fix(xmtp_mls): stabilize should_reconnect flaky test in d14n suite

[xmtp-coder-agent]

Resolves #3438

Summary

xmtp_mls::client::tests::should_reconnect exhausts its retry budget in the --features d14n nextest suite with a Status::Cancelled / "connection closed" error on the second stream_conversations call — the one that verifies reconnection after the toxiproxy blackhole is lifted.

Root cause

1. During the blackhole, HTTP/2 keep-alive pings time out and tonic marks the Channel as disconnected.
2. The test removes the toxics and asserts the old stream is done.
3. The test then opens a new stream. tonic::transport::Channel reconnects lazily on the first RPC after a transient failure — and that first RPC can observe hyper::Error(Canceled, "connection closed") → tonic::Status::Cancelled if it fires while the channel is still transitioning out of the failure state.

Why d14n flakes more than v3

The ToxicOnly d14n test client is D14nClient<ReadWriteClient<ToxicXmtpdClient, ToxicGatewayClient>>. ReadWriteClient holds two independent tonic Channels (one per backend) and for_each applies/removes toxics on both. With two channels to recover in lock-step, the reconnect race is roughly twice as likely to land inside the "Channel still transitioning" window. The v3 toxic client has one proxy / one channel.

Fix

Wrap the final stream_conversations call in the existing xmtp_common::retry_async! macro with an ExponentialBackoff strategy (5 retries, 100ms initial backoff). GrpcError is already universally retryable, so the retry mechanism correctly handles transient transport errors. Non-transport errors short-circuit immediately via RetryableError, so the test still fails loudly on genuine failures.

Why retry_async! and not a longer sleep

A longer sleep would need to be sized for the worst case, increasing test duration on every run. retry_async! already exists, already honours RetryableError, and only waits as long as the channel actually needs.

Scope

Test-only change. No production code is touched:

• ❌ No changes to stream_conversations
• ❌ No changes to tonic/hyper keep-alive settings
• ❌ No changes to for_each_proxy or ReadWriteClient

The earlier assertions that verify the stream errored out during the blackhole and the assert_stream_done! check are unchanged. Same test name, same final assertions on the new stream's first item.

Test plan

• cargo clippy -p xmtp_mls --features test-utils,d14n --tests --no-deps -- -Dwarnings
• cargo clippy -p xmtp_mls --features test-utils --tests --no-deps -- -Dwarnings (v3 build)
• cargo fmt --check
• just test d14n -E 'test(should_reconnect)' — run the single test in the failing suite (requires backend; please run in CI)
• just test v3 -E 'test(should_reconnect)' — confirm no regression in v3
• Run the d14n test in a loop (≥10 iterations) to confirm stability

🤖 Generated with Claude Code

Note

Retry subscription setup in subscribe_group_messages and subscribe_welcome_messages to fix flaky test

Wraps the initial subscription API calls in mls.rs with retry_async! using the existing retry_strategy, so transient transport errors during subscription setup are retried rather than immediately failing. This stabilizes the should_reconnect test in the d14n suite.

^{Macroscope summarized b7bf1de.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.16%. Comparing base (701cc12) to head (b7bf1de).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3446      +/-   ##
==========================================
+ Coverage   85.12%   85.16%   +0.04%     
==========================================
  Files         411      412       +1     
  Lines       63290    64141     +851     
==========================================
+ Hits        53873    54626     +753     
- Misses       9417     9515      +98     

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[tylerhawkes]

Even though the test-workspace workflow passed, there were still retries for the should_reconnect test. It is not fixed.

[insipx]

@xmtp-coder-agent pull in latest and resolve conflicts

[insipx]

@xmtp-coder-agent you didn't resolve all the conflicts yet

[neekolas]

Resolved and pushed (b7bf1dee9). My earlier push had merged a stale main; since then 81 more commits landed (including the #3828 transport-reconnect / BidiTransport rework), which is what re-introduced the conflict.

The only real conflict was in crates/xmtp_api/src/mls.rs: main added #[xmtp_common::rpc_span] to the subscribe_* methods, right where this PR adds the retry wrapping. Resolved by keeping both — every subscribe_* method now has the rpc_span attribute and the retry_async! wrap. GitHub now reports the PR as mergeable.

One thing worth a look on your end: #3828 adds transparent reconnect at the d14n transport layer. If that already covers the post-reconnect first-RPC failure this PR targets, the wrapper-level retry here may be redundant — happy to close in favor of it, or keep it as defense-in-depth. Your call.

[neekolas]

The failing check (test-browser-sdk / shard 4 → DebugInformation.test.ts > should return network API statistics) was a flake, not a regression from this PR — it passed on re-run of the same commit (b7bf1dee9) with no code change, and the whole run is now green.

Root cause (unrelated to this PR's change):

• The test calls clearAllStatistics() and then immediately asserts apiStats2.queryGroupMessages === 0n.
• queryGroupMessages is incremented once per group sync_with_conn() / receive(). On a freshly-created client, the DeviceSync worker spawns automatically at client build and its sync_init → get_sync_group creates + syncs the sync group on first run (crates/xmtp_mls/src/worker/device_sync/mod.rs:368), issuing one query_group_messages.
• When that background sync races in between clearAllStatistics() and the stats read, the counter reads 1n instead of 0n. It's the same class of background-worker-vs-exact-count race the test already documents for the identity stats (lines 21–28), which were softened to lower-bound assertions for exactly this reason.

This PR only changes crates/xmtp_api/src/mls.rs (wrapping the three subscribe_* methods in retry_async!). Those methods increment subscribeMessages / subscribeWelcomes, never queryGroupMessages, so this change cannot affect the failing assertion.

No code change needed here to make CI pass. If maintainers want to de-flake the test itself, the fix would mirror the identity-stats treatment — assert a bound / await worker quiescence after clearAllStatistics() rather than an exact 0n — but that's a separate change to the newly-migrated browser-sdk suite, out of scope for this PR.