Record the repository's default branch as the merge base outside pull requests

__tests__/revision.test.ts

@@ -6,6 +6,7 @@ import {
   checkedOutRevision,
   firstParent,
   resolveBuildId,
+  resolveMergeBaseBranch,
   resolveParentRevision,
   resolveRevision,
   upstreamMergeBase,
@@ -243,3 +244,25 @@ describe("resolveBuildId", () => {
     });
   });
 });
+
+describe("resolveMergeBaseBranch", () => {
+  test("a pull request's base branch", () => {
+    const pr = { head: { ref: "topic", sha: SHA_A }, base: { ref: "dev" } };
+    expect(
+      resolveMergeBaseBranch({
+        pull_request: pr,
+        repository: { default_branch: "trunk" },
+      }),
+    ).toEqual("dev");
+  });
+
+  test("the repository's default branch outside a pull request", () => {
+    expect(
+      resolveMergeBaseBranch({ repository: { default_branch: "trunk" } }),
+    ).toEqual("trunk");
+  });
+
+  test("main when the payload names no repository", () => {
+    expect(resolveMergeBaseBranch({})).toEqual("main");
+  });
+});

dist/index.js

@@ -113,7 +113,7 @@ function getConfig() {
         revisionSource,
         parentRevision,
         parentRevisionSource,
-        mergeBaseBranchName: eventPullRequest ? eventPullRequest.base.ref : "main",
+        mergeBaseBranchName: (0, revision_1.resolveMergeBaseBranch)(event),
     };
 }
 /**
@@ -397,7 +397,7 @@ var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, ge
     });
 };
 Object.defineProperty(exports, "__esModule", ({ value: true }));
-exports.resolveBuildId = exports.upstreamMergeBase = exports.resolveParentRevision = exports.firstParent = exports.resolveRevision = exports.checkedOutRevision = void 0;
+exports.resolveMergeBaseBranch = exports.resolveBuildId = exports.upstreamMergeBase = exports.resolveParentRevision = exports.firstParent = exports.resolveRevision = exports.checkedOutRevision = void 0;
 const child_process_1 = __nccwpck_require__(2081);
 /**
  * The commit the workspace actually has checked out. Never throws: when `cwd`
@@ -563,6 +563,16 @@ function resolveBuildId(explicit, mergeBase) {
     return { buildId: "", source: "none" };
 }
 exports.resolveBuildId = resolveBuildId;
+/**
+ * The branch a run is compared against: a pull request's base, else the
+ * repository's default branch (in every event payload that names the
+ * repository), else "main".
+ */
+function resolveMergeBaseBranch(event) {
+    var _a, _b;
+    return (((_a = event.pull_request) === null || _a === void 0 ? void 0 : _a.base.ref) || ((_b = event.repository) === null || _b === void 0 ? void 0 : _b.default_branch) || "main");
+}
+exports.resolveMergeBaseBranch = resolveMergeBaseBranch;
 
 
 /***/ }),

src/github.ts

@@ -0,0 +1,15 @@
+/**
+ * The parts of the event payload (`GITHUB_EVENT_PATH`) the action reads. Each
+ * object is required when present by GitHub's webhook payload schemas, but
+ * absent on events that lack it (`pull_request` outside pull requests;
+ * `repository` is not guaranteed on `schedule`).
+ */
+export interface GitHubEvent {
+  pull_request?: {
+    head: { ref: string; sha: string };
+    base: { ref: string };
+  };
+  repository?: {
+    default_branch: string;
+  };
+}

src/main.ts

@@ -15,10 +15,12 @@ import {
   ParentRevisionSource,
   resolveParentRevision,
   resolveBuildId,
+  resolveMergeBaseBranch,
   resolveRevision,
   RevisionSource,
   upstreamMergeBase,
 } from "./revision";
+import { GitHubEvent } from "./github";
 import { mayhemProjectSlug } from "./project";
 
 const mayhemUrl: string =
@@ -95,7 +97,7 @@ function getConfig(): Config {
 
   const ghRepo = `${process.env["GITHUB_SERVER_URL"]}:443/${repo}/`;
   const eventPath = process.env["GITHUB_EVENT_PATH"] || "event.json";
-  const event = JSON.parse(readFileSync(eventPath, "utf-8")) || {};
+  const event: GitHubEvent = JSON.parse(readFileSync(eventPath, "utf-8")) || {};
   const eventPullRequest = event.pull_request;
 
   // Optional typed run duration (in seconds). When set it must be a positive
@@ -159,7 +161,7 @@ function getConfig(): Config {
     revisionSource,
     parentRevision,
     parentRevisionSource,
-    mergeBaseBranchName: eventPullRequest ? eventPullRequest.base.ref : "main",
+    mergeBaseBranchName: resolveMergeBaseBranch(event),
   };
 }
 

src/revision.ts

@@ -1,4 +1,5 @@
 import { execFileSync } from "child_process";
+import { GitHubEvent } from "./github";
 
 /** Where the revision recorded on a Mayhem run came from, for the action log. */
 export type RevisionSource =
@@ -208,3 +209,14 @@ export function resolveBuildId(
   }
   return { buildId: "", source: "none" };
 }
+
+/**
+ * The branch a run is compared against: a pull request's base, else the
+ * repository's default branch (in every event payload that names the
+ * repository), else "main".
+ */
+export function resolveMergeBaseBranch(event: GitHubEvent): string {
+  return (
+    event.pull_request?.base.ref || event.repository?.default_branch || "main"
+  );
+}