firmware: POST /ota always fails — stack overflow in task httpd during esp_ota_end() image validation (ESP32-S3, v0.8.8)

Summary

POST /ota never completes on an ESP32-S3-DevKitC-1: the upload transfers fully, then the node hits ***ERROR*** A stack overflow in task httpd has been detected. during image validation and reboots into the old slot. The endpoint is effectively unusable, and the failure is silent from the client side (curl just sees the connection drop).

Environment

Reproduction

curl -X POST -H "Authorization: Bearer $PSK" \
     -H "Content-Type: application/octet-stream" \
     --data-binary @esp32-csi-node.bin http://<node>:8032/ota

Client result: HTTP 000 sent=911232 t=58.7s (connection closed, no response body).

GET /ota/status before and after shows the same running_partition, so nothing was switched.

Console output

I (20046) ota_update: OTA update started, content_length=911232
I (31246) esp_image: segment 0: paddr=00220020 vaddr=3c0a0020 size=23dd8h (146904) map

***ERROR*** A stack overflow in task httpd has been detected.

Backtrace: 0x40375c19:0x3fcc9470 0x4037d579:0x3fcc9490 0x4037e276:0x3fcc94b0
           0x4037f837:0x3fcc9530 0x4037e3bc:0x3fcc9550 0x4037e3b2:0x3c0a0020 |<-CORRUPTED

Rebooting...

The overflow happens after the whole body is received, at the point esp_ota_end() runs esp_image_verify() — that validation executes on the httpd task.

Cause

ota_start_server() in main/ota_update.c uses HTTPD_DEFAULT_CONFIG() and overrides max_uri_handlers and recv_wait_timeout, but leaves stack_size at the 4096-byte default. Image verification needs more than that.

This is present both on v0.8.8-esp32 and on main (b370d4f).

Note that docs/TROUBLESHOOTING.md §6 already prescribes this class of fix ("Increase httpd stack from 4KB to 8KB") for a different OTA crash, but ota_update.c never sets it.

Fix that works

httpd_config_t config = HTTPD_DEFAULT_CONFIG();
config.server_port = OTA_PORT;
config.stack_size = 12288;          // <-- added
config.max_uri_handlers = 12;
config.recv_wait_timeout = 30;

With this change OTA completes normally on the same hardware and image:

{"status":"ok","message":"OTA update successful. Rebooting..."}

and /ota/status afterwards reports the other slot (ota_0 -> ota_1). Verified repeatedly across three nodes.

8192 may well be enough; 12288 was chosen for headroom and not minimised.

Start in main/ota_update.c at ota_start_server() and inspect the HTTPD_DEFAULT_CONFIG() values, especially stack_size. Reproduce with the provided curl upload on an ESP32-S3 using the stated firmware setup, then verify that OTA returns success and /ota/status reports the other slot without a reboot. Check docs/TROUBLESHOOTING.md §6 for the existing guidance on HTTPD stack sizing.