fix: wake MCP session::recv when SSE stream closes

[cursor]

Bug and impact

http::mcp::session::recv() with the default unbounded timeout used cv.wait. That wait is only notified by POST enqueue or erase_session. When the SSE stream becomes not good (client disconnect after a write sets failbit, or any path that makes sse::session::closed() true), ready()'s m_stream.closed() clause was never re-checked.

Custom sse_transport::attach handlers that block in recv() can hang forever after the stream dies, leaving the SSE handler thread alive so http::server::stop() → wait_for_handlers() never completes.

mcp::server::run_session already avoids this with a 1s timed recv + send_comment probe; the public session::recv() default did not.

Root cause

cv.wait only re-evaluates its predicate on notify. Stream close/failbit never notifies the session CV.

Fix

Slice the unbounded wait with wait_for (50ms) so m_stream.closed() is re-checked. Timed recv(timeout > 0) was already correct.

Complementary to open PR #68 (shutdown latch for poll-only closed()): #68 makes stop() visible without a write; this PR makes recv() notice once closed() is true.

Validation

• Unit regression: unbounded recv() returns nullopt within 2s after badbit is set on the underlying stream (no CV notify).
• Full MCP suite: test_case.*MCP — 25/25 passed (89 assertions).

  

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Note

Medium Risk
Changes blocking/wakeup semantics on the MCP SSE session path; incorrect behavior could still strand handler threads during shutdown, but the change is narrow and covered by a regression test.

Overview
Fixes a hang when session::recv() is called with the default unbounded timeout while the SSE stream dies without a condition-variable notify (client disconnect / failbit).

recv() no longer uses a single cv.wait for that path. It loops on wait_for(50ms) so ready() periodically re-evaluates m_stream.closed(), matching the behavior callers already get with a positive timeout. Timed recv is unchanged.

Adds a regression test: unbounded recv() on a stringstream-backed SSE session returns nullopt within 2s after badbit is set, with no POST or session erase.

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