[Server] Stop overwriting client responses while polling for them

[chr-hertel]

A request from the server to the client (sampling, elicitation, roots) intermittently timed out over Streamable HTTP when the server runs in more than one process. In CI this showed up as rare 30–90s hangs in the multi-worker php -S tests, so far in DualEraEndpointTest::testAsksForInput (#540) and in HttpClientCommunicationTest on #541.

Cause

While such a request is pending, the process holding the SSE stream polls the session every 100 ms. Each poll calls Protocol::consumeOutgoingMessages(), which:

1. reads the session,
2. empties the outgoing queue,
3. saves the whole session back, even when the queue was already empty.

The client's answer arrives as a separate POST, usually handled by another process, which stores it in the same session. If that store lands between steps 1 and 3, step 3 writes the old copy back, and the answer is lost. The waiting request never sees it and times out. FileSessionStore writes atomically, but nothing protects a read-modify-write, so the session's last writer wins.

Fix

consumeOutgoingMessages() leaves the session untouched when there is nothing to dequeue. While it waits, the polling loop then only reads, until the answer is there.

Test

ProtocolSessionRaceTest interleaves the two processes deterministically. A store decorator runs the answering worker's processInput() right after the waiting worker's read.

• On main, the answer is gone (checkResponse() returns null).
• With this change, it is found.

Existing unit tests, including the ones counting save() calls with messages queued, are unchanged and green.

Not covered

Writes that do have something to persist are still unprotected read-modify-writes of the whole session. Two processes changing a session at the same moment can still lose one change. Closing that fully needs locking or a compare-and-swap in SessionStoreInterface, which is a larger change. This PR removes the write that happened on every poll, which is the one that made this race likely.

🤖 Generated with Claude Code

https://claude.ai/code/session_014RjzbQfz2DVm9xHmGsqM7d

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[chr-hertel]

Side effect of introducing client tests - will be closed in favor of #535