feat(github-actions): create a local branch manager github action

.github/local-actions/branch-manager/BUILD.bazel

@@ -0,0 +1,10 @@
+load("//tools:defaults.bzl", "esbuild_checked_in")
+
+esbuild_checked_in(
+    name = "main",
+    entry_point = "//.github/local-actions/branch-manager/lib:main.ts",
+    target = "node16",
+    deps = [
+        "//.github/local-actions/branch-manager/lib",
+    ],
+)

.github/local-actions/branch-manager/action.yml

@@ -0,0 +1,19 @@
+name: 'Branch Manager'
+description: 'Determine if a provided pull request is able to merge into its target branches'
+author: 'Angular'
+inputs:
+  angular-robot-key:
+    description: 'The private key for the Angular Robot Github app.'
+    required: true
+  pr:
+    description: 'The pull request number'
+    required: true
+  repo:
+    description: 'The name of the repo for the pull request'
+    required: true
+  owner:
+    description: 'The owner of the repo for the pull request'
+    required: true
+runs:
+  using: 'node16'
+  main: 'main.js'

.github/local-actions/branch-manager/lib/BUILD.bazel

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+load("//tools:defaults.bzl", "ts_library")
+
+package(default_visibility = ["//.github/local-actions/branch-manager:__subpackages__"])
+
+exports_files([
+    "main.ts",
+])
+
+ts_library(
+    name = "lib",
+    srcs = glob(
+        ["*.ts"],
+        exclude = ["*.spec.ts"],
+    ),
+    deps = [
+        "//github-actions:utils",
+        "//ng-dev/pr/common",
+        "//ng-dev/pr/common:labels",
+        "//ng-dev/pr/config",
+        "//ng-dev/pr/merge",
+        "//ng-dev/utils",
+        "@npm//@actions/core",
+        "@npm//@actions/github",
+        "@npm//@octokit/rest",
+        "@npm//@types/node",
+        "@npm//typed-graphqlify",
+    ],
+)

