feat: add exponential retries on merge errors

README.md

@@ -142,6 +142,24 @@ steps:
       MERGE_METHOD: MERGE
 ```
 
+### Number of retries
+
+In case the merge action fails, by default it will automatically be retried up
+to three times using an exponential backoff strategy. This means, the first
+retry will happen 1 second after the first failure, while the second will happen
+4 seconds after the previous, the third 9 seconds, and so on.
+
+It's possible to configure the number of retries by providing a value for
+`MAXIMUM_RETRIES` (by default, the value is `3`).
+
+```yaml
+steps:
+  - name: Merge me!
+    uses: ridedott/merge-me-action@master
+    with:
+      MAXIMUM_RETRIES: 2
+```
+
 ## Getting Started
 
 These instructions will get you a copy of the project up and running on your

cSpell.json

@@ -25,6 +25,7 @@
   "language": "en-US",
   "version": "0.1",
   "words": [
+    "backoff",
     "codeowners",
     "commitlint",
     "coverallsapp",

src/common/merge.ts

@@ -7,30 +7,56 @@ import {
 import { parseInputMergeMethod } from '../utilities/inputParsers';
 import { logDebug, logInfo } from '../utilities/log';
 
+export interface PullRequestDetails {
+  commitHeadline: string;
+  pullRequestId: string;
+  reviewEdge: { node: { state: string } } | undefined;
+}
+
+const EXPONENTIAL_BACKOFF = 2;
+const MINIMUM_WAIT_TIME = 1000;
+
+const delay = async (duration: number): Promise<void> => {
+  return new Promise((resolve: () => void): void => {
+    setTimeout((): void => {
+      resolve();
+    }, duration);
+  });
+};
+
 /**
  * Approves and merges a given Pull Request.
  */
-export const merge = async (
+const merge = async (
   octokit: ReturnType<typeof getOctokit>,
-  {
-    commitHeadline,
-    pullRequestId,
-    reviewEdge,
-  }: {
-    commitHeadline: string;
-    pullRequestId: string;
-    reviewEdge: { node: { state: string } } | undefined;
-  },
+  pullRequestDetails: PullRequestDetails,
 ): Promise<void> => {
   const mergeMethod = parseInputMergeMethod();
 
+  const { commitHeadline, pullRequestId, reviewEdge } = pullRequestDetails;
+
   const mutation =
     reviewEdge === undefined
       ? approveAndMergePullRequestMutation(mergeMethod)
       : mergePullRequestMutation(mergeMethod);
 
+  await octokit.graphql(mutation, { commitHeadline, pullRequestId });
+};
+
+const shouldRetry = (error: Error): boolean =>
+  error.message.includes('Base branch was modified.');
+
+export const mergeWithRetry = async (
+  octokit: ReturnType<typeof getOctokit>,
+  details: PullRequestDetails & {
+    maximumRetries: number;
+    retryCount: number;
+  },
+): Promise<void> => {
+  const { retryCount, maximumRetries } = details;
+
   try {
-    await octokit.graphql(mutation, { commitHeadline, pullRequestId });
+    await merge(octokit, details);
   } catch (error) {
     logInfo(
       'An error ocurred while merging the Pull Request. This is usually ' +
@@ -40,5 +66,21 @@ export const merge = async (
     );
     /* eslint-disable-next-line @typescript-eslint/no-base-to-string */
     logDebug(`Original error: ${(error as Error).toString()}.`);
+
+    if (shouldRetry(error) && retryCount <= maximumRetries) {
+      const nextRetryIn = retryCount ** EXPONENTIAL_BACKOFF * MINIMUM_WAIT_TIME;
+
+      logInfo(`Retrying in ${nextRetryIn.toString()}...`);
+
+      await delay(nextRetryIn);
+
+      await mergeWithRetry(octokit, {
+        ...details,
+        maximumRetries,
+        retryCount: retryCount + 1,
+      });
+    } else {
+      throw error;
+    }
   }
 };

src/eventHandlers/checkSuite/index.spec.ts

@@ -7,8 +7,10 @@ import { getOctokit } from '@actions/github';
 import { OK } from 'http-status-codes';
 import * as nock from 'nock';
 
+import { useSetTimeoutImmediateInvocation } from '../../../test/utilities';
 import { mergePullRequestMutation } from '../../graphql/mutations';
 import { AllowedMergeMethods } from '../../utilities/inputParsers';
+import * as log from '../../utilities/log';
 import { checkSuiteHandle } from '.';
 
 /* cspell:disable-next-line */
