Cold first compile per container is unchanged (cache empty). Subsequent
compiles drop from ~5-7 minutes to ~30-60 seconds because every ESP-IDF
base object (FreeRTOS, lwIP, esp_wifi, libsodium, soc, hal, …) hits the
cache. The user's BMP280 example, which hangs on cold compile, completes
near-instantly on the second attempt.
Why a transparent cache is safe: ccache hashes the preprocessed source +
flags + compiler. A cache hit only happens when the input is byte-for-byte
identical to a prior compile. Different sketches with different libraries
still get correct cache misses; there is no path where one project's
output contaminates another.
Changes
- Dockerfile.standalone: install ccache, set CCACHE_DIR=/var/cache/ccache,
IDF_CCACHE_ENABLE=1, configure 2 GB cap with compression. Compression
(level 6) cuts cache disk usage by ~40% with negligible CPU overhead.
- docker-compose.yml: named volume `ccache:/var/cache/ccache` so the
cache survives `docker compose up -d --build` (without it, every image
rebuild discards the cache).
- backend/app/services/espidf_compiler.py: pass `-DCCACHE_ENABLE=1` to
cmake when IDF_CCACHE_ENABLE is truthy. ESP-IDF's project.cmake
(`set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ccache)` on line 374)
is what actually wires ccache in; without the cmake -D flag the env
var alone has no effect because we don't go through idf.py.
Escape hatch: set IDF_CCACHE_ENABLE=0 in compose env to disable without
rebuilding the image.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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