refactor: build the DIAL LOCATION rewrite in a reusable scratch buffer - #67
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The rewrite allocated a fresh ~4 KiB string per rewritten datagram, on the packet path. Build into a scratch reserved once to MAX_UDP_PAYLOAD_SIZE instead, and compute the rewritten size up front so the overflow check needs no copy and the splice cannot reallocate. DialRewrite now carries a view into that scratch; both callers send it before returning.
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The LOCATION rewrite allocated a fresh ~4 KiB string per rewritten datagram, on the packet path. It now builds into a scratch reserved once to
MAX_UDP_PAYLOAD_SIZE.The rewritten size is computed up front from the payload length, the spliced span and the formatted authority, so the overflow check rejects without copying and the splice that follows fits the reserved scratch by construction — the previous order built the string first, which in the overflow case would have reallocated past the reserve, exactly where it matters. The authority itself is formatted into a stack array, so the replacement text doesn't allocate either.
DialRewritecarries a view into the scratch; both callers send it before returning.Native (889), docker/Linux (876) and e2e (33 cases) green. The new test covers the reused buffer's real risk — a stale tail from a longer predecessor — and was verified to fail when the reset is suppressed, while the other DIAL rewrite tests still pass (their payloads are similar lengths).