fix(tcp): prevent select error watchdog loop (EPROT-79)

[zyk6271]

Summary

• validate descriptors before adding them to an fd_set
• monitor exceptional conditions only for connected sockets and route them through the normal connection error path
• add bounded exponential backoff and actionable errno logging when select() repeatedly fails
• release every accepted socket and copied address string when the client table is full or node creation fails
• recover stale TCP slave clients after fatal lwIP/network errors without deleting persistent TCP master node definitions
• make close-all transaction cleanup and sparse-node timeout scanning deterministic

Root cause

Two failure modes combined in the reported long-running failure:

1. The TCP slave accepted a socket and allocated its address string before checking the five-node limit. The rejection path closed the socket but did not release the copied address, so repeated connection attempts steadily exhausted internal heap. Under heap pressure, accept()/select() returned ENOMEM, Wi-Fi allocation failed, and connectivity was lost.
2. The driver retried select() immediately on persistent errors. The high-priority mb_drv_tcp_task could therefore starve CPU0's IDLE task and trigger the task watchdog.

The previous code also copied the full read set into exceptfds, including non-socket descriptors, and did not consume exceptional client conditions.

Changes

• check for a free virtual descriptor before allocating a node
• centralize cleanup of rejected accepted sockets and address metadata
• preserve accept() errno, avoid close(-1), and handle address/strdup() failures
• close all stale slave clients on fatal network-stack errors, then purge their transactions
• keep TCP master node descriptors intact so their reconnect configuration is not lost
• use the virtual node index, never a negative socket descriptor, with open_set
• scan all virtual descriptors during timeout processing so holes cannot skip active nodes
• apply a 1-tick to 1-second bounded exponential backoff to repeated select() failures

Hardware validation

Target: ESP32-D0WD-V3, ESP-IDF v6.0.2, esp-modbus v2.1.3 component sources, Modbus TCP slave with five client slots.

A/B resource-pressure fault injection

The fault injector holds all five clients open, then repeatedly creates extra TCP connections.

Unpatched firmware:

overflow=100   free_internal=91836
overflow=800   free_internal=76084
overflow=2300  free_internal=4202
overflow=3600  free_internal=1264
held clients lost at approximately 3700 overflows
E (...) mb_driver: task select error, errno=12 (Not enough space)
W (...) wifi:mem fail
W (...) wifi:m f null
E (...) task_wdt: IDLE0 did not reset; CPU 0: mb_drv_tcp_task

Patched firmware, with the same temporary heap telemetry:

overflow=100   free_internal=94328
overflow=5000  free_internal=94348
PASS: attempts=6000 rejected=5999 admitted=1; service recovered

A fresh run after the final source-review fixes:

holding 5 active client(s)
progress=6000/6000 rejected=6000 admitted=0 held_alive=5
PASS: attempts=6000 rejected=6000 admitted=0; service recovered

Additional fault injection/regression

PASS: node-table holes/refill cycles=100
malformed MBAP/PID/UID/truncated frame cases: 4/4; service alive after each
Wi-Fi disabled for 15 seconds: held clients discarded; original IP and Modbus service recovered

During the patched test windows there was no errno=12, wifi:mem fail, wifi:m f null, select busy loop, or task WDT. A single errno=113 while Wi-Fi was physically unavailable was expected and recovery completed after reassociation.

Build validation

• rebuilt TCP master, TCP slave, TCP driver, and TCP utility objects
• linked and flashed the complete application successfully
• firmware size: 0xfdb30; smallest app partition remained 32% free
• git diff --check passed for all five changed library files

[alisitsyn]

Hi @ zyk6271,

Thanks for the PR proposal. Unfortunately, I don't have time to review and test the related cases for now. I will take a look once have time for this. Please consider to extend your PR with the logs of stress testing with fault injection.

[zyk6271]

Implemented the requested stress/fault-injection follow-up in commit 466cdb57f72f69a52707ba2b574a5939dd2dfc2a and added the complete A/B data to the PR description.

Key results on ESP32-D0WD-V3 / ESP-IDF v6.0.2:

unpatched:
  free_internal: 91836 @ 100 overflows
                 76084 @ 800
                  4202 @ 2300
                  1264 @ 3600
  held clients lost at ~3700, followed by errno=12 / Wi-Fi allocation failures / task WDT

patched:
  free_internal: 94328 @ 100 overflows
                 94348 @ 5000
  PASS: 6000/6000 extra connections rejected, all 5 held clients alive
  PASS: service recovered after held clients were released
  PASS: 100 sparse-node close/refill cycles
  PASS: 4 malformed-frame cases, service alive after each
  PASS: 15-second Wi-Fi outage and Modbus recovery

No errno=12, wifi:mem fail, wifi:m f null, select busy loop, or task WDT was observed in the patched windows.

The follow-up also fixes the accepted-socket/address leak itself, preserves TCP master reconnect definitions during fatal-error recovery, and removes negative-fd FD_ISSET calls in both master and slave close handlers.