refactor(dev-infra): move common versioning tooling to shared location (#38656)

We initially added logic for determining active release trains into
the merge script. Given we now build more tools that rely on this
information, we move the logic into a more general "versioning" folder
that can contain common logic following the versioning document for the
Angular organization.

PR Close #38656

dev-infra/release/versioning/version-branches.ts

@@ -0,0 +1,89 @@
+/**
+ * @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 {GithubClient, GithubRepo} from '../../utils/git/github';
+
+/** Type describing a Github repository with corresponding API client. */
+export interface GithubRepoWithApi extends GithubRepo {
+  /** API client that can access the repository. */
+  api: GithubClient;
+}
+
+/** Type describing a version-branch. */
+export interface VersionBranch {
+  /** Name of the branch in Git. e.g. `10.0.x`. */
+  name: string;
+  /**
+   * Parsed SemVer version for the version-branch. Version branches technically do
+   * not follow the SemVer format, but we can have representative SemVer versions
+   * that can be used for comparisons, sorting and other checks.
+   */
+  parsed: semver.SemVer;
+}
+
+/** Regular expression that matches version-branches. */
+const versionBranchNameRegex = /(\d+)\.(\d+)\.x/;
+
+/** Gets the version of a given branch by reading the `package.json` upstream. */
+export async function getVersionOfBranch(
+    repo: GithubRepoWithApi, branchName: string): Promise<semver.SemVer> {
+  const {data} = await repo.api.repos.getContents(
+      {owner: repo.owner, repo: repo.name, path: '/package.json', ref: branchName});
+  const {version} = JSON.parse(Buffer.from(data.content, 'base64').toString());
+  const parsedVersion = semver.parse(version);
+  if (parsedVersion === null) {
+    throw Error(`Invalid version detected in following branch: ${branchName}.`);
+  }
+  return parsedVersion;
+}
+
+/** Whether the given branch corresponds to a version branch. */
+export function isVersionBranch(branchName: string): boolean {
+  return versionBranchNameRegex.test(branchName);
+}
+
+/**
+ * Converts a given version-branch into a SemVer version that can be used with SemVer
+ * utilities. e.g. to determine semantic order, extract major digit, compare.
+ *
+ * For example `10.0.x` will become `10.0.0` in SemVer. The patch digit is not
+ * relevant but needed for parsing. SemVer does not allow `x` as patch digit.
+ */
+export function getVersionForVersionBranch(branchName: string): semver.SemVer|null {
+  // Convert a given version-branch into a SemVer version that can be used
+  // with the SemVer utilities. i.e. to determine semantic order.
+  return semver.parse(branchName.replace(versionBranchNameRegex, '$1.$2.0'));
+}
+
+/**
+ * Gets the version branches for the specified major versions in descending
+ * order. i.e. latest version branches first.
+ */
+export async function getBranchesForMajorVersions(
+    repo: GithubRepoWithApi, majorVersions: number[]): Promise<VersionBranch[]> {
+  const {data: branchData} =
+      await repo.api.repos.listBranches({owner: repo.owner, repo: repo.name, protected: true});
+  const branches: VersionBranch[] = [];
+
+  for (const {name} of branchData) {
+    if (!isVersionBranch(name)) {
+      continue;
+    }
+    // Convert the version-branch into a SemVer version that can be used with the
+    // SemVer utilities. e.g. to determine semantic order, compare versions.
+    const parsed = getVersionForVersionBranch(name);
+    // Collect all version-branches that match the specified major versions.
+    if (parsed !== null && majorVersions.includes(parsed.major)) {
+      branches.push({name, parsed});
+    }
+  }
+
+  // Sort captured version-branches in descending order.
+  return branches.sort((a, b) => semver.rcompare(a.parsed, b.parsed));
+}