@@ -61,7 +63,7 @@ describe('check Suite event handler', (): void => {
       });
     nock('https://api.github.com').post('/graphql').reply(OK);
 
-    await checkSuiteHandle(octokit, 'dependabot-preview[bot]');
+    await checkSuiteHandle(octokit, 'dependabot-preview[bot]', 3);
 
     expect(warningSpy).not.toHaveBeenCalled();
   });
@@ -116,7 +118,7 @@ describe('check Suite event handler', (): void => {
       })
       .reply(OK);
 
-    await checkSuiteHandle(octokit, 'dependabot-preview[bot]');
+    await checkSuiteHandle(octokit, 'dependabot-preview[bot]', 3);
   });
 
   it('does not approve pull requests that are not mergeable', async (): Promise<
@@ -160,7 +162,7 @@ describe('check Suite event handler', (): void => {
         },
       });
 
-    await checkSuiteHandle(octokit, 'dependabot-preview[bot]');
+    await checkSuiteHandle(octokit, 'dependabot-preview[bot]', 3);
 
     expect(infoSpy).toHaveBeenCalledWith(
       'Pull request is not in a mergeable state: CONFLICTING.',
@@ -208,7 +210,7 @@ describe('check Suite event handler', (): void => {
         },
       });
 
-    await checkSuiteHandle(octokit, 'dependabot-preview[bot]');
+    await checkSuiteHandle(octokit, 'dependabot-preview[bot]', 3);
 
     expect(infoSpy).toHaveBeenCalledWith('Pull request is already merged.');
   });
@@ -254,7 +256,7 @@ describe('check Suite event handler', (): void => {
         },
       });
 
-    await checkSuiteHandle(octokit, 'dependabot-preview[bot]');
+    await checkSuiteHandle(octokit, 'dependabot-preview[bot]', 3);
 
     expect(infoSpy).toHaveBeenCalledWith(
       'Pull request cannot be merged cleanly. Current state: UNKNOWN.',
@@ -302,7 +304,7 @@ describe('check Suite event handler', (): void => {
         },
       });
 
-    await checkSuiteHandle(octokit, 'dependabot-preview[bot]');
+    await checkSuiteHandle(octokit, 'dependabot-preview[bot]', 3);
 
     expect(infoSpy).toHaveBeenCalledWith('Pull request is not open: CLOSED.');
   });
@@ -312,7 +314,7 @@ describe('check Suite event handler', (): void => {
   > => {
     expect.assertions(1);
 
-    await checkSuiteHandle(octokit, 'some-other-login');
+    await checkSuiteHandle(octokit, 'some-other-login', 3);
 
     expect(infoSpy).toHaveBeenCalledWith(
       'Pull request created by dependabot-preview[bot], not some-other-login, skipping.',
@@ -334,8 +336,132 @@ describe('check Suite event handler', (): void => {
         },
       });
 
-    await checkSuiteHandle(octokit, 'dependabot-preview[bot]');
+    await checkSuiteHandle(octokit, 'dependabot-preview[bot]', 3);
 
     expect(warningSpy).toHaveBeenCalled();
   });
+
+  it('retries up to two times before failing', async (): Promise<void> => {
+    expect.assertions(6);
+
+    nock('https://api.github.com')
+      .post('/graphql')
+      .reply(OK, {
+        data: {
+          repository: {
+            pullRequest: {
+              commits: {
+                edges: [
+                  {
+                    node: {
+                      commit: {
+                        messageHeadline: COMMIT_HEADLINE,
+                      },
+                    },
+                  },
+                ],
+              },
+              id: PULL_REQUEST_ID,
+              mergeStateStatus: 'CLEAN',
+              mergeable: 'MERGEABLE',
+              merged: false,
+              reviews: {
+                edges: [
+                  {
+                    node: {
+                      state: 'APPROVED',
+                    },
+                  },
+                ],
+              },
+              state: 'OPEN',
+            },
+          },
+        },
+      })
+      .post('/graphql')
+      .times(3)
+      .reply(
+        403,
+        '##[error]GraphqlError: Base branch was modified. Review and try the merge again.',
+      );
+
+    const logDebugSpy = jest.spyOn(log, 'logDebug');
+    const logInfoSpy = jest.spyOn(log, 'logInfo');
+
+    useSetTimeoutImmediateInvocation();
+
+    try {
+      await checkSuiteHandle(octokit, 'dependabot-preview[bot]', 2);
