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fix: bounded notification writes — a non-reading MCP client must never wedge the embedder - #22
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…r wedge the embedder displayxr-runtime#928: broadcast_notification did a BLOCKING write per connected client, on whatever thread the embedder called it from. The DisplayXR runtime broadcasts from app main threads inside OpenXR calls (xrSetMCPAppInfoDXR registers tools -> notify_tools_changed), so one connected client that stopped reading (a displayxr-voice instance, live or orphaned) filled the pipe buffer and froze every subsequently launched app before its render loop started. - mcp_conn_write_bounded(conn, buf, len, timeout_ms): frame-scoped deadline on both arms (Windows: overlapped wait + CancelIoEx on timeout; POSIX: poll(POLLOUT) with remaining-deadline before each send). A timed-out write ABORTS the connection — a partial frame corrupts Content-Length framing, so the stream must not be reused; the serve thread notices and unwinds the slot. - broadcast_notification: 250 ms budget per client per write, and TRYlock on write_mutex — if the client's own serve thread is stuck writing a response to the same dead peer, the notification is dropped instead of queueing behind it (notifications are fire-and-forget by contract). A slow consumer now costs the embedder at most ~500 ms once, then loses its connection. Request/response writes on serve threads are unchanged (self-contained; server stop already aborts them). Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
…e OVERLAPPED on sync handles Live validation of the bounded write caught it still blocking inside ZwWriteFile: CreateNamedPipeA never passed FILE_FLAG_OVERLAPPED, so every ReadFile/WriteFile on server-side conns ran synchronously and ignored the OVERLAPPED structs the I/O paths were already passing — the IO_PENDING branches (and the bounded write's timeout+CancelIoEx) were dead code. The accept path moves to an overlapped ConnectNamedPipe (NULL overlapped on an overlapped handle can misreport completion); semantics unchanged — blocking accept, still woken by mcp_listener_close's CancelIoEx. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Live validation on the SR box (runtime FetchContent pointed at f0f1e13, DisplayXRClient.dll deployed, displayxr-voice ALIVE — the deterministic wedge precondition): app launch sailed through xrSetMCPAppInfoDXR, session began, render loop running, main thread sampled clean (Win32 message pump). Under v0.5.0 this froze inside ZwWriteFile 100% of the time. Note the second commit is load-bearing: the first (bounded write alone) still wedged live because the server pipes were synchronous handles — WriteFile ignored the OVERLAPPED entirely. FILE_FLAG_OVERLAPPED on CreateNamedPipe is what makes both the existing I/O paths and the new timeout real. |
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Fixes the framework side of DisplayXR/displayxr-runtime#928 (launch-blocker: with any displayxr-voice client connected, every DisplayXR app launch wedged at xrSetMCPAppInfoDXR inside a blocking notification write; captured live with full symbols during the 2026-08-14 close gauntlet).
mcp_conn_write_bounded(both transports) with frame-scoped deadline + abort-on-timeout (partial frames corrupt LSP framing — the stream must die, and its serve thread unwinds the slot).broadcast_notification: 250 ms budget per client + trylock on write_mutex (drops the notification rather than queueing behind a serve thread stuck on the same dead peer).Validation: MSVC lib+adapter build clean locally; POSIX smoke tests ride CI (they don't compile under MSVC — pre-existing
strings.hinclude). End-to-end validation against the live wedge repro (runtime FetchContent pointed at this branch, voice client alive, app launch) is being run on the SR box and will be reported on runtime#928.🤖 Generated with Claude Code