feat: expose merge strategy for configuration when automerging

Expose the underlying merge strategy used by our platforms as a
configurable option for each repository to configure with the option
`automergeStrategy`. Additionally implement `automergeStrategy` for the
Bitbucket Cloud platform, as the only platform that doesn't currently
respect the remote repository's configured merge strategy.

docs/usage/configuration-options.md

@@ -173,6 +173,26 @@ Example use:
 }
 ```
 
+## automergeStrategy
+
+This setting is only applicable if you opt in to configure `automerge` to `true` and `automergeType` to `pr` for any of
+your dependencies
+
+The automerge strategy defaults to `auto`, in which Renovate will make its best guess as to how to pull requests. This
+generally results in Renovate respecting a strategy configured in the platform itself for the repository where possible.
+Acceptable values are:
+
+- `auto`, in which the choice is left to Renovate.
+- `fast-forward`, which generally involves no new commits in the resultant tree, but "fast-forwarding" the main branch reference.
+- `merge-commit`, which generally involves synthesizing a new merge commit.
+- `rebase`, which generally involves rewriting history as part of the merge — but usually retaining the individual commits.
+- `squash`, which generally involves flattening the commits that are being merged into a new commit.
+
+Not all platforms support all pull request merge strategies. In the cases where a merge strategy is explicitly
+unsupported by the platform, Renovate will hold off on merging instead of silently merging in a way you didn't wish for.
+
+The only supported platform for this option at the moment is Bitbucket Cloud.
+
 ## automergeType
 
 This setting is only applicable if you opt in to configure `automerge` to `true` for any of your dependencies.

lib/config/definitions.ts

@@ -1277,6 +1277,14 @@ const options: RenovateOptions[] = [
     allowedValues: ['branch', 'pr', 'pr-comment'],
     default: 'pr',
   },
+  {
+    name: 'automergeStrategy',
+    description:
+      'The merge strategy to use when automerging PRs. Used only if automergeType=pr or automergeType=pr-comment',
+    type: 'string',
+    allowedValues: ['auto', 'merge-commit', 'rebase', 'squash'],
+    default: 'auto',
+  },
   {
     name: 'automergeComment',
     description:

lib/config/types.ts

@@ -21,6 +21,7 @@ export interface GroupConfig extends Record<string, unknown> {
 export interface RenovateSharedConfig {
   $schema?: string;
   automerge?: boolean;
+  automergeStrategy?: MergeStrategy;
   branchPrefix?: string;
   branchName?: string;
   manager?: string;
@@ -228,6 +229,13 @@ export type UpdateType =
 
 export type MatchStringsStrategy = 'any' | 'recursive' | 'combination';
 
+export type MergeStrategy =
+  | 'auto'
+  | 'fast-forward'
+  | 'merge-commit'
+  | 'rebase'
+  | 'squash';
+
 // TODO: Proper typings
 export interface PackageRule
   extends RenovateSharedConfig,

lib/platform/azure/index.md

@@ -0,0 +1,7 @@
+# Azure DevOps and Azure DevOps Server
+
+## Features awaiting implementation
+
+- The `automergeStrategy` configuration option has not been implemented for this platform, and all values behave as if
+  the value `auto` was used. Renovate will use the merge strategy configured in the Azure Repos repository itself, and
+  this cannot be overridden yet.

lib/platform/bitbucket-server/index.md

@@ -8,6 +8,9 @@
 ## Features awaiting implementation
 
 - Creating issues not implemented yet, used to alert users when there is a config error
+- The `automergeStrategy` configuration option has not been implemented for this platform, and all values behave as if
+  the value `auto` was used. Renovate will implicitly use the merge strategy configured as 'default' in the Bitbucket
+  Server repository itself, and this cannot be overridden yet.
