ci: apt mirror failover, retried LLVM install, no fail-fast between build variants

[robobun]

Problem

• On 2026-08-19 about a dozen Preview Build runs failed early in the docker build. 22 of 30 failed jobs end in E: Package 'python3' has no installation candidate. 8 end in llvm.sh: [error] GPG key not reachable, a failed script download, or Distribution 'ubuntu' ... is not supported by this script.
• Cause 1, Dockerfile:41: azure.archive.ubuntu.com timed out for an hour. The fallback apt-get update || <switch mirror> never runs, because apt-get update exits 0 when a mirror is down.
• Cause 2: the five apt based Dockerfiles make each apt.llvm.org request once, with no retry. With fail-fast, each failed variant also cancelled its siblings.

Fix

• Dockerfile: dylan-conway's b840022 from [JSC] Let an embedder's global object intercept global-scope variable accesses and declarations #471 (also in Error stack GC finalizer: render only frame lines, add the name/message header at materialization #302), unchanged, so this merges cleanly with both and lands the fix either way. apt gets a mirror list (azure, then archive) through mirror+file: and fetches a failed index or .deb from the other mirror, in every apt-get run.
• scripts/install-llvm.sh, used by all five Dockerfiles: one attempt is key, script, llvm.sh, retried up to five times with a growing pause. The two downloads have time limits, so a stalled server fails the attempt. llvm.sh is idempotent and skips its own key download when the key file exists. The installed packages do not change.
• build-reusable.yml: fail-fast: false on every matrix, the same hunk as 418804c in Upgrade LLVM toolchain from 21.1.8 to 22.1.8 #296.
• Verified: focal's real apt 2.0.2 fails over (InRelease, Packages, .deb) when the first mirror refuses, answers 503, is unreachable, or hangs, and the control run reproduces cause 1. The first preview build (41 of 42 green, no sibling cancelled) hit a connect failure on apt.llvm.org in the one fetch the script did not retry yet. It does now. Preview builds 32276671889 and 32282832625 (this version) built all 42 variants.

Background

• A run is about 25 docker builds with no layer cache. Each build installs its toolchain from the network.
• llvm.sh is apt.llvm.org's installer. It probes the repo and the key with wget --method=HEAD, which does not retry an HTTP error.
• curl --retry covers timeouts, a few HTTP codes and ECONNREFUSED only. The first preview build failed on another connect error, so the retry is a shell loop.

Notes

Related commits in open PRs. The Dockerfile hunk is identical to #471 b840022 and #302 672dd8f. The workflow hunk is identical to #296 418804c and to the workflow half of #297 3946a08. The Dockerfile half of #297, and #296 f63d3da and 8c35fe7, address the arm64 side of cause 1 (ports.ubuntu.com was down for hours on 2026-07-16, with the same dead apt-get update || diagnosis). This PR does not cover arm64 or the 25 noble based builds, which still use one host each. A mirror list for those is a follow-up to settle with the maintainers. For the focal image, dylan-conway decided on #471 to keep security.ubuntu.com as the only source of the security pocket, and this PR follows that. #296 replaces the LLVM step on arm64 and #314 edits next to it, so both get a small rebase onto install-llvm.sh.

Follow-ups that would remove these downloads instead of retrying them: mirror the llvm-21 debs to a sha256 pinned release asset, as gcc-13-focal-debs and compiler-rt-* already are, and --cache-from and --cache-to for the buildx invocations in the *-release.sh scripts.

Failed jobs examined, with the duration of the Run step: 96008539971 linux-amd64-debug (35 s), 95995258770 linux-amd64-debug-asan (34 s), 96007762013 linux-amd64-debug (35 s), 95997209112 linux-amd64-lto (50 s, the index fetch succeeded and the .deb downloads from the azure mirror failed), 95997310102 linux-amd64-lto (5.5 min, same), 96008995721 linux-arm64 (GPG key not reachable), 95998064531 freebsd-amd64-debug and 95998175656 windows-amd64-asan (no output at all from wget -qO- llvm.sh | bash, so the script download failed), 95998064664 macos-arm64-debug-asan (the key download failed after its HEAD probe passed), 95998213101 macos-arm64-lto (Distribution 'ubuntu' in version '24.04.4 LTS' is not supported, so the HEAD probe of apt.llvm.org/noble/ failed). All 30 failures in the 11 runs fail in one of these two steps. The musl images (alpine, distro LLVM) did not fail in any run. Docker Hub was not involved, every base image pull took under a second.