.github/local-actions/branch-manager/lib/main.ts

@@ -0,0 +1,169 @@
+import * as core from '@actions/core';
+import {PullRequestValidationConfig} from '../../../../ng-dev/pr/common/validation/validation-config.js';
+import {
+  assertValidPullRequestConfig,
+  PullRequestConfig,
+} from '../../../../ng-dev/pr/config/index.js';
+import {loadAndValidatePullRequest} from '../../../../ng-dev/pr/merge/pull-request.js';
+import {AutosquashMergeStrategy} from '../../../../ng-dev/pr/merge/strategies/autosquash-merge.js';
+import {
+  assertValidGithubConfig,
+  getConfig,
+  GithubConfig,
+  setConfig,
+} from '../../../../ng-dev/utils/config.js';
+import {AuthenticatedGitClient} from '../../../../ng-dev/utils/git/authenticated-git-client.js';
+import {
+  ANGULAR_ROBOT,
+  getAuthTokenFor,
+  revokeActiveInstallationToken,
+} from '../../../../github-actions/utils.js';
+import {MergeConflictsFatalError} from '../../../../ng-dev/pr/merge/failures.js';
+import {chdir} from 'process';
+import {spawnSync} from 'child_process';
+
+interface CommmitStatus {
+  state: 'pending' | 'error' | 'failure' | 'success';
+  description: string;
+}
+
+/** The directory name for the temporary repo used for validation. */
+const tempRepo = 'branch-mananger-repo';
+/** The context name used for the commmit status applied. */
+const statusContextName = 'mergeability';
+/** The branch used as the primary branch for the temporary repo. */
+const mainBranchName = 'main';
+
+async function main(repo: {owner: string; repo: string}, token: string, pr: number) {
+  // Because we want to perform this check in the targetted repository, we first need to check out the repo
+  // and then move to the directory it is cloned into.
+  chdir('/tmp');
+  console.log(
+    spawnSync('git', [
+      'clone',
+      '--depth=1',
+      `https://github.com/${repo.owner}/${repo.repo}.git`,
+      `./${tempRepo}`,
+    ]).output.toString(),
+  );
+  chdir(`/tmp/${tempRepo}`);
+
+  // Manually define the configuration for the pull request and github to prevent having to
+  // checkout the repository before defining the config.
+  // TODO(josephperrott): Load this from the actual repository.
+  setConfig(<{pullRequest: PullRequestConfig; github: GithubConfig}>{
+    github: {
+      mainBranchName,
+      owner: repo.owner,
+      name: repo.repo,
+    },
+    pullRequest: {
+      githubApiMerge: false,
+    },
+  });
+  /** The configuration used for the ng-dev tooling. */
+  const config = await getConfig([assertValidGithubConfig, assertValidPullRequestConfig]);
+
+  AuthenticatedGitClient.configure(token);
+  /** The git client used to perform actions. */
+  const git = await AuthenticatedGitClient.get();
+
+  /** The pull request after being retrieved and validated. */
+  const pullRequest = await loadAndValidatePullRequest(
+    {git, config},
+    pr,
+    new PullRequestValidationConfig(),
+  );
+  core.info('Validated PR information:');
+  core.info(JSON.stringify(pullRequest));
+  /** Whether any fatal validation failures were discovered. */
+  let hasFatalFailures = false;
+  /** The status information to be pushed as a status to the pull request. */
+  let statusInfo: CommmitStatus = await (async () => {
+    // Log validation failures and check for any fatal failures.
+    if (pullRequest.validationFailures.length !== 0) {
+      core.info(`Found ${pullRequest.validationFailures.length} failing validation(s)`);
+      for (const failure of pullRequest.validationFailures) {
+        hasFatalFailures = !failure.canBeForceIgnored || hasFatalFailures;
+        await core.group('Validation failures', async () => {
+          core.info(failure.message);
+        });
+      }
+    }
+
+    // With any fatal failure the check is not necessary to do.
+    if (hasFatalFailures) {
+      core.info('One of the validations was fatal, setting the status as pending for the pr');
+      return {
+        description: 'Waiting to check mergeability due to failing status(es)',
+        state: 'pending',
+      };
+    }
+
+    try {
+      git.run(['checkout', mainBranchName]);
+      /**
+       * A merge strategy used to perform the merge check.
+       * Any concrete class implementing MergeStrategy is sufficient as all of our usage is
+       * defined in the abstract base class.
+       * */
+      const strategy = new AutosquashMergeStrategy(git);
+      await strategy.prepare(pullRequest);
+      await strategy.check(pullRequest);
+      core.info('Merge check passes, setting a passing status on the pr');
+      return {
+        description: `Merges cleanly to ${pullRequest.targetBranches.join(', ')}`,
+        state: 'success',
+      };
+    } catch (e) {
+      // As the merge strategy class will express the failures during checks, any thrown error is a
+      // failure for our merge check.
+      let description: string;
+      if (e instanceof MergeConflictsFatalError) {
+        core.info('Merge conflict found');
+        description = `Unable to merge into: ${e.failedBranches.join(', ')}`;
+      } else {
+        core.info('Unknown error found when checking merge:');
+        core.error(e as Error);
+        description =
+          'Cannot cleanly merge to all target branches, please update changes or PR target';
+      }
+      return {
+        description,
+        state: 'failure',
+      };
+    }
+  })();
+
+  await git.github.repos.createCommitStatus({
+    ...repo,
+    state: statusInfo.state,
+    // Status descriptions are limited to 140 characters.
+    description: statusInfo.description.substring(0, 139),
+    sha: pullRequest.headSha,
+    context: statusContextName,
+  });
+}
+
+/** The repository name for the pull request. */
+const repo = core.getInput('repo', {required: true, trimWhitespace: true});
+/** The owner of the repository for the pull request. */
+const owner = core.getInput('owner', {required: true, trimWhitespace: true});
+/** The pull request number. */
+const pr = Number(core.getInput('pr', {required: true, trimWhitespace: true}));
+// If the provided pr is not a number, we cannot evaluate the mergeability.
+if (isNaN(pr)) {
+  core.setFailed('The provided pr value was not a number');
+  process.exit();
+}
+/** The token for the angular robot to perform actions in the requested repo. */
+const token = await getAuthTokenFor(ANGULAR_ROBOT, {repo, owner});
+
+try {
+  await main({repo, owner}, token, pr).catch((e: Error) => {
+    core.error(e);
+    core.setFailed(e.message);
+  });
+} finally {
+  await revokeActiveInstallationToken(token);
+}