feat(view issues-and-mrs): checkout local branch for merge request

Credit: [@Musisimaru](https://gitlab.com/Musisimaru) (originally
    introduced in
    [!229](https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/merge_requests/229))

This is a copy paste of the code contributed by Mussimaru in
https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/merge_requests/229
This code doesn't compile, but I wanted to include it as a record that
he's done most of the work.

package.json

@@ -169,6 +169,10 @@
         "command": "gl.createComment",
         "title": "Add comment now",
         "category": "GitLab"
+      },
+      {
+        "command": "gl.checkoutMrBranch",
+        "title": "Checkout MR branch"
       }
     ],
     "menus": {
@@ -201,6 +205,10 @@
           "command": "gl.createComment",
           "when": "false"
         },
+        {
+          "command": "gl.checkoutMrBranch",
+          "when": "false"
+        },
         {
           "command": "gl.showIssuesAssignedToMe",
           "when": "gitlab:validState"
@@ -281,6 +289,12 @@
           "group": "navigation"
         }
       ],
+      "view/item/context": [
+        {
+          "command": "gl.checkoutMrBranch",
+          "when": "view =~ /issuesAndMrs/ && viewItem == mr-item-from-branch"
+        }
+      ],
       "comments/comment/title": [
         {
           "command": "gl.startEditingComment",

src/command_names.ts

@@ -31,6 +31,7 @@ export const USER_COMMANDS = {
   CANCEL_EDITING_COMMENT: 'gl.cancelEditingComment',
   SUBMIT_COMMENT_EDIT: 'gl.submitCommentEdit',
   CREATE_COMMENT: 'gl.createComment',
+  CHECKOUT_MR_BRANCH: 'gl.checkoutMrBranch',
 };
 
 /*

src/commands/checkout_mr_branch.test.ts

+import * as vscode from 'vscode';
+import * as sidebar from '../sidebar.js';
+import { GitExtension, Repository } from '../api/git';
+import { MrItemModel } from '../data_providers/items/mr_item_model';
+import { anotherWorkspace, mr, workspace } from '../test_utils/entities';
+import { createFakeRepository, FakeGitExtension } from '../test_utils/fake_git_extension';
+import { checkoutMrBranch } from './checkout_mr_branch';
+import { gitExtensionWrapper } from '../git/git_extension_wrapper';
+
+jest.mock('../sidebar.js');
+vscode.window.showInformationMessage = jest.fn();
+vscode.window.showErrorMessage = jest.fn();
+
+describe('MR branch commands', () => {
+  describe('Checkout branch by Merge request', () => {
+    let commandData: MrItemModel;
+
+    let fakeExtension: FakeGitExtension;
+
+    let firstWorkspace: GitLabWorkspace;
+    let secondWorkspace: GitLabWorkspace;
+
+    let firstRepository: Repository;
+    let secondRepository: Repository;
+
+    beforeEach(() => {
+      firstWorkspace = { ...workspace };
+      secondWorkspace = { ...anotherWorkspace };
+    });
+
+    afterEach(() => {
+      (vscode.window.showInformationMessage as jest.Mock).mockReset();
+      (vscode.window.showWarningMessage as jest.Mock).mockReset();
+    });
+
+    describe('If merge request from local branch', () => {
+      describe('Basic functionality', () => {
+        beforeEach(() => {
+          firstRepository = createFakeRepository(firstWorkspace.uri);
+          secondRepository = createFakeRepository(secondWorkspace.uri);
+
+          fakeExtension = new FakeGitExtension(firstRepository, secondRepository);
+          jest
+            .spyOn(gitExtensionWrapper, 'Extension', 'get')
+            .mockReturnValue((fakeExtension as unknown) as GitExtension);
+
+          commandData = new MrItemModel(mr, firstWorkspace);
+
+          checkoutByMRFromLocalBranch(commandData);
+        });
+
+        it('(local-branch) Branch fetch message', () => {
+          expect((vscode.window.showInformationMessage as jest.Mock).mock.calls[0]).toEqual([
+            'Fetching branches...',
+          ]);
+        });
+
+        it('(local-branch) Was checkout', async () => {
+          await expect(firstRepository.checkout).toBeCalled();
+        });
+
+        it('(local-branch) Was fetching before checkout', async () => {
+          await expect(firstRepository.checkout).toBeCalled();
+          expect(firstRepository.fetch).toBeCalled();
+        });
+
+        it('(local-branch) There were no error messages', () => {
+          expect(vscode.window.showErrorMessage).not.toBeCalled();
+        });
+
+        it('(local-branch) Sidebar was refreshed', () => {
+          expect(sidebar.refresh).toBeCalled();
+        });
+
+        it('(local-branch) Message about success', () => {
+          const callsCount = (vscode.window.showInformationMessage as jest.Mock).mock.calls.length;
+          expect(
+            (vscode.window.showInformationMessage as jest.Mock).mock.calls[callsCount - 1],
+          ).toEqual([`Branch successfully changed to ${mr.source_branch}`]);
+        });
+      });
+      describe('Multi-root Workspaces', () => {
+        beforeEach(() => {
+          commandData = new MrItemModel(mr, secondWorkspace);
+
+          checkoutByMRFromLocalBranch(commandData);
+        });
+
+        it('(multi-root) The branch was checkout  from right repository', async () => {
+          await expect(secondRepository.checkout).toBeCalled();
+        });
+
+        it('(multi-root) The branch from second repository was not checkout', async () => {
+          await expect(secondRepository.checkout).toBeCalled();
+          expect(firstRepository.fetch).not.toBeCalled();
+          expect(firstRepository.checkout).not.toBeCalled();
+        });
+      });
+    });
+  });
+});

