[CI] iOS/tvOS 16 simulator runtime downloads intermittently fail with curl 56 timeout

[dalexsoto]

Failure signature

Provision system dependencies falls back to Apple's downloadable simulator index, then a legacy iOS or tvOS 16 runtime download fails after roughly 11-12 minutes with:

curl: (56) Recv failure: Operation timed out
Failed to download the <platform> 16.0 simulator runtime.
System check failed

Confirmed occurrences

PR / head	Build and job	Runtime	Outcome
#26203 / abd0ea0058d6285dccbdcc9c0fc4837d264b45d0	14718863 - monotouch_macos	tvOS 16.0	Download timed out with curl 56 after about 11 minutes. Simulator-tests attempt 2 was accepted at 19:35 UTC and remains queued.
#26200 / 966f7cc95666adf0926c09964990868e5f161864	14718622 - cecil, attempt 2	iOS 16.0	Download timed out with curl 56 after about 12 minutes. This occurred during the currently active simulator-tests retry.

First observed: 2026-07-21 15:40 UTC. Latest observed: 2026-07-21 23:13 UTC. Recurrence: 2 jobs across 2 distinct monitored PRs.

Classification and impact

This is a CI infrastructure/network failure, not a source or test regression. Both jobs fail while provisioning Apple simulator runtimes before tests start. The failures occur in unrelated PRs and projects, and the stable symptom is an Apple CDN transfer timing out with curl 56.

Investigation

• Inspected both Azure timelines and the failing Provision system dependencies task logs.
• Confirmed that Xcode platform/component setup completed before the legacy runtime download.
• Searched open and closed dotnet/macios issues using the exact curl error, iOS/tvOS runtime messages, and simulator provisioning terms; no matching issue was found.

Workaround

Rerun failed jobs after Apple CDN/network conditions recover, subject to mergeability and retry-budget gates. Do not submit another retry while an equivalent stage attempt is queued or running. Azure task logs have limited retention.

[dalexsoto]

PR #26200 retry outcome: build 14718622 simulator_tests attempt 2 completed failed because cecil hit the iOS 16 runtime curl-56 timeout; the other three retried jobs succeeded. Attempt 3 was accepted at 00:49 UTC to rerun only cecil. Budget: 2 lifetime/current-head retries.

[dalexsoto]

Final retry outcome: PR #26200 build 14718622 simulator_tests attempt 3 completed successfully at 01:24 UTC. Cecil passed, the overall build result is succeeded, and the PR merged at 01:28 UTC.