Allow setting merge method through labels

README.md

@@ -96,6 +96,16 @@ The following merge options are supported:
   or [`squash`](https://help.github.com/en/articles/about-pull-request-merges#squash-and-merge-your-pull-request-commits)
   (squash all commits into a single commit). The default option is `merge`.
 
+- `MERGE_METHOD_LABELS`: Set to allow labels to determine the merge method
+  (see `MERGE_METHOD` for possible values).
+  For example, `automerge=merge,autosquash=squash`. If no such label is present,
+  the method set by `MERGE_METHOD` will be used.
+
+  The default value is `""`.
+
+- `MERGE_METHOD_LABEL_REQUIRED`: Set to `"true"` to require one of the
+  `MERGE_METHOD_LABELS` to be set.
+
 - `MERGE_COMMIT_MESSAGE`: The commit message to use when merging the pull
   request into the base branch. Possible values are `automatic` (use GitHub's
   default message), `pull-request-title` (use the pull request's title),

lib/common.js

@@ -69,6 +69,21 @@ function createConfig(env = {}) {
     };
   }
 
+  function parseMergeMethodLabels(str) {
+    if (!str) {
+      return [];
+    }
+    return str.split(",").map(lm => {
+      const [label, method] = lm.split("=");
+      if (!label || !method) {
+        throw new Error(
+          `Couldn't parse "${lm}" as "<label>=<method>" expression`
+        );
+      }
+      return { label, method };
+    });
+  }
+
   function parseMergeRemoveLabels(str, defaultValue) {
     return (str == null ? defaultValue : str).split(",").map(s => s.trim());
   }
@@ -97,6 +112,8 @@ function createConfig(env = {}) {
   const mergeRetries = parsePositiveInt("MERGE_RETRIES", 6);
   const mergeRetrySleep = parsePositiveInt("MERGE_RETRY_SLEEP", 5000);
   const mergeDeleteBranch = env.MERGE_DELETE_BRANCH === "true";
+  const mergeMethodLabels = parseMergeMethodLabels(env.MERGE_METHOD_LABELS);
+  const mergeMethodLabelRequired = env.MERGE_METHOD_LABEL_REQUIRED === "true";
 
   const updateLabels = parseMergeLabels(env.UPDATE_LABELS, "automerge");
   const updateMethod = env.UPDATE_METHOD || "merge";
@@ -107,6 +124,8 @@ function createConfig(env = {}) {
     mergeLabels,
     mergeRemoveLabels,
     mergeMethod,
+    mergeMethodLabels,
+    mergeMethodLabelRequired,
     mergeForks,
     mergeCommitMessage,
     mergeCommitMessageRegex,

lib/merge.js

@@ -17,7 +17,8 @@ async function merge(context, pullRequest) {
   const {
     octokit,
     config: {
-      mergeMethod,
+      mergeMethod: defaultMergeMethod,
+      mergeMethodLabels,
       mergeCommitMessage,
       mergeCommitMessageRegex,
       mergeFilterAuthor,
@@ -55,6 +56,11 @@ async function merge(context, pullRequest) {
   }
 
   const commitMessage = getCommitMessage(mergeCommitMessage, pullRequest);
+  const mergeMethod = getMergeMethod(
+    defaultMergeMethod,
+    mergeMethodLabels,
+    pullRequest
+  );
   const merged = await tryMerge(
     octokit,
     pullRequest,
@@ -143,7 +149,12 @@ async function deleteBranch(octokit, pullRequest) {
 
 function skipPullRequest(context, pullRequest) {
   const {
-    config: { mergeForks, mergeLabels }
+    config: {
+      mergeForks,
+      mergeLabels,
+      mergeMethodLabelRequired,
+      mergeMethodLabels
+    }
   } = context;
 
   let skip = false;
@@ -181,6 +192,14 @@ function skipPullRequest(context, pullRequest) {
     }
   }
 
+  const numberMethodLabelsFound = mergeMethodLabels
+    .map(lm => labels.includes(lm.label))
+    .filter(x => x).length;
+  if (mergeMethodLabelRequired && numberMethodLabelsFound === 0) {
+    logger.info("Skipping PR merge, required merge method label missing");
+    skip = true;
+  }
+
   return skip;
 }
 
