test(code): sandbox creation must not run on the blockbuster-guarded loop

[Jack Michaud (jack-michaud)]

Summary

Failing regression test for a server-startup bug: sandbox creation runs synchronously on the LangGraph dev event loop and is killed by the blockbuster blocking-I/O guard.

Failure: dcode --sandbox <provider> (reproduced on langsmith, agentcore, and previously reported for daytona in #5278) aborts before the readiness check passes:

Error: Server failed to start: RuntimeError: Server graph 'agent' failed readiness
check (status: 500): Sandbox creation failed for 'langsmith': Failed to list
snapshots: Blocking call to socket.socket.connect

Root cause: langgraph dev arms blockbuster (langgraph_runtime_inmem/queue.py -> _enable_blockbuster()), which raises BlockingError for patched blocking calls (socket connect, time.sleep, ...) on the asyncio loop. _make_graphs is async and calls the sync create_sandbox(...) / __enter__() directly on that loop (server_graph.py), which invokes the provider's sync get_or_create(...) — real blocking I/O. Every other blockbuster-hostile step in _make_graphs is already offloaded via asyncio.to_thread (with comments citing #5043); sandbox creation is the one that was missed. Note SandboxProvider.aget_or_create exists but has zero call sites.

What this test does

Mimics the server's guard with BlockBuster().activate() around a real _make_graphs run (modules stubbed per the existing test_interpreter_settings_apply_before_agent_construction pattern). The fake provider's sync get_or_create does a time.sleep — deterministic, and safe under pytest-socket's --disable-socket, unlike a real socket connect. On main, blockbuster raises BlockingError, _make_graphs hits its startup-error path, and the test fails with SystemExit(1). Once sandbox creation is offloaded off the event loop (e.g. asyncio.to_thread, matching the existing pattern in this file), the provider runs in a worker thread and the test passes. Verified locally: fails on main, passes with the to_thread fix applied.

Implementation notes

• Uses explicit BlockBuster().activate()/deactivate() rather than blockbuster_ctx: blockbuster <1.5.27 lacks the try/finally in that helper, so the guard leaks into later tests when the body raises. The repo's uv.lock currently pins 1.5.26. The explicit form is version-stable and always restores.
• Test lives in test_server_graph.py (the server-side seam where the fix belongs), reusing the existing _import_fresh_server_graph / _backend_with_offload helpers.

Out of scope

Deliberately a test-only PR (per the bug template guidance). The fix itself is straightforward — offload sandbox creation off the event loop — and will be tracked by the follow-up issue referencing this PR.

Fixes #6018

[github-actions]

This PR has been automatically closed because you are not assigned to the linked issue.

Opening a PR is not an indication that it will be accepted. This process exists so maintainers can confirm a change is aligned with the project direction before you invest time implementing it. Please:

1. Comment on the linked issue explaining the approach you would like to take and why — include enough detail for a maintainer to evaluate the design. Do not post a drive-by "please assign me" comment with no substance; those will be ignored.
2. Wait for a maintainer to assign you. Once assigned, your PR will be reopened automatically.

Maintainers: reopen this PR or remove the missing-issue-link label to bypass this check.