fix(root): deepen main history for pr change detection

Author: shepherdjerred

packages/docs/logs/2026-05-11_renovate-dashboard-481-max-updates.md

@@ -16,6 +16,7 @@ User asked to work in a new dissociated clone, update as much of Renovate dashbo
 - Created branch `chore/renovate-481-max` from fresh `origin/main`.
 - Opened draft PR #773: <https://github.com/shepherdjerred/monorepo/pull/773>.
 - Pushed commit `a7ee0bdbc` (`chore(root): update renovate dashboard 481 dependencies`).
+- Fixed the Buildkite PR pickup failure by deepening shallow `origin/main` history when the CI pipeline generator cannot compute a merge base.
 - Applied the Renovate dashboard sweep across Bun/npm package manifests and locks, Rust crates, Maven deps, Swift packages, Go modules, Terraform/OpenTofu providers, tool pins, Gradle wrapper, and Docker/image digests.
 - Migrated Prisma 7 generation/runtime setup for Birmel and Scout backend, including Prisma config files and libSQL adapters.
 - Updated Zod 4 call sites and replaced the DPP config loader's `zconf` dependency with TOML parsing plus explicit Zod validation.
@@ -32,5 +33,6 @@ User asked to work in a new dissociated clone, update as much of Renovate dashbo
 ### Caveats
 
 - Validation passed: `bun run scripts/setup.ts`, `bun run lint`, `bun run typecheck`, `bun run test`, Clauderon `cargo check`, Scout desktop `cargo check`, Castle Casters `mvn test`, ASUSWRT provider `go test ./...`, all three updated Tofu `validate` runs, and Android `./gradlew help` with the local Android SDK exported.
+- CI pickup fix validation passed: `cd scripts/ci && bun test src/__tests__/change-detection.test.ts`, `cd scripts/ci && bun run typecheck`, `cd scripts/ci && bun test`, and local PR pipeline generation with `BUILDKITE_PULL_REQUEST=773`.
 - Android validation installed NDK `26.1.10909125` into `/Users/jerred/Library/Android/sdk` because the updated Gradle project requested it.
 - Some validators still print non-fatal warnings/hints: SwiftLint renamed-rule warnings, Astro inline-script hints, Gradle deprecation warnings, and KSP warnings under Kotlin 2.3.20.

scripts/ci/src/__tests__/change-detection.test.ts

@@ -593,20 +593,49 @@ describe("fail-fast base detection", () => {
   it("rejects when merge-base cannot be computed", async () => {
     process.env["BUILDKITE_BRANCH"] = "feature";
     process.env["BUILDKITE_PULL_REQUEST"] = "42";
-    // Pretend origin/main is already resolvable so ensureOriginMain short-circuits;
-    // merge-base then fails (e.g. shallow clone history doesn't reach the common ancestor).
     const execFn: ExecFn = async (cmd) => {
       if (cmd[1] === "rev-parse") {
         return { stdout: "deadbeef", exitCode: 0 };
       }
+      if (cmd[1] === "fetch") {
+        return { stdout: "", exitCode: 0 };
+      }
       return { stdout: "", exitCode: 1 };
     };
 
     await expect(_getBaseRevision(execFn)).rejects.toThrow(
-      "Unable to compute merge-base",
+      "Unable to compute merge-base with origin/main after deepening shallow history",
     );
   });
 
+  it("deepens origin/main history when the shallow checkout lacks a merge-base", async () => {
+    process.env["BUILDKITE_BRANCH"] = "feature";
+    process.env["BUILDKITE_PULL_REQUEST"] = "42";
+    const calls: string[][] = [];
+    let mergeBaseCalls = 0;
+    const execFn: ExecFn = async (cmd) => {
+      calls.push(cmd);
+      if (cmd[1] === "rev-parse") {
+        return { stdout: "deadbeef", exitCode: 0 };
+      }
+      if (cmd[1] === "fetch") {
+        return { stdout: "", exitCode: 0 };
+      }
+      if (cmd[1] === "merge-base") {
+        mergeBaseCalls += 1;
+        if (mergeBaseCalls === 1) {
+          return { stdout: "", exitCode: 1 };
+        }
+        return { stdout: "abc123", exitCode: 0 };
+      }
+      return { stdout: "", exitCode: 1 };
+    };
+
+    const base = await _getBaseRevision(execFn);
+    expect(base).toBe("abc123");
+    expect(calls.some((c) => c.includes("--deepen=1000"))).toBe(true);
+  });
+
   it("fetches origin/main when missing before merge-base", async () => {
     process.env["BUILDKITE_BRANCH"] = "feature";
     process.env["BUILDKITE_PULL_REQUEST"] = "42";
@@ -629,6 +658,7 @@ describe("fail-fast base detection", () => {
     expect(base).toBe("abc123");
     const fetchCmd = calls.find((c) => c[1] === "fetch");
     expect(fetchCmd).toBeDefined();
+    expect(fetchCmd).toContain("--depth=100");
     expect(fetchCmd).toContain("+refs/heads/main:refs/remotes/origin/main");
   });
 