 
 ## Testing
 

lib/platform/bitbucket/__snapshots__/index.spec.ts.snap

@@ -1254,7 +1254,7 @@ Array [
 
 exports[`platform/bitbucket/index massageMarkdown() returns diff files 1`] = `"**foo**bartext"`;
 
-exports[`platform/bitbucket/index mergePr() posts Merge 1`] = `
+exports[`platform/bitbucket/index mergePr() posts Merge with auto 1`] = `
 Array [
   Object {
     "headers": Object {
@@ -1268,7 +1268,67 @@ Array [
     "url": "https://api.bitbucket.org/2.0/repositories/some/repo",
   },
   Object {
-    "body": "{\\"close_source_branch\\":true,\\"merge_strategy\\":\\"merge_commit\\",\\"message\\":\\"auto merged\\"}",
+    "body": "{\\"close_source_branch\\":true,\\"message\\":\\"auto merged\\"}",
+    "headers": Object {
+      "accept": "application/json",
+      "accept-encoding": "gzip, deflate, br",
+      "authorization": "Basic YWJjOjEyMw==",
+      "content-length": "52",
+      "content-type": "application/json",
+      "host": "api.bitbucket.org",
+      "user-agent": "RenovateBot/0.0.0-semantic-release (https://github.com/renovatebot/renovate)",
+    },
+    "method": "POST",
+    "url": "https://api.bitbucket.org/2.0/repositories/some/repo/pullrequests/5/merge",
+  },
+]
+`;
+
+exports[`platform/bitbucket/index mergePr() posts Merge with fast-forward 1`] = `
+Array [
+  Object {
+    "headers": Object {
+      "accept": "application/json",
+      "accept-encoding": "gzip, deflate, br",
+      "authorization": "Basic YWJjOjEyMw==",
+      "host": "api.bitbucket.org",
+      "user-agent": "RenovateBot/0.0.0-semantic-release (https://github.com/renovatebot/renovate)",
+    },
+    "method": "GET",
+    "url": "https://api.bitbucket.org/2.0/repositories/some/repo",
+  },
+  Object {
+    "body": "{\\"close_source_branch\\":true,\\"message\\":\\"auto merged\\",\\"merge_strategy\\":\\"fast_forward\\"}",
+    "headers": Object {
+      "accept": "application/json",
+      "accept-encoding": "gzip, deflate, br",
+      "authorization": "Basic YWJjOjEyMw==",
+      "content-length": "84",
+      "content-type": "application/json",
+      "host": "api.bitbucket.org",
+      "user-agent": "RenovateBot/0.0.0-semantic-release (https://github.com/renovatebot/renovate)",
+    },
+    "method": "POST",
+    "url": "https://api.bitbucket.org/2.0/repositories/some/repo/pullrequests/5/merge",
+  },
+]
+`;
+
+exports[`platform/bitbucket/index mergePr() posts Merge with merge-commit 1`] = `
+Array [
+  Object {
+    "headers": Object {
+      "accept": "application/json",
+      "accept-encoding": "gzip, deflate, br",
+      "authorization": "Basic YWJjOjEyMw==",
+      "host": "api.bitbucket.org",
+      "user-agent": "RenovateBot/0.0.0-semantic-release (https://github.com/renovatebot/renovate)",
+    },
+    "method": "GET",
+    "url": "https://api.bitbucket.org/2.0/repositories/some/repo",
+  },
+  Object {
+    "body": "{\\"close_source_branch\\":true,\\"message\\":\\"auto merged\\",\\"merge_strategy\\":\\"merge_commit\\"}",
     "headers": Object {
       "accept": "application/json",
       "accept-encoding": "gzip, deflate, br",
@@ -1284,6 +1344,66 @@ Array [
 ]
 `;
 
+exports[`platform/bitbucket/index mergePr() posts Merge with optional merge strategy 1`] = `
+Array [
+  Object {
+    "headers": Object {
+      "accept": "application/json",
+      "accept-encoding": "gzip, deflate, br",
+      "authorization": "Basic YWJjOjEyMw==",
+      "host": "api.bitbucket.org",
+      "user-agent": "RenovateBot/0.0.0-semantic-release (https://github.com/renovatebot/renovate)",
+    },
+    "method": "GET",
+    "url": "https://api.bitbucket.org/2.0/repositories/some/repo",
+  },
+  Object {
+    "body": "{\\"close_source_branch\\":true,\\"message\\":\\"auto merged\\"}",
+    "headers": Object {
+      "accept": "application/json",
+      "accept-encoding": "gzip, deflate, br",
+      "authorization": "Basic YWJjOjEyMw==",
+      "content-length": "52",
+      "content-type": "application/json",
