From 473820bba77792efc01d64cfc2fed21f547fe768 Mon Sep 17 00:00:00 2001
From: Alistair Smith <hi@alistair.sh>
Date: Sat, 15 Aug 2026 11:51:58 -0700
Subject: [PATCH] ci: give PR builds one darwin aarch64 test lane that any mac
 agent can take

PRs lost darwin tests entirely in #39191 because the version-specific
lanes queue for hours. Capacity is not actually short, it is fragmented:
the 26 lane's agents sit idle on PR-heavy days while the 14 lane backs
up. So PR builds now get a single untiered aarch64 lane (no release-tier
selector) that every arm64 mac agent serves; main, manual and
[macos tests] builds keep the 26 / 14 / x64 lanes for version coverage.

No-Verification-Needed: pipeline generator only; checked with --dry-run
---
 .buildkite/ci.mjs | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/.buildkite/ci.mjs b/.buildkite/ci.mjs
index 224642f86f29..aba15736e6bf 100755
--- a/.buildkite/ci.mjs
+++ b/.buildkite/ci.mjs
@@ -186,8 +186,9 @@ const testPlatforms = [
   // The darwin test suite runs on real macOS agents against the Linux-built
   // artifacts from the `darwin-<arch>-build-bun` steps (the only darwin build
   // lanes — see buildPlatforms).
-  // All three darwin test lanes run on main and on opt-in only for now (see
-  // darwinTestsEnabled): the mac agent pool cannot keep up with PR volume.
+  // These three version-specific lanes run on main and on opt-in (see
+  // darwinTestsEnabled). PR builds instead get one aarch64 lane that any mac
+  // agent can take (prDarwinTestPlatforms), so the whole arm64 pool serves PRs.
   { os: "darwin", arch: "aarch64", release: "26", tier: "latest" },
   { os: "darwin", arch: "aarch64", release: "14", tier: "previous" },
   { os: "darwin", arch: "x64", release: "14", tier: "latest" },
@@ -409,7 +410,7 @@ function getTestAgent(platform, options) {
       queue: `test-${os}`,
       os,
       arch,
-      ...(arch === "aarch64" ? { "release-tier": tier } : {}),
+      ...(arch === "aarch64" && tier ? { "release-tier": tier } : {}),
     };
   }
 
@@ -1543,11 +1544,16 @@ async function getPipeline(options = {}) {
   // Tests run on main too so the canary release step below can gate on them.
   // ASAN is PR-only (see includeASAN above), so the asan test lane is dropped
   // on main along with its build.
+  // Untiered: any arm64 mac agent, whatever macOS it runs, can take it.
+  /** @type {Platform[]} */
+  const prDarwinTestPlatforms = [{ os: "darwin", arch: "aarch64", release: "any" }];
   const darwinTestsEnabled =
     isMainBranch() || isBuildManual() || /\[(macos|darwin) tests?\]/i.test(getCommitMessage());
   const relevantTestPlatforms = (
     includeASAN ? testPlatforms : testPlatforms.filter(({ profile }) => profile !== "asan")
-  ).filter(({ os }) => os !== "darwin" || darwinTestsEnabled);
+  )
+    .filter(({ os }) => os !== "darwin" || darwinTestsEnabled)
+    .concat(darwinTestsEnabled ? [] : prDarwinTestPlatforms);
   /** @type {string[]} */
   const testStepKeys = [];
   {