # We sometimes add a title prefix when cherry-picking commits to a
+    # release branch. Attempt to remove these prefixes so that we can
+    # match them to the corresponding master branch.
+    if m := re.match(r"\[release [0-9.]+\] *", title, flags=re.I):
+        title = title.replace(m.group(0), "")
+    return title
+
+
+def filter_out_commits_from_old_release_branch(
+    new_commits: list[CommitInfo], old_commits: list[CommitInfo]
+) -> list[CommitInfo]:
+    old_titles = {normalize_title(commit.title) for commit in old_commits}
+    result = []
+    for commit in new_commits:
+        drop = False
+        if normalize_title(commit.title) in old_titles:
+            drop = True
+        if normalize_title(f"{commit.title} (#{commit.pr_number})") in old_titles:
+            drop = True
+        if not drop:
+            result.append(commit)
+        else:
+            print(f'NOTE: Drop "{commit.title}", since it was in previous release branch')
+    return result
+
+
+def find_changes_between_releases(old_branch: str, new_branch: str) -> list[CommitInfo]:
+    merge_base = git_merge_base(old_branch, new_branch)
+    print(f"Merge base: {merge_base}")
+    new_commits = git_commit_log(merge_base, new_branch)
+    old_commits = git_commit_log(merge_base, old_branch)
+
+    # Filter out some commits that won't be mentioned in release notes.
+    new_commits = filter_omitted_commits(new_commits)
+
+    # Filter out commits cherry-picked to old branch.
+    new_commits = filter_out_commits_from_old_release_branch(new_commits, old_commits)
+
+    return new_commits
+
+
+def format_changelog_entry(c: CommitInfo) -> str:
+    """
+    s = f" * {c.commit[:9]} - {c.title}"
+    if c.pr_number:
+        s += f" (#{c.pr_number})"
+    s += f" ({c.author})"
+    """
+    s = f" * {c.title} ({c.author}"
+    if c.pr_number:
+        s += f", PR [{c.pr_number}](https://github.com/python/mypy/pull/{c.pr_number})"
+    s += ")"
+
+    return s
+
+
+def main() -> None:
+    parser = argparse.ArgumentParser()
+    parser.add_argument("version", help="target mypy version (form X.Y)")
+    parser.add_argument("--local", action="store_true")
+    args = parser.parse_args()
+    version: str = args.version
+    local: bool = args.local
+
+    if not re.match(r"[0-9]+\.[0-9]+$", version):
+        sys.exit(f"error: Release must be of form X.Y (not {version!r})")
+    major, minor = (int(component) for component in version.split("."))
+
+    if not local:
+        print("Running 'git fetch' to fetch all release branches...")
+        subprocess.run(["git", "fetch"], check=True)
+
+    if minor > 0:
+        prev_major = major
+        prev_minor = minor - 1
+    else:
+        # For a x.0 release, the previous release is the most recent (x-1).y release.
+        all_releases = sorted(find_all_release_branches())
+        if (major, minor) not in all_releases:
+            sys.exit(f"error: Can't find release branch for {major}.{minor} at origin")
+        for i in reversed(range(len(all_releases))):
+            if all_releases[i][0] == major - 1:
+                prev_major, prev_minor = all_releases[i]
+                break
+        else:
+            sys.exit("error: Could not determine previous release")
+    print(f"Generating changelog for {major}.{minor}")
+    print(f"Previous release was     {prev_major}.{prev_minor}")
+
+    new_branch = f"origin/release-{major}.{minor}"
+    old_branch = f"origin/release-{prev_major}.{prev_minor}"
+
+    changes = find_changes_between_releases(old_branch, new_branch)
+
+    print()
+    for c in changes:
+        print(format_changelog_entry(c))
+
+
+if __name__ == "__main__":
+    main()