Guard TCP API writes after Wi-Fi reconnects

[Copilot]

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• Problem

  • The TCP API server could keep writing to stale WiFi sockets after reconnects, causing repeated errno 11 (EAGAIN) write failures, noisy logs, and degraded API stability.

• Change

  • Stream write path
    • Consolidates framed output into a shared writeFrame() path in StreamAPI.
    • Stops draining queued packets as soon as a frame write fails.
  • TCP socket health checks
    • Adds transport-specific pre-write checks in ServerAPI:
      • reject disconnected clients
      • reject non-writable clients when availableForWrite() is supported
    • Closes the client and disables further writes on pre-write failure or short write.
  • Transport compatibility
    • Uses compile-time detection for availableForWrite() so non-TCP / non-WiFi stream types keep existing behavior.
  • Coverage
    • Adds focused unit coverage for the socket write-readiness helper logic.

• Example

  if (!canWriteFrame(totalLen))
      return false;
  
  size_t written = stream->write(buf, totalLen);
  if (written != totalLen) {
      onFrameWriteFailed(totalLen, written);
      return false;
  }

🤝 Attestations

• I have tested that my proposed changes behave as described.
• I have tested that my proposed changes do not cause any obvious regressions on the following devices:
  • Heltec (Lora32) V3
  • LilyGo T-Deck
  • LilyGo T-Beam
  • RAK WisBlock 4631
  • Seeed Studio T-1000E tracker card
  • Other (please specify below)

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• api.registry.nm1.platformio.org
  • Triggering command: /home/REDACTED/.local/bin/pio /home/REDACTED/.local/bin/pio test -e native -f test_mqtt (dns block)
  • Triggering command: /home/REDACTED/.local/bin/pio /home/REDACTED/.local/bin/pio test -e native -f test_mqtt -vvv (dns block)
  • Triggering command: /home/REDACTED/.local/bin/pio /home/REDACTED/.local/bin/pio test -e native -f test_client_write_checks -vvv (dns block)
• api.registry.platformio.org
  • Triggering command: /home/REDACTED/.local/bin/pio /home/REDACTED/.local/bin/pio test -e native -f test_mqtt (dns block)
  • Triggering command: /home/REDACTED/.local/bin/pio /home/REDACTED/.local/bin/pio test -e native -f test_mqtt -vvv (dns block)
  • Triggering command: /home/REDACTED/.local/bin/pio /home/REDACTED/.local/bin/pio test -e native -f test_client_write_checks -vvv (dns block)
• collector.platformio.org
  • Triggering command: /home/REDACTED/.local/bin/pio /home/REDACTED/.local/bin/pio test -e native -f test_mqtt (dns block)
  • Triggering command: /home/REDACTED/.local/bin/pio /home/REDACTED/.local/bin/pio test -e native -f test_mqtt -vvv (dns block)
  • Triggering command: /home/REDACTED/.local/bin/pio /home/REDACTED/.local/bin/pio test -e native -f test_client_write_checks -vvv (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

[Copilot]

Pull request overview

This PR improves TCP-based API robustness by preventing StreamAPI/ServerAPI from continuing to write framed protobuf output to stale or non-writable WiFi/TCP clients after reconnects, reducing repeated write failures and log noise.

Changes:

• Refactors StreamAPI framed output into a shared writeFrame() path and stops draining queued frames after a write failure.
• Adds ServerAPI transport-specific pre-write checks (disconnected / non-writable) and closes the client on pre-check failure or short writes.
• Introduces a compile-time availableForWrite() detection helper and adds unit tests for that helper logic.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/mesh/StreamAPI.h	Adds frame-write hook points and changes emitTxBuffer() to return success/failure.
src/mesh/StreamAPI.cpp	Consolidates framing + write/flush into writeFrame() and aborts queue draining on failed writes.
src/mesh/api/ClientWriteChecks.h	New SFINAE-based helpers to detect and query availableForWrite() safely when present.
src/mesh/api/ServerAPI.h	Wires ServerAPI into the new StreamAPI frame-write hooks and includes the helper header.
src/mesh/api/ServerAPI.cpp	Implements pre-write readiness checks and closes/disables the API service on failures.
test/test_client_write_checks/test_main.cpp	Adds unit coverage for availableForWrite() detection and readiness helper behavior.

[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• api.registry.nm1.platformio.org
  • Triggering command: /home/REDACTED/.local/bin/pio /home/REDACTED/.local/bin/pio test -e native (dns block)
  • Triggering command: /home/REDACTED/.local/bin/pio /home/REDACTED/.local/bin/pio run -e native (dns block)
• api.registry.platformio.org
  • Triggering command: /home/REDACTED/.local/bin/pio /home/REDACTED/.local/bin/pio test -e native (dns block)
  • Triggering command: /home/REDACTED/.local/bin/pio /home/REDACTED/.local/bin/pio run -e native (dns block)
• collector.platformio.org
  • Triggering command: /home/REDACTED/.local/bin/pio /home/REDACTED/.local/bin/pio run -e native (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)