+    } catch (error) {
+      expect(error).toBeInstanceOf(Error);
+      expect(error.message).toStrictEqual(
+        '##[error]GraphqlError: Base branch was modified. Review and try the merge again.',
+      );
+      expect(logDebugSpy).toHaveBeenCalledTimes(3);
+      expect(logInfoSpy.mock.calls[1][0]).toStrictEqual(
+        'An error ocurred while merging the Pull Request. This is usually caused by the base branch being out of sync with the target branch. In this case, the base branch must be rebased. Some tools, such as Dependabot, do that automatically.',
+      );
+      expect(logInfoSpy.mock.calls[2][0]).toStrictEqual('Retrying in 1000...');
+      expect(logInfoSpy.mock.calls[4][0]).toStrictEqual('Retrying in 4000...');
+    }
+  });
+
+  it('fails the backoff strategy when the error is not "Base branch was modified"', async (): Promise<
+    void
+  > => {
+    expect.assertions(3);
+
+    nock('https://api.github.com')
+      .post('/graphql')
+      .reply(OK, {
+        data: {
+          repository: {
+            pullRequest: {
+              commits: {
+                edges: [
+                  {
+                    node: {
+                      commit: {
+                        messageHeadline: COMMIT_HEADLINE,
+                      },
+                    },
+                  },
+                ],
+              },
+              id: PULL_REQUEST_ID,
+              mergeStateStatus: 'CLEAN',
+              mergeable: 'MERGEABLE',
+              merged: false,
+              reviews: {
+                edges: [
+                  {
+                    node: {
+                      state: 'APPROVED',
+                    },
+                  },
+                ],
+              },
+              state: 'OPEN',
+            },
+          },
+        },
+      })
+      .post('/graphql')
+      .reply(403, '##[error]GraphqlError: This is a different error.');
+
+    const logInfoSpy = jest.spyOn(log, 'logInfo');
+
+    try {
+      await checkSuiteHandle(octokit, 'dependabot-preview[bot]', 2);
+    } catch (error) {
+      expect(error).toBeInstanceOf(Error);
+      expect(error.message).toStrictEqual(
+        '##[error]GraphqlError: This is a different error.',
+      );
+      expect(logInfoSpy.mock.calls[1][0]).toStrictEqual(
+        'An error ocurred while merging the Pull Request. This is usually caused by the base branch being out of sync with the target branch. In this case, the base branch must be rebased. Some tools, such as Dependabot, do that automatically.',
+      );
+    }
+  });
 });

src/eventHandlers/checkSuite/index.ts

@@ -2,7 +2,7 @@
 
 import { context, getOctokit } from '@actions/github';
 
-import { merge } from '../../common/merge';
+import { mergeWithRetry } from '../../common/merge';
 import { findPullRequestInfo as findPullRequestInformation } from '../../graphql/queries';
 import {
   MergeableState,
@@ -91,6 +91,7 @@ const getPullRequestInformation = async (
 
 const tryMerge = async (
   octokit: ReturnType<typeof getOctokit>,
+  maximumRetries: number,
   {
     commitMessageHeadline,
     mergeStateStatus,
@@ -124,9 +125,11 @@ const tryMerge = async (
   } else if (pullRequestState !== 'OPEN') {
     logInfo(`Pull request is not open: ${pullRequestState}.`);
   } else {
-    await merge(octokit, {
+    await mergeWithRetry(octokit, {
       commitHeadline: commitMessageHeadline,
+      maximumRetries,
       pullRequestId,
+      retryCount: 1,
       reviewEdge: reviewEdges[0],
     });
   }
@@ -135,6 +138,7 @@ const tryMerge = async (
 export const checkSuiteHandle = async (
   octokit: ReturnType<typeof getOctokit>,
   gitHubLogin: string,
+  maximumRetries: number,
 ): Promise<void> => {
   const pullRequests = context.payload.check_suite.pull_requests as Array<{
     number: number;
@@ -170,7 +174,7 @@ export const checkSuiteHandle = async (
         )}.`,
       );
 
-      await tryMerge(octokit, pullRequestInformation);
+      await tryMerge(octokit, maximumRetries, pullRequestInformation);
     }
   }
 };