The azure failure is a connect timeout, Could not connect to azure.archive.ubuntu.com:80 (52.147.219.192), connection timed out (also 52.252.75.106), after 30 s, which is the 34 to 35 s Run step. apt remembers a failed connection per IP for the rest of the run, so a dead mirror costs about 30 s once per apt-get run, and then every file goes to the other mirror.

A successful build (job 95952576742) requests only InRelease and Packages per suite plus 312 debs from the archive, and no optional files (the image sets Acquire::Languages none). This matters because the mirror transport reports a file that is missing on every mirror as an error, not as an ignorable 404. The rewritten sources.list was checked against the stock focal file from the OCI rootfs: the 7 archive lines become deb mirror+file:/etc/apt/mirrors.txt <suite> <components>, the 3 security lines and the arm64 ports.ubuntu.com lines are unchanged. add-apt-repository (kitware step, llvm.sh) picks its template by codename and ignores entries with a URI it does not know, so the mirror+file: lines do not affect it (focal python-apt 2.0.1, aptsources/distro.py).

Acquire::Retries alone does not cover cause 2: focal's apt 2.0.2 with Acquire::Retries=3 makes exactly one request for an index or a .deb that answers 503. The shell level retry of the whole LLVM install covers it on focal and noble alike.

The apt check: apt 2.0.2 and libapt-pkg 6.0 from the focal pool, extracted and run with Dir::Bin::Methods pointing at their methods, a one package repo signed with a throwaway key, the mirror list in the exact format of this hunk, the broken mirror on its own loopback address at priority:1 and the live one at priority:2. Output per scenario: Ign: <broken> InRelease, Get: <live> InRelease, Get: <live> Packages, then Ign: <broken> hello-wk, Get: <live> hello-wk, Download complete. The control run (one unreachable mirror, no list) gives exit 0 from apt-get update and exit 100 from the install, as in the CI logs.

The script check, with a fake curl and a fake llvm.sh on PATH: 2 curl calls and 1 run when everything works, a retry after 5 s when the key fetch fails (the first preview build's failure, job 96029579592: curl: (7) Failed to connect to apt.llvm.org port 443 after 187 ms), 3 attempts when llvm.sh fails twice, exit 1 after 5 attempts when nothing answers. Against a server that accepts and never answers, a download gives up after 60 s (curl exit 28), which fails the attempt.

The first version of this PR listed security.ubuntu.com as a third mirror and routed the security pocket through the list as well. It was replaced with the hunk already agreed on #471.

[coderabbitai]

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Walkthrough

Changes

Build reliability updates

Layer / File(s)	Summary
Shared LLVM installer scripts/install-llvm.sh	Adds LLVM version validation, timed downloads, up to five installation retries, increasing delays, and failure reporting.
Container LLVM and APT integration Dockerfile, Dockerfile.android, Dockerfile.freebsd, Dockerfile.macos, Dockerfile.windows	Container builds use the shared installer. The Linux image adds prioritized APT mirrors and retry settings.
Non-fail-fast build matrices .github/workflows/build-reusable.yml	Five build matrices continue running remaining variants after a variant fails.

🚥 Pre-merge checks | ✅ 3 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description is detailed and relevant but omits the required Bugzilla link, review status, and template-style changed-file listing.	Add the bug title and Bugzilla URL, include “Reviewed by NOBODY (OOPS!).”, and provide the required changed-file and function entries.

✅ Passed checks (3 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 three main changes: APT mirror failover, LLVM retries, and disabled matrix fail-fast behavior.

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[robobun]

Status: ready for review. Head 85d8f38, mergeable, no open review threads.

Preview builds:

• 32282832625 (85d8f38, the version in this PR): all 42 variants built, release autobuild-preview-pr-473-85d8f38b has 42 assets.
• 32276671889 (eae9e246, the same without the download time limits): all 42 variants built.
• 32240423748 (first version): 41 of 42 built, none cancelled. The failure, windows-amd64 (job 96029579592), was curl: (7) Failed to connect to apt.llvm.org port 443 after 187 ms in the key fetch, which that version ran outside the retry loop. The loop now covers the key, the script and llvm.sh. The Dockerfile mirror hunk was also replaced with the one from [JSC] Let an embedder's global object intercept global-scope variable accesses and declarations #471 (b840022), unchanged.

