[atomicdeploy]

Summary

Keep AsyncAbstractResponse's final local buffer alive until its bytes have actually been admitted to the TCP queue, and finish only after admitted header/body bytes have been acknowledged.

The current implementation marks RESPONSE_END while filling the final source buffer. A short AsyncClient::add() can leave a tail in _send_buffer; the next request ACK then considers the response finished and closes it. Partial body adds also bypass _writtenLength accounting, while headers can be counted twice.

Changes

• Separate source EOF from response completion with one boolean.
• Account for every partial TCP add exactly once; retain zero/partial tails.
• Preserve the current RESPONSE_TRY_AGAIN credit fix and avoid spending a header credit when zero bytes were admitted.
• Wait for the final actual ACK; a poll or partial ACK cannot finish an unacknowledged tail.
• Treat a failed send as an explicit transport failure, not successful completion.

Reproduction and verification

Prepared against exact upstream main 4d4eaca1833c3c11940a988fc8a6c0bdf734bbd2.

tests/run-response-backpressure.mjs extracts and compiles the actual write_send_buffs implementation from src/WebResponses.cpp with small transport/source doubles. It is not a separately rewritten response algorithm. Requires Node.js and a C++17 g++ (or CXX).

node tests/run-response-backpressure.mjs

The optional --baseline SOURCE --output EXECUTABLE mode verifies the pinned original source hash before reproduction. The original method fails deterministically with an eight-byte final source buffer, one admitted body byte and seven pending bytes while already exposing RESPONSE_END. Both in-flight-credit configurations reproduce the failure. Both patched configurations pass:

• Partial/zero header and body adds, zero TCP window, and partial final ACKs/polls.
• Empty, known-length, unknown-length, chunked/terminator and 224 KiB bodies with byte-exact reconstruction.
• RESPONSE_TRY_AGAIN, failed sends and invalid sources.
• Consistent actual admitted/acknowledged byte accounting.

This is focused host/mocked-transport evidence, not a full Arduino/lwIP build, packet-level measurement, hardware/Wi-Fi soak or proof that every reported network failure has this cause. No installed SDK was modified; this upstream patch follows a downstream immutable-response workaround and its reproducible short-add regression.

[mathieucarbou]

@atomicdeploy please open an issue first describing the bug you saw and an MRE to reproduce it. Ideally written similar to a project exemple.
We’ll discuss in this issue how to fix it. Opening a PR from AI like that does not help understand the issue you had and also does not help us.
Thank you !