@@ -255,6 +274,26 @@ function tryMerge(
   );
 }
 
+function getMergeMethod(defaultMergeMethod, mergeMethodLabels, pullRequest) {
+  const foundMergeMethodLabels = pullRequest.labels.flatMap(l =>
+    mergeMethodLabels.filter(ml => ml.label === l.name)
+  );
+  if (foundMergeMethodLabels.length > 0) {
+    const first = foundMergeMethodLabels[0];
+    if (foundMergeMethodLabels.length > 1) {
+      throw new Error(
+        `Discovered multiple merge method labels, only one is permitted!`
+      );
+    } else {
+      logger.info(
+        `Discovered ${first.label}, will merge with method ${first.method}`
+      );
+    }
+    return first.method;
+  }
+  return defaultMergeMethod;
+}
+
 function getCommitMessage(mergeCommitMessage, pullRequest) {
   if (mergeCommitMessage === "automatic") {
     return undefined;

test/common.test.js

@@ -13,6 +13,8 @@ test("createConfig", () => {
       blocking: [],
       required: []
     },
+    mergeMethodLabels: [],
+    mergeMethodLabelRequired: false,
     mergeForks: true,
     mergeCommitMessage: "automatic",
     mergeCommitMessageRegex: "",

test/merge.test.js

@@ -2,12 +2,13 @@ const { merge } = require("../lib/merge");
 const { createConfig } = require("../lib/common");
 const { pullRequest } = require("./common");
 
-let octokit;
+let octokit, mergeMethod;
 
 beforeEach(() => {
+  mergeMethod = undefined;
   octokit = {
     pulls: {
-      merge: jest.fn()
+      merge: jest.fn(({ merge_method }) => (mergeMethod = merge_method))
     }
   };
 });
@@ -109,3 +110,97 @@ test("MERGE_FILTER_AUTHOR when set but do not match current author should not me
   // WHEN
   expect(await merge({ config, octokit }, pr)).toEqual(false);
 });
+
+test("Merge method can be set by env variable", async () => {
+  // GIVEN
+  const pr = pullRequest();
+
+  const config = createConfig({
+    MERGE_METHOD: "rebase"
+  });
+
+  // WHEN
+  expect(await merge({ config, octokit }, pr)).toEqual(true);
+  expect(mergeMethod).toEqual("rebase");
+});
+
+test("Merge method can be set by a merge method label", async () => {
+  // GIVEN
+  const pr = pullRequest();
+  pr.labels = [{ name: "autosquash" }, { name: "reallyautomerge" }];
+
+  const config = createConfig({
+    MERGE_METHOD_LABELS: "automerge=merge,autosquash=squash,autorebase=rebase",
+    MERGE_METHOD: "merge",
+    MERGE_LABELS: "reallyautomerge"
+  });
+
+  // WHEN
+  expect(await merge({ config, octokit }, pr)).toEqual(true);
+  expect(mergeMethod).toEqual("squash");
+});
+
+test("Merge method can be required", async () => {
+  // GIVEN
+  const pr = pullRequest();
+  pr.labels = [{ name: "autosquash" }];
+
+  const config = createConfig({
+    MERGE_METHOD_LABELS: "automerge=merge,autosquash=squash,autorebase=rebase",
+    MERGE_METHOD_LABEL_REQUIRED: "true",
+    MERGE_METHOD: "merge",
+    MERGE_LABELS: ""
+  });
+
+  // WHEN
+  expect(await merge({ config, octokit }, pr)).toEqual(true);
+  expect(mergeMethod).toEqual("squash");
+});
+
+test("Missing require merge method skips PR", async () => {
+  // GIVEN
+  const pr = pullRequest();
+  pr.labels = [{ name: "mergeme" }];
+
+  const config = createConfig({
+    MERGE_METHOD_LABELS: "automerge=merge,autosquash=squash,autorebase=rebase",
+    MERGE_METHOD_LABEL_REQUIRED: "true",
+    MERGE_METHOD: "merge",
+    MERGE_LABELS: "mergeme"
+  });
+
+  // WHEN
+  expect(await merge({ config, octokit }, pr)).toEqual(false);
+});
+
+test("Merge method doesn't have to be required", async () => {
+  // GIVEN
+  const pr = pullRequest();
+  pr.labels = [{ name: "mergeme" }];
+
+  const config = createConfig({
+    MERGE_METHOD_LABELS: "automerge=merge,autosquash=squash,autorebase=rebase",
+    MERGE_METHOD_LABEL_REQUIRED: "false",
+    MERGE_METHOD: "merge",
+    MERGE_LABELS: "mergeme"
+  });
+
+  // WHEN
+  expect(await merge({ config, octokit }, pr)).toEqual(true);
+  expect(mergeMethod).toEqual("merge");
+});
+
+test("Multiple merge method labels throw an error", async () => {
+  // GIVEN
+  const pr = pullRequest();
+  pr.labels = [{ name: "automerge" }, { name: "autosquash" }];
+
+  const config = createConfig({
+    MERGE_METHOD_LABELS: "automerge=merge,autosquash=squash,autorebase=rebase",
+    MERGE_METHOD_LABEL_REQUIRED: "true",
+    MERGE_METHOD: "merge",
+  });
+
+  // WHEN
+  expect(merge({ config, octokit }, pr)).rejects.toThrow("merge method labels");
+});