How the two causes were reproduced without CI:

• Cause 1: focal's apt 2.0.2 (extracted from the focal pool) against a local signed repo. With one unreachable mirror, apt-get update exits 0 and the following apt-get install exits 100, the sequence in the failed jobs. With the mirror list from this PR, the InRelease, Packages and .deb fetches fail over to the second mirror when the first one refuses, answers 503, is unreachable, or hangs.
• Cause 2: read from the logs of the failed llvm.sh jobs listed in the PR notes and from the current llvm.sh, which probes with wget --method=HEAD and downloads the key without retrying. The retry loop was run against a fake curl and llvm.sh, including a failing key fetch and a stalled download.

[claude]

LGTM — CI reliability fix, no product code touched.

What was reviewed:

• mirror+file: rewrite in Dockerfile: sed regex matches only archive/security.ubuntu.com lines (arm64 ports.ubuntu.com untouched), and .dockerignore does not exclude scripts/ so the bind mount resolves.
• install-llvm.sh: curl is present in every image that mounts it (added to android/freebsd/macos, already in windows/focal); retry loop exits 0/1/2 correctly.
• fail-fast: false added to the five matrices that lacked it; windows-cross already had it and windows has a single entry.

Extended reasoning...

Overview

CI-only change across 7 files: five Dockerfiles, the reusable build workflow, and a new scripts/install-llvm.sh. The focal Dockerfile replaces a broken apt-get update || <switch mirror> fallback with apt's mirror+file: transport (three prioritized Ubuntu mirrors). All five apt-based images now install LLVM via a bind-mounted wrapper that pre-fetches the GPG key and llvm.sh with curl --retry and retries llvm.sh up to five times with backoff. The workflow adds fail-fast: false to each remaining multi-entry matrix so one flaked variant no longer cancels its siblings.

Security risks

None introduced. The wrapper fetches from the same https://apt.llvm.org/ endpoints as before and writes the key to the same path llvm.sh uses (/etc/apt/trusted.gpg.d/apt.llvm.org.asc). Package selection is unchanged — the wrapper only adds retries around the existing install. The mirror list contains only Canonical-operated hosts already trusted by the image.

Level of scrutiny

Low-to-medium. This is build/CI infrastructure with no changes to JSC, WTF, or bmalloc, and it is self-verifying: the PR's own preview build exercises all 42 variants through the modified Dockerfiles and workflow. A regression would surface as a failed docker build, not as shipped-artifact behavior.

Other factors

• All Dockerfiles already use BuildKit (--mount=type=tmpfs is present throughout, and release.sh/siblings invoke docker buildx build), so --mount=type=bind is available.
• curl was added to the three images that lacked it; Dockerfile.windows and the focal Dockerfile already installed it.
• Checked .dockerignore — scripts/ is not excluded, so the bind-mount source resolves in the build context.
• The windows-cross matrix already had fail-fast: false prior to this PR, and the native windows job has a single matrix entry, so neither needed a hunk.
• The PR description documents local verification of the mirror failover against focal's apt 2.0.2 and of the retry loop against a mock llvm.sh.

[claude]

LGTM — CI reliability fix, no product code touched.

What was reviewed:

• install-llvm.sh: retry loop is guarded by if attempt, so set -e doesn't fire mid-attempt; key path matches what upstream llvm.sh checks before its own download.
• All five Dockerfiles use buildx (checked *release*.sh) so RUN --mount=type=bind is supported, and scripts/ is not in .dockerignore.
• curl is added to the three Dockerfiles that lacked it; windows/focal already had it.
• fail-fast: false added to the five matrices missing it; windows-cross already had it and single-entry windows doesn't need it.

Extended reasoning...

Overview

This PR hardens the CI docker builds against transient network failures on azure.archive.ubuntu.com and apt.llvm.org. Seven files change: five Dockerfiles swap their one-shot llvm.sh fetch for a new scripts/install-llvm.sh that wraps the same install in a 5-attempt retry loop; Dockerfile (focal) additionally moves apt to a mirror+file: list with Acquire::Retries (the exact hunk already agreed on #471); and .github/workflows/build-reusable.yml gets fail-fast: false on every multi-entry matrix. No JavaScriptCore, WTF, or bmalloc source is touched.

Security risks

None identified. The install script fetches the same GPG key and installer from apt.llvm.org over HTTPS that the previous wget -qO- ... | bash step fetched, and writes the key to /etc/apt/trusted.gpg.d/ where llvm.sh itself would put it. The apt mirror list only lists the two canonical Ubuntu archive hosts and leaves security.ubuntu.com untouched. No new external hosts, secrets, or permissions are introduced.