+      "host": "api.bitbucket.org",
+      "user-agent": "RenovateBot/0.0.0-semantic-release (https://github.com/renovatebot/renovate)",
+    },
+    "method": "POST",
+    "url": "https://api.bitbucket.org/2.0/repositories/some/repo/pullrequests/5/merge",
+  },
+]
+`;
+
+exports[`platform/bitbucket/index mergePr() posts Merge with squash 1`] = `
+Array [
+  Object {
+    "headers": Object {
+      "accept": "application/json",
+      "accept-encoding": "gzip, deflate, br",
+      "authorization": "Basic YWJjOjEyMw==",
+      "host": "api.bitbucket.org",
+      "user-agent": "RenovateBot/0.0.0-semantic-release (https://github.com/renovatebot/renovate)",
+    },
+    "method": "GET",
+    "url": "https://api.bitbucket.org/2.0/repositories/some/repo",
+  },
+  Object {
+    "body": "{\\"close_source_branch\\":true,\\"message\\":\\"auto merged\\",\\"merge_strategy\\":\\"squash\\"}",
+    "headers": Object {
+      "accept": "application/json",
+      "accept-encoding": "gzip, deflate, br",
+      "authorization": "Basic YWJjOjEyMw==",
+      "content-length": "78",
+      "content-type": "application/json",
+      "host": "api.bitbucket.org",
+      "user-agent": "RenovateBot/0.0.0-semantic-release (https://github.com/renovatebot/renovate)",
+    },
+    "method": "POST",
+    "url": "https://api.bitbucket.org/2.0/repositories/some/repo/pullrequests/5/merge",
+  },
+]
+`;
+
 exports[`platform/bitbucket/index setBranchStatus() posts status 1`] = `
 Array [
   Object {

lib/platform/bitbucket/index.md

@@ -3,8 +3,11 @@
 ## Unsupported platform features/concepts
 
 - Adding assignees to PRs not supported (does not seem to be a Bitbucket concept)
+- `automergeStrategy=rebase` [is unsupported][bcloud-16610] by Bitbucket Cloud.
 
 ## Features requiring implementation
 
 - Creating issues not implemented yet, e.g. when there is a config error
 - Adding comments to PRs not implemented yet, e.g. when a PR has been edited or has a lockfile error
+
+[bcloud-16610]: https://jira.atlassian.com/browse/BCLOUD-16610

lib/platform/bitbucket/index.spec.ts

@@ -819,12 +819,45 @@ describe(getName(), () => {
   });
 
   describe('mergePr()', () => {
-    it('posts Merge', async () => {
+    it('posts Merge with optional merge strategy', async () => {
       const scope = await initRepoMock();
       scope.post('/2.0/repositories/some/repo/pullrequests/5/merge').reply(200);
       await bitbucket.mergePr(5, 'branch');
       expect(httpMock.getTrace()).toMatchSnapshot();
     });
+
+    it('posts Merge with auto', async () => {
+      const scope = await initRepoMock();
+      scope.post('/2.0/repositories/some/repo/pullrequests/5/merge').reply(200);
+      await bitbucket.mergePr(5, 'branch', 'auto');
+      expect(httpMock.getTrace()).toMatchSnapshot();
+    });
+
+    it('posts Merge with merge-commit', async () => {
+      const scope = await initRepoMock();
+      scope.post('/2.0/repositories/some/repo/pullrequests/5/merge').reply(200);
+      await bitbucket.mergePr(5, 'branch', 'merge-commit');
+      expect(httpMock.getTrace()).toMatchSnapshot();
+    });
+
+    it('posts Merge with squash', async () => {
+      const scope = await initRepoMock();
+      scope.post('/2.0/repositories/some/repo/pullrequests/5/merge').reply(200);
+      await bitbucket.mergePr(5, 'branch', 'squash');
+      expect(httpMock.getTrace()).toMatchSnapshot();
+    });
+
+    it('does not post Merge with rebase', async () => {
+      await bitbucket.mergePr(5, 'branch', 'rebase');
+      expect(httpMock.getTrace()).toEqual([]);
+    });
+
+    it('posts Merge with fast-forward', async () => {
+      const scope = await initRepoMock();
+      scope.post('/2.0/repositories/some/repo/pullrequests/5/merge').reply(200);
+      await bitbucket.mergePr(5, 'branch', 'fast-forward');
+      expect(httpMock.getTrace()).toMatchSnapshot();
+    });
   });
 
   describe('getVulnerabilityAlerts()', () => {

