Issue #101 reproducer: an ESP32 sketch that pulls in Adafruit_SSD1306 +
Adafruit_GFX produced a "No response from server. Is the backend
running on port 8001?" error in the browser. The compile actually
succeeded backend-side, but the JSON response carrying the firmware
was ~5.5 MB of base64 — the ESP-IDF compiler builds a full 4 MB merged
flash image (mostly 0xFF padding), encodes it whole, and ships it. In
prod that response goes through nginx + Cloudflare, which buffer-fail
or RST the connection on payloads that big — axios then lands in the
"no response" branch with no HTTP status to surface.
Fix: trim the trailing 0xFF padding before serializing, re-pad to a
valid QEMU flash size (2/4/8/16 MB) just before mtd attach. Lossless:
bytes after `last_used` in the merge are 0xFF by construction, so
trim → pad reproduces the original image byte-for-byte.
Numbers from the reproducer (Adafruit_SSD1306 + Adafruit_GFX,
esp32:esp32:esp32 board):
before: ~5.5 MB JSON response
after: 539 KB JSON response (10× smaller)
backend/app/services/espidf_compiler.py
_merge_flash_image now tracks `last_used` across the three placed
sections (bootloader / partitions / app) and writes only
flash[:last_used] to merged_flash.bin.
backend/app/services/esp32_flash_image.py (new)
Shared `pad_to_flash_size(bytes) -> bytes` helper. Rounds up to the
next valid QEMU flash size with a 4 MB minimum, matches the
frontend's existing padToFlashSize logic in Esp32MicroPythonLoader.
Raises ValueError on >16 MB inputs (would indicate a broken upstream
merge, not anything user-recoverable).
backend/app/services/esp32_lib_bridge.py
backend/app/services/esp32_worker.py
Both QEMU consumer paths (in-process and subprocess) call
pad_to_flash_size right after base64.b64decode, before writing the
tmp .bin that QEMU attaches with `-drive if=mtd,format=raw`.
Verified: smoke test confirms trim → pad → original is byte-exact.
Edge cases covered: small payloads pad up to the 4 MB minimum;
firmwares >16 MB are rejected loudly.
Closes#101
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
David Montero Crespo