src/commands/checkout_mr_branch.ts

+import * as vscode from 'vscode';
+import * as assert from 'assert';
+import * as sidebar from '../sidebar.js';
+import { MrItemModel } from '../data_providers/items/mr_item_model';
+import { gitExtensionWrapper } from '../git/git_extension_wrapper';
+import { UserFriendlyError } from '../errors/user_friendly_error';
+import { handleError } from '../log';
+
+/**
+ * Command will checkout source branch by merge request in current git. Merge request must be from local branch.
+ * @param data item of view from the left sidebar
+ */
+export const checkoutMrBranch = async (data: MrItemModel): Promise<void> => {
+  // todo: Change data.workspace to data.repository (issue #345)
+  assert(data.mr.target_project_id === data.mr.source_project_id);
+  const { showInformationMessage } = vscode.window;
+  const sourceBranchName = data.mr.source_branch as string;
+  let branchNameForCheckout: string;
+  try {
+    const repos = gitExtensionWrapper.getRepositoriesByWorkspace(data.workspace);
+    if (repos.length > 1) {
+      throw new Error(
+        `You have more then one repos in one workspace. Extension can't work with this case yet. But we will fix it on soon.`,
+      );
+    }
+    const repo = repos[0];
+    showInformationMessage('Fetching branches...');
+
+    // merge from local branch
+    branchNameForCheckout = sourceBranchName;
+    await repo.fetch();
+    await repo.checkout(sourceBranchName);
+
+    assert(
+      repo.state.HEAD,
+      "We can't read repository HEAD. We suspect that your `git head` command fails and we can't continue till it succeeds",
+    );
+
+    const currentBranchName = repo.state.HEAD.name;
+    if (currentBranchName !== branchNameForCheckout) {
+      throw new Error(
+        `The branch name after the checkout (${currentBranchName}) is not the branch that the extension tried to check out (${branchNameForCheckout}). This is an unexpected error, please inspect your repository before making any further changes.`,
+      );
+    }
+
+    sidebar.refresh();
+    showInformationMessage(`Branch successfully changed to ${sourceBranchName}`);
+  } catch (e) {
+    handleError(
+      new UserFriendlyError(
+        e.stderr || `${(e as Error).message}` || `Couldn't checkout branch ${sourceBranchName}`,
+        e,
+      ),
+    );
+  }
+};

src/extension.js

@@ -29,6 +29,7 @@ const {
 } = require('./commands/mr_discussion_commands');
 const { fileDecorationProvider } = require('./review/file_decoration_provider');
 const { checkVersion } = require('./utils/check_version');
+const { checkoutMrBranch } = require('./commands/checkout_mr_branch');
 
 vscode.gitLabWorkflow = {
   sidebarDataProviders: [],
@@ -87,6 +88,7 @@ const registerCommands = (context, outputChannel) => {
     [USER_COMMANDS.CANCEL_EDITING_COMMENT]: cancelEdit,
     [USER_COMMANDS.SUBMIT_COMMENT_EDIT]: submitEdit,
     [USER_COMMANDS.CREATE_COMMENT]: createComment,
+    [USER_COMMANDS.CHECKOUT_MR_BRANCH]: checkoutMrBranch,
     [PROGRAMMATIC_COMMANDS.NO_IMAGE_REVIEW]: () =>
       vscode.window.showInformationMessage("GitLab MR review doesn't support images yet."),
   };

src/test_utils/entities.ts

@@ -32,6 +32,8 @@ export const mr: RestMr = {
   sha: '69ad609e8891b8aa3db85a35cd2c5747705bd76a',
   source_project_id: 9999,
   target_project_id: 9999,
+  source_branch: 'feature-a',
+  target_branch: 'main',
 };
 
 export const diffFile: RestDiffFile = {

src/types.d.ts

@@ -25,6 +25,8 @@ interface RestMr extends RestIssuable {
   sha: string;
   source_project_id: number;
   target_project_id: number;
+  target_branch: string;
+  source_branch: string;
 }
 
 interface RestMrVersion {