Level of scrutiny

Low. This is build-infrastructure only, confined to Docker image construction and GitHub Actions matrix behaviour. The installed package set is unchanged (same llvm.sh <version> [all] invocation), and the workflow change is the standard fail-fast: false toggle. Failure modes are visible immediately in CI, and the first preview build already ran 41/42 green with no sibling cancellations.

Other factors

I checked that the RUN --mount=type=bind,source=scripts/install-llvm.sh mount is viable: every *-release.sh invokes docker buildx build (BuildKit) with the repo root as context, and .dockerignore does not exclude scripts/. The attempt() function's && chain runs under if, so set -e correctly does not abort the script on a failed attempt. The three noble-based Dockerfiles that previously lacked curl now install it before the LLVM step. The PR description documents local reproduction of both failure modes against real focal apt 2.0.2, and the Dockerfile/workflow hunks are byte-identical to ones already reviewed on #471 and #296.

[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@scripts/install-llvm.sh`:
- Around line 28-41: Update the curl invocations in attempt to include finite
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[github-actions]

Preview Builds

Commit	Release	Date
85d8f38b	autobuild-preview-pr-473-85d8f38b	2026-08-19 18:11:42 UTC
eae9e246	autobuild-preview-pr-473-eae9e246	2026-08-19 17:33:03 UTC

[coderabbitai]

Note

GitHub couldn't provide a complete incremental comparison for this pull request, so CodeRabbit is performing a full review instead. This review may take a little longer.

[claude]

LGTM — CI-only reliability change, verified by a full 42-variant preview build.

What was reviewed:

• install-llvm.sh: retry loop under set -e — attempt runs in if context so errexit is suppressed and the && chain returns non-zero to drive retries; key path matches where llvm.sh checks so its own unretried fetch is skipped.
• Dockerfiles: curl added where the new script needs it; --mount=type=bind is fine since BuildKit is already required (existing --mount=type=tmpfs).
• fail-fast: false: the release job still needs: every build job, so a failed variant still blocks release — only sibling cancellation is removed, as intended.

Extended reasoning...

Overview

CI/build-infrastructure reliability fix touching seven files: a new scripts/install-llvm.sh (50-line retry wrapper around apt.llvm.org's llvm.sh), five Dockerfiles updated to bind-mount and call it instead of wget -qO- | bash, an apt mirror+file: failover list in the focal Dockerfile, and fail-fast: false on five matrices in build-reusable.yml. No runtime, engine, or library code is touched.

Security risks

None material. The script fetches the same signing key and installer over HTTPS from the same host as before; the key lands in the same trusted.gpg.d path llvm.sh itself uses. No new external sources, no relaxed verification, no new secrets. The apt mirror list adds only archive.ubuntu.com alongside the existing azure.archive.ubuntu.com, both official Ubuntu mirrors, with security.ubuntu.com deliberately kept single-source per the maintainer decision on #471.

Level of scrutiny

Low-to-moderate. This is build-container plumbing whose failure mode is a red CI job, not a shipped defect. The Dockerfile mirror hunk and the workflow hunk are stated to be byte-identical to commits already agreed on in #471 and #296. The new logic (install-llvm.sh) is short, well-commented, and its edge cases were exercised: the PR description documents testing against focal's real apt 2.0.2, a fake curl/llvm.sh harness, and a stalled server, and preview build 32276671889 built all 42 variants green.

Other factors

• The one CodeRabbit finding (missing curl timeouts) was addressed in 85d8f38 and the thread is resolved.
• I checked the set -euo pipefail interaction: attempt is called as an if condition, which disables errexit for its body, so a failing curl correctly propagates through the && chain and triggers a retry rather than aborting the script.
• curl was added to the base apt-get install in Dockerfile.android/freebsd/macos (previously wget-only); Dockerfile and Dockerfile.windows already had it.
• fail-fast: false does not risk publishing a partial release: the release job needs: every build job and only runs if all succeed; the change just stops one flaky variant from cancelling its siblings so they can be individually retried.

[robobun]

Data point for prioritising this: the preview build for #456 (a two-file data change) has now failed 4 of 4 attempts over the last day, each time one of the Linux x64 jobs dying a few minutes in (linux-amd64-debug-asan twice, linux-amd64-debug and linux-amd64-asan once each) and fail-fast cancelling the rest, so no release gets published. The arm64 and cross-compiled jobs pass every time. Since build-reusable.yml is shared with the main branch builds, the same failure mode presumably applies to the autobuild of anything merged to main until this lands.