lib/platform/bitbucket/index.ts

@@ -1,6 +1,7 @@
 import URL from 'url';
 import is from '@sindresorhus/is';
 import parseDiff from 'parse-diff';
+import { MergeStrategy } from '../../config/types';
 import { REPOSITORY_NOT_FOUND } from '../../constants/error-messages';
 import { PLATFORM_TYPE_BITBUCKET } from '../../constants/platforms';
 import { logger } from '../../logger';
@@ -29,7 +30,7 @@ import { smartTruncate } from '../utils/pr-body';
 import { readOnlyIssueBody } from '../utils/read-only-issue-body';
 import * as comments from './comments';
 import * as utils from './utils';
-import { PrResponse, RepoInfoBody } from './utils';
+import { PrResponse, RepoInfoBody, mergeBodyTransformer } from './utils';
 
 const bitbucketHttp = new BitbucketHttp();
 
@@ -395,7 +396,7 @@ export async function setBranchStatus({
   const sha = await getBranchCommit(branchName);
 
   // TargetUrl can not be empty so default to bitbucket
-  const url = targetUrl || /* istanbul ignore next */ 'http://bitbucket.org';
+  const url = targetUrl || /* istanbul ignore next */ 'https://bitbucket.org';
 
   const body = {
     name: context,
@@ -752,19 +753,22 @@ export async function updatePr({
 
 export async function mergePr(
   prNo: number,
-  branchName: string
+  branchName: string,
+  mergeStrategy: MergeStrategy
 ): Promise<boolean> {
-  logger.debug(`mergePr(${prNo}, ${branchName})`);
+  logger.debug(`mergePr(${prNo}, ${branchName}, ${mergeStrategy})`);
+
+  // Bitbucket Cloud does not support a rebase-alike; https://jira.atlassian.com/browse/BCLOUD-16610
+  if (mergeStrategy === 'rebase') {
+    logger.warn('Bitbucket Cloud does not support a "rebase" strategy.');
+    return false;
+  }
 
   try {
     await bitbucketHttp.postJson(
       `/2.0/repositories/${config.repository}/pullrequests/${prNo}/merge`,
       {
-        body: {
-          close_source_branch: true,
-          merge_strategy: 'merge_commit',
-          message: 'auto merged',
-        },
+        body: mergeBodyTransformer(mergeStrategy),
       }
     );
     logger.debug('Automerging succeeded');

lib/platform/bitbucket/utils.ts

@@ -1,4 +1,5 @@
 import url from 'url';
+import { MergeStrategy } from '../../config/types';
 import { BranchStatus, PrState } from '../../types';
 import { HttpOptions, HttpPostOptions, HttpResponse } from '../../util/http';
 import { BitbucketHttp } from '../../util/http/bitbucket';
@@ -56,6 +57,36 @@ export function repoInfoTransformer(repoInfoBody: RepoInfoBody): RepoInfo {
   };
 }
 
+export type BitbucketMergeStrategy = 'fast_forward' | 'merge_commit' | 'squash';
+const bitbucketMergeStrategies: Map<MergeStrategy, BitbucketMergeStrategy> =
+  new Map([
+    ['squash', 'squash'],
+    ['merge-commit', 'merge_commit'],
+    ['fast-forward', 'fast_forward'],
+  ]);
+
+export interface MergeRequestBody {
+  close_source_branch?: boolean;
+  message: string;
+  merge_strategy?: BitbucketMergeStrategy;
+}
+
+export function mergeBodyTransformer(
+  mergeStrategy: MergeStrategy
+): MergeRequestBody {
+  const body: MergeRequestBody = {
+    close_source_branch: true,
+    message: 'auto merged',
+  };
+
+  // The `auto` strategy will use the strategy configured inside Bitbucket.
+  if (mergeStrategy !== 'auto') {
+    body.merge_strategy = bitbucketMergeStrategies.get(mergeStrategy);
+  }
+
+  return body;
+}
+
 export const prStates = {
   open: ['OPEN'],
   notOpen: ['MERGED', 'DECLINED', 'SUPERSEDED'],

lib/platform/gitea/index.md

@@ -3,3 +3,9 @@
 ## Unsupported platform features/concepts
 
 - **Adding reviewers to PRs not supported**: Gitea versions older than v1.14.0 do not have the required API.
+
+## Features awaiting implementation
+
+- The `automergeStrategy` configuration option has not been implemented for this platform, and all values behave as if
+  the value `auto` was used. Renovate will use the merge strategy configured in the Gitea repository itself, and this
+  cannot be overridden yet.