ci(v8): gate Windows source builds on relevant changes (#27715)

Avoid rebuilding sandboxed Windows MSVC V8 artifacts for unrelated
changes to `codex-rs/Cargo.toml`.

The V8 canary now compares the resolved V8 version between the base and
head commits and only runs the Windows source-build matrix when:

- the resolved V8 crate version changes;
- Windows artifact-production scripts or workflows change; or
- the workflow is manually dispatched.

The existing Bazel V8 matrix is unchanged.

## Why

The Windows MSVC source builds take roughly two to three hours and
currently run whenever any entry in the broad `v8-canary` path filter
changes.

.github/scripts/test_v8_canary_changes.py

@@ -0,0 +1,98 @@
+import subprocess
+import tempfile
+import unittest
+from pathlib import Path
+
+from v8_canary_changes import changed_files
+from v8_canary_changes import merge_base
+from v8_canary_changes import resolved_v8_version
+from v8_canary_changes import windows_source_required
+
+
+class V8CanaryChangesTest(unittest.TestCase):
+    def test_resolved_v8_version(self) -> None:
+        cargo_lock = b"""\
+[[package]]
+name = "other"
+version = "1.0.0"
+
+[[package]]
+name = "v8"
+version = "149.2.0"
+"""
+
+        self.assertEqual(resolved_v8_version(cargo_lock), "149.2.0")
+
+    def test_unrelated_cargo_manifest_change_does_not_require_source_build(
+        self,
+    ) -> None:
+        self.assertFalse(
+            windows_source_required(
+                {"codex-rs/Cargo.toml"},
+                "149.2.0",
+                "149.2.0",
+            )
+        )
+
+    def test_v8_version_change_requires_source_build(self) -> None:
+        self.assertTrue(windows_source_required(set(), "149.2.0", "150.0.0"))
+
+    def test_module_helper_change_requires_source_build(self) -> None:
+        self.assertTrue(
+            windows_source_required(
+                {".github/scripts/rusty_v8_module_bazel.py"},
+                "149.2.0",
+                "149.2.0",
+            )
+        )
+
+    def test_manual_dispatch_requires_source_build(self) -> None:
+        self.assertTrue(
+            windows_source_required(
+                set(),
+                "149.2.0",
+                "149.2.0",
+                force=True,
+            )
+        )
+
+    def test_changed_files_excludes_changes_made_only_on_base_branch(self) -> None:
+        with tempfile.TemporaryDirectory() as temp_dir:
+            root = Path(temp_dir)
+            self.run_git(root, "init", "--initial-branch=main")
+            self.run_git(root, "config", "user.name", "Test User")
+            self.run_git(root, "config", "user.email", "test@example.com")
+
+            self.write_and_commit(root, "initial", "initial.txt")
+            common = self.run_git(root, "rev-parse", "HEAD")
+            self.run_git(root, "switch", "-c", "feature")
+            self.run_git(root, "switch", "main")
+            self.write_and_commit(root, "base-only", "base-only.txt")
+            base = self.run_git(root, "rev-parse", "HEAD")
+
+            self.run_git(root, "switch", "feature")
+            self.write_and_commit(root, "feature-only", "feature-only.txt")
+            head = self.run_git(root, "rev-parse", "HEAD")
+
+            self.assertEqual(
+                changed_files(base, head, root=root),
+                {"feature-only.txt"},
+            )
+            self.assertEqual(merge_base(base, head, root=root), common)
+
+    def write_and_commit(self, root: Path, contents: str, path: str) -> None:
+        (root / path).write_text(contents)
+        self.run_git(root, "add", path)
+        self.run_git(root, "commit", "-m", contents)
+
+    def run_git(self, root: Path, *args: str) -> str:
+        return subprocess.check_output(
+            ["git", *args],
+            cwd=root,
+            stderr=subprocess.PIPE,
+            text=True,
+        ).strip()
+
+
+if __name__ == "__main__":
+    unittest.main()

.github/scripts/v8_canary_changes.py

@@ -0,0 +1,104 @@
+#!/usr/bin/env python3
+
+import argparse
+import subprocess
+import tomllib
+from pathlib import Path
+
+
+ROOT = Path(__file__).resolve().parents[2]
+WINDOWS_SOURCE_BUILD_PATHS = {
+    ".github/scripts/rusty_v8_bazel.py",
+    ".github/scripts/rusty_v8_module_bazel.py",
+    ".github/scripts/v8_canary_changes.py",
+    ".github/workflows/rusty-v8-release.yml",
+    ".github/workflows/v8-canary.yml",
+}
+
+
+def resolved_v8_version(cargo_lock: bytes) -> str:
+    versions = sorted(
+        {
+            package["version"]
+            for package in tomllib.loads(cargo_lock.decode())["package"]
+            if package["name"] == "v8"
+        }
+    )
+    if len(versions) != 1:
+        raise ValueError(f"expected exactly one resolved v8 version, found: {versions}")
+    return versions[0]
+
+
+def windows_source_required(
+    changed_files: set[str],
+    base_v8_version: str,
+    head_v8_version: str,
+    *,
+    force: bool = False,
+) -> bool:
+    return (
+        force
+        or base_v8_version != head_v8_version
+        or not changed_files.isdisjoint(WINDOWS_SOURCE_BUILD_PATHS)
+    )
+
+
+def git_output(*args: str, root: Path = ROOT) -> bytes:
+    return subprocess.check_output(["git", *args], cwd=root)
+
+
+def v8_version_at_revision(revision: str, *, root: Path = ROOT) -> str:
+    return resolved_v8_version(
+        git_output("show", f"{revision}:codex-rs/Cargo.lock", root=root)
+    )
+
+
+def merge_base(base: str, head: str, *, root: Path = ROOT) -> str:
+    return git_output("merge-base", base, head, root=root).decode().strip()
+
+
+def changed_files(base: str, head: str, *, root: Path = ROOT) -> set[str]:
+    output = git_output(
+        "diff",
+        "--name-only",
+        "--no-renames",
+        f"{base}...{head}",
+        root=root,
+    )
+    return set(output.decode().splitlines())
+
+
+def parse_args() -> argparse.Namespace:
+    parser = argparse.ArgumentParser()
+    parser.add_argument("--base")
+    parser.add_argument("--head")
+    parser.add_argument("--force", action="store_true")
+    return parser.parse_args()
+
+
+def main() -> None:
+    args = parse_args()
+    if args.force:
+        required = True
+        reason = "manual workflow dispatch"
+    elif not args.base or not args.head:
+        raise SystemExit("--base and --head are required unless --force is set")
+    else:
+        files = changed_files(args.base, args.head)
+        base_version = v8_version_at_revision(merge_base(args.base, args.head))
+        head_version = v8_version_at_revision(args.head)
+        required = windows_source_required(files, base_version, head_version)
+        if base_version != head_version:
+            reason = f"v8 version changed from {base_version} to {head_version}"
+        else:
+            matched_paths = sorted(files & WINDOWS_SOURCE_BUILD_PATHS)
+            reason = (
+                ", ".join(matched_paths) if matched_paths else "no relevant changes"
+            )
+
+    print(f"windows_source_required={str(required).lower()}")
+    print(f"windows_source_reason={reason}")
+
+
+if __name__ == "__main__":
+    main()