feat(dev-infra): expose script for determining merge branches (#37217)

The components repo and framework repository follow the same patch
branch concept. We should be able to share a script for determining
these merge branches.

Additonally the logic has been improved compared to the old merge script because
we no longer consult `git ls-remote` unless really needed. Currently,
`git ls-remote` is always consulted, even though not necessarily needed.

This can slow down the merge script and the caretaker process when a
couple of PRs are merged (personally saw around ~4 seconds per merge).

Additionally, the new logic is more strict and will ensure (in most
cases) that no wrong patch/minor branch is determined. Previously,
the script just used the lexicographically greatest patch branch.
This _could_ be wrong when a new patch branch has been created too
early, or by accident.

PR Close #37217

dev-infra/pr/merge/BUILD.bazel

@@ -11,6 +11,7 @@ ts_library(
         "@npm//@octokit/rest",
         "@npm//@types/inquirer",
         "@npm//@types/node",
+        "@npm//@types/semver",
         "@npm//@types/yargs",
         "@npm//chalk",
     ],

dev-infra/pr/merge/determine-merge-branches.ts

@@ -0,0 +1,68 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import * as semver from 'semver';
+import {exec} from '../../utils/shelljs';
+
+/**
+ * Helper function that can be used to determine merge branches based on a given
+ * project version. The function determines merge branches primarily through the
+ * specified version, but falls back to consulting the NPM registry when needed.
+ *
+ * Consulting the NPM registry for determining the patch branch may slow down merging,
+ * so whenever possible, the branches are determined statically based on the current
+ * version. In some cases, consulting the NPM registry is inevitable because for major
+ * pre-releases, we cannot determine the latest stable minor version from the current
+ * pre-release version.
+ */
+export function determineMergeBranches(
+    currentVersion: string, npmPackageName: string): {minor: string, patch: string} {
+  const projectVersion = semver.parse(currentVersion);
+  if (projectVersion === null) {
+    throw Error('Cannot parse version set in project "package.json" file.');
+  }
+  const {major, minor, patch, prerelease} = projectVersion;
+  const isMajor = minor === 0 && patch === 0;
+  const isMinor = minor !== 0 && patch === 0;
+
+  // If there is no prerelease, then we compute patch and minor branches based
+  // on the current version major and minor.
+  if (prerelease.length === 0) {
+    return {minor: `${major}.x`, patch: `${major}.${minor}.x`};
+  }
+
+  // If current version is set to a minor prerelease, we can compute the merge branches
+  // statically. e.g. if we are set to `9.3.0-next.0`, then our merge branches should
+  // be set to `9.x` and `9.2.x`.
+  if (isMinor) {
+    return {minor: `${major}.x`, patch: `${major}.${minor - 1}.x`};
+  } else if (!isMajor) {
+    throw Error('Unexpected version. Cannot have prerelease for patch version.');
+  }
+
+  // If we are set to a major prerelease, we cannot statically determine the stable patch
+  // branch (as the latest minor segment is unknown). We determine it by looking in the NPM
+  // registry for the latest stable release that will tell us about the current minor segment.
+  // e.g. if the current major is `v10.0.0-next.0`, then we need to look for the latest release.
+  // Let's say this is `v9.2.6`. Our patch branch will then be called `9.2.x`.
+  const latestVersion = exec(`yarn -s info ${npmPackageName} dist-tags.latest`).trim();
+  if (!latestVersion) {
+    throw Error('Could not determine version of latest release.');
+  }
+  const expectedMajor = major - 1;
+  const parsedLatestVersion = semver.parse(latestVersion);
+  if (parsedLatestVersion === null) {
+    throw Error(`Could not parse latest version from NPM registry: ${latestVersion}`);
+  } else if (parsedLatestVersion.major !== expectedMajor) {
+    throw Error(
+        `Expected latest release to have major version: v${expectedMajor}, ` +
+        `but got: v${latestVersion}`);
+  }
+
+  return {patch: `${expectedMajor}.${parsedLatestVersion.minor}.x`, minor: `${expectedMajor}.x`};
+}

dev-infra/tmpl-package.json

@@ -16,6 +16,7 @@
     "inquirer": "<from-root>",
     "minimatch": "<from-root>",
     "multimatch": "<from-root>",
+    "semver": "<from-root>",
     "shelljs": "<from-root>",
     "typed-graphqlify": "<from-root>",
     "yaml": "<from-root>",