ci: log a host health snapshot when the artifact download stalls and on the bare-metal darwin agents

[robobun]

Problem

• :darwin: any aarch64 - test-bun fails with Error: buildkite-agent artifact download timed out after 120s for step 'darwin-aarch64-build-bun'. Refusing to continue with a partial download (would silently fall back to the wrong binary). at getExecPathFromBuildKite (scripts/runner.node.mjs:2745). A manual retry on another box passes.
• All 62 such failures in 3 days (builds 107078 to 108064) are on the two bare-metal 16 GB minis, biscuit and hardtack. Each box goes bad 15 to 18 hours after its daily 06:27 reboot. From then on every job on it fails here or crawls into the 45 minute job timeout, until the next reboot. The other minis and the Tart guests never fail here.
• The job log says nothing about the host at that moment. The last time this happened (CI: darwin-26-aarch64 test-bun fails on most PR builds with 'buildkite-agent artifact download timed out after 120s' #33116), the cause was found on the box itself: bun-profile processes from earlier jobs wedged in sendfile(2), pinning kernel socket buffers until the mbuf pool was empty (Bun.serve: stop using sendfile(2) for file responses on macOS #33728). The fleet ssh route is not reachable from this session, so the log has to carry that evidence.

Fix

• getHostHealthSnapshot() in scripts/utils.mjs returns a few lines: uptime and load, memory and swap, processes in an uninterruptible wait or stuck exiting (stat U or E on macOS, D or Z on Linux), processes that run a binary under the agent's build directory (only a CI job starts those, so at job start they are leftovers), and on macOS the netstat -m mbuf pool and TCP retransmit counters.
• The runner prints it as a --- Host health group in the header of every job on an agent tagged ephemeral=false (the bare-metal minis), and from any agent right before it fails on the download timeout. Healthy runs add about eight lines and a few ps/sysctl calls; the header of the minis then shows the mbuf pool and the stuck process count drift over the day, and the first failed job shows the state at the stall.
• No policy change. A first version of this PR paused the agent and exited a status the pipeline retried. ci: recover test-bun from Buildkite artifact-download failures #33117 proposed the same retry for the same symptom and was closed after Bun.serve: stop using sendfile(2) for file responses on macOS #33728 fixed the host cause and ci: scope automatic retry to agent loss via signal_reason #34684 narrowed automatic retries to agent loss. That half waits for one episode's snapshot, and for ci: retry a timed out artifact download and bound the darwin guest warmup installs #40500, which edits the same timeout branch.
• Verified: test/internal/runner-host-health.test.ts covers the agent tag check and the snapshot format. The runner was run against a fake buildkite-agent whose artifact download hangs: the group appears in the header with ephemeral=false, and before the error on every agent. ps -eo was checked against the busybox source (alpine has no -x).

Background

• scripts/runner.node.mjs is the test step's command. It downloads the build step's zips with buildkite-agent artifact download before it runs any test. ci: fail loudly when artifact download times out #29039 made a timeout fatal so a truncated zip is never used.
• scripts/agent.mjs registers the bare-metal macOS agents with the tag ephemeral=false and reboots them daily at 06:27 local. Cloud agents are ephemeral=true and leave after one job. The darwin Tart agents boot a fresh guest per job and carry tart=true. Only the bare-metal kernels carry state from one job to the next.
• XNU's sendfile(2) allocates its mbuf chain with an uninterruptible wait. When the pool is empty the process cannot be killed, and it keeps what it holds. Bun.serve: stop using sendfile(2) for file responses on macOS #33728 removed the server-side sendfile on macOS for that reason. fetch() with a Bun.file() body over plain HTTP still uses it (src/http/SendFile.rs), and test/js/bun/http/fetch-file-upload.test.ts pushes a 128 MiB file through that path on every darwin job.
• macOS derives kern.maxprocperuid from physical memory. The job header's max user processes is 1333 on biscuit and hardtack and 2666 on the other minis, so these two are the 16 GB boxes with the smallest mbuf pool.

Notes

Data from the Buildkite API over builds 107078 to 108064 (2026-08-27T22:56Z to 2026-08-29T01:36Z), :darwin: any aarch64 - test-bun jobs, retried jobs included:

• Download timeouts per host: biscuit 28 (agent darwin-aarch64-26.6.1-1), hardtack 34 (darwin-aarch64-26.6.2-1), crouton 0, breadstick 0, bingus 0, every Tart agent 0. The one other exit 1 under 250 s on breadstick was a job cut by the 06:27 reboot, with a 3 s download.
• biscuit episode 1: 23:52Z on 08-27 to the 05:27Z reboot. Episode 2: from 21:20Z on 08-28, still going at the end of the window. hardtack: 03:53Z to about 10:18Z on 08-28, ended by an agent restart at 11:30Z. In an episode, jobs are only download timeouts (exit 1, 124 to 204 s), 45 minute timeouts, or long failures (1191 to 2520 s). Job durations creep up before an episode: 450 s, 510 s, 585 s, 748 s, 2520 s.
• Downloads inside an episode are intermittent: build 107185 took 41 s for the 24 MiB zip and 2 s for the 120 MiB zip, build 107190 took 4 s for both, build 107956 (20:02Z, before the episode) took 41 s and 85 s. Normal is 2 to 4 s.
• The tests that fail inside an episode all move bytes over loopback or the network: The socket connection was closed unexpectedly, ConnectionClosed downloading tarball from a local registry, test/package.json (bun install) hitting its timeout, Failed to install dependencies: SIGTERM, and uploads roundtrip with sendfile() in fetch-file-upload.test.ts. curl https://checkip.amazonaws.com in the runner's own header took 257 ms ten seconds before the stalled download in build 108064.
• The agent runs jobs in a PTY, so the job shell's exit sends SIGHUP to the job's process group and ordinary leftovers die (checked on Linux with pty.fork()). That does not reach a process in an uninterruptible wait, which is the case Bun.serve: stop using sendfile(2) for file responses on macOS #33728 describes.
• If the snapshot shows stuck bun-profile processes and a full pool, the product fix is the one Bun.serve: stop using sendfile(2) for file responses on macOS #33728 left open: stop SendFile::is_eligible from choosing sendfile on macOS, which needs the fallback at fetch.rs:1566 (a synchronous whole-file read on the JS thread) to become asynchronous first. Shrinking the 128 MiB upload in fetch-file-upload.test.ts would lower the pressure from that one test but not remove the hazard.
• Related: ci: retry a timed out artifact download and bound the darwin guest warmup installs #40500 retries the download three times before it fails, for a Tart guest whose NAT stalled for two minutes. It edits the same timeout branch; the snapshot call belongs before its rmSync of the release directory.
• Not done: the on-host check the handoff asked for. The fleet ssh route (farm-tailscale SOCKS proxy) does not resolve from this session's container.

[robobun]

Status: ready for review. Reworked after self-review: this PR now only adds the host health snapshot. The agent pause and the exit status 4 retry from the first version are dropped (see the PR body: #33117, #33728, #34684).

How the failure was characterized: Buildkite API data for the :darwin: any aarch64 - test-bun jobs of builds 107078 to 108064. All 62 download timeouts are on two 16 GB bare-metal minis (biscuit, hardtack), in episodes that start 15 to 18 hours after the daily reboot and end at the next reboot. Inside an episode every job on the box fails at the download or runs into the 45 minute timeout. The last time this happened (#33116), the cause was bun-profile processes wedged in sendfile(2) exhausting the mbuf pool (#33728); the client-side sendfile path that PR left in place is still there.

Verification: test/internal/runner-host-health.test.ts, plus the runner run against a fake buildkite-agent whose artifact download hangs: the --- Host health group appears in the header with ephemeral=false, and before the download error on every agent.

[robobun]

^{Updated 1:41 AM PT - Aug 29th, 2026}

❌ @robobun, your commit fd4ecdf has 1 failures in Build #108224 (All Failures):

• test/js/web/url/url.test.ts - code 1: TypeError: Invalid URL on 🍎 any x64

---

🧪   To try this PR locally:

bunx bun-pr 40865

That installs a local version of the PR into your bun-40865 executable, so you can run:

bun-40865 --bun

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Walkthrough

Changes

The runner adds platform-specific host-health diagnostics for bare-metal Buildkite agents. It prints these diagnostics during environment reporting and before failing artifact downloads that exceed the 120-second timeout. Tests cover agent classification and platform behavior.

Host health diagnostics

Layer / File(s)	Summary
Host health collection scripts/utils.mjs, test/internal/runner-host-health.test.ts	The utilities collect uptime, load, memory, swap, process, and macOS network statistics. Tests cover bare-metal classification and platform-specific snapshots.
Host health rendering scripts/utils.mjs	Environment reporting detects bare-metal agents. printHostHealth emits available diagnostics in a Host health log group.
Artifact timeout diagnostics scripts/runner.node.mjs	The runner prints host-health diagnostics before it throws the existing partial-download error after an artifact timeout.

Suggested reviewers: jarred-sumner, alii, dylan-conway

Merge Risk: ⚪ Minimal · up to fd4ec

This change adds host-health diagnostics to selected job headers and artifact-download timeout failures without changing the retry or execution policy; no actionable merge-blocking risk remains after normal checks and review.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly summarizes the main change: logging host-health diagnostics for stalled artifact downloads and bare-metal Darwin agents. It is specific and related to the changeset.
Description check	✅ Passed	The description thoroughly explains the problem, implementation, verification steps, background, scope, and limitations. Although it does not use the exact template headings, it provides the required …

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The description thoroughly explains the problem, implementation, verification steps, background, scope, and limitations. Although it does not use the exact template headings, it provides the required change summary and verification information.

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[claude]

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Inline comments:
In `@scripts/utils.mjs`:
- Line 3035: Update the process-listing flow around run(["ps", "-eo", ...]) to
preserve diagnostics when BusyBox does not support etime: use a supported
fallback format that still returns process rows, or otherwise verify and enforce
FEATURE_PS_TIME in the deployed image. Ensure nonzero ps execution does not
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[robobun]

On-host data for these episodes, from darwin-arm64-hardtack during the Sep 18 one: the kernel held 64,728 TCP sockets that no process owned (sysctl net.inet.tcp.pcbcount, while netstat -an listed 66). The mbuf pool was 2 % in use. macOS 26 caps TCP memory at 1/32 of RAM (tcp_init in xnu bsd/netinet/tcp_subr.c, bsd/kern/mem_acct.c), and past 80 % of the cap tcp_input drops received data, which fits the artifact download timeout. hardtack has 8 GB (hw.memsize), not 16 GB, so it reaches the cap first. Details and a guard in the runner are in #43462.