The per-write deadline that severs a stalled SSE reader was never cleared on
return. Server.WriteTimeout is deliberately unset -- a WriteTimeout would sever
a healthy long-lived stream -- and with it unset net/http never resets the
connection write deadline between keep-alive requests. So the deadline the last
writeChunk left set leaks onto the next request that reuses the pooled
connection and fails its first write for no reason. Clear it to the zero value
on return via a deferred rc.SetWriteDeadline; best-effort, a no-op on writers
without deadline support.
Also record honestly at the header flush that the connect-time stall stays
bounded only by the per-principal stream cap, not severed by this guard -- only
the mid-stream stall is closed. Adds a test pinning the clear (fails closed:
neutering the deferred clear leaves a +writeTimeout deadline set on return).
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