scripts/ci/src/change-detection.ts

@@ -282,6 +282,25 @@ async function exec(cmd: string[]): Promise<ExecResult> {
   return { stdout: stdout.trim(), exitCode };
 }
 
+const ORIGIN_MAIN_REFSPEC = "+refs/heads/main:refs/remotes/origin/main";
+
+async function fetchOriginMain(
+  execFn: ExecFn,
+  depthArg: string,
+): Promise<void> {
+  const fetch = await execFn([
+    "git",
+    "fetch",
+    "--no-tags",
+    depthArg,
+    "origin",
+    ORIGIN_MAIN_REFSPEC,
+  ]);
+  if (fetch.exitCode !== 0) {
+    throw new Error("Unable to fetch origin/main for merge-base computation");
+  }
+}
+
 // Buildkite's checkout uses --depth=100 and only fetches the PR HEAD ref, so
 // `origin/main` is often missing as a remote-tracking ref. Fetch it explicitly
 // before any merge-base call.
@@ -296,43 +315,47 @@ async function ensureOriginMain(execFn: ExecFn): Promise<void> {
   if (check.exitCode === 0 && check.stdout !== "") {
     return;
   }
-  const fetch = await execFn([
-    "git",
-    "fetch",
-    "--no-tags",
-    "--depth=100",
-    "origin",
-    "+refs/heads/main:refs/remotes/origin/main",
-  ]);
-  if (fetch.exitCode !== 0) {
-    throw new Error("Unable to fetch origin/main for merge-base computation");
+  await fetchOriginMain(execFn, "--depth=100");
+}
+
+async function getMergeBaseWithOriginMain(execFn: ExecFn): Promise<string> {
+  await ensureOriginMain(execFn);
+
+  const initial = await execFn(["git", "merge-base", "HEAD", "origin/main"]);
+  if (initial.exitCode === 0 && initial.stdout !== "") {
+    return initial.stdout;
   }
+
+  // The remote-tracking ref can exist while the shallow clone still lacks the
+  // common ancestor. Deepen in bounded steps so normal PRs stay cheap while
+  // long-lived branches still get a fair retry before failing.
+  for (const depthArg of ["--deepen=1000", "--deepen=10000"]) {
+    await fetchOriginMain(execFn, depthArg);
+    const result = await execFn(["git", "merge-base", "HEAD", "origin/main"]);
+    if (result.exitCode === 0 && result.stdout !== "") {
+      return result.stdout;
+    }
+  }
+
+  throw new Error(
+    "Unable to compute merge-base with origin/main after deepening shallow history",
+  );
 }
 
 async function getBaseRevision(execFn: ExecFn = exec): Promise<string> {
   const branch = process.env["BUILDKITE_BRANCH"] ?? "";
   const pullRequest = process.env["BUILDKITE_PULL_REQUEST"] ?? "false";
 
   if (pullRequest && pullRequest !== "false") {
-    await ensureOriginMain(execFn);
-    const result = await execFn(["git", "merge-base", "HEAD", "origin/main"]);
-    if (result.exitCode !== 0 || result.stdout === "") {
-      throw new Error("Unable to compute merge-base with origin/main");
-    }
-    return result.stdout;
+    return getMergeBaseWithOriginMain(execFn);
   }
 
   if (branch === "main") {
     return getLastGreenCommit();
   }
 
   // Feature branch without PR
-  await ensureOriginMain(execFn);
-  const result = await execFn(["git", "merge-base", "HEAD", "origin/main"]);
-  if (result.exitCode !== 0 || result.stdout === "") {
-    throw new Error("Unable to compute merge-base with origin/main");
-  }
-  return result.stdout;
+  return getMergeBaseWithOriginMain(execFn);
 }
 
 async function getChangedFiles(execFn: ExecFn = exec): Promise<string[]> {