ESP32 compile on Windows: 'Cannot create temporary file in C:\Windows\' — uncovered tempfile callsite #875

Symptom

bash compile esp32s3 --examples Blink (or any ESP32-family compile) fails on Windows with:

build error: build failed: compilation failed for
  C:\Users\niteris\.fbuild\prod\cache\platforms\framework-arduinoespressif32\9ef436ac06b7bf7f\3.3.7\esp32-core-3.3.7\cores\esp32\FunctionalInterrupt.cpp:
  Cannot create temporary file in C:\Windows\: Permission denied

Then:

ERROR fbuild::output: …
running-process broker: direct daemon fallback (failed to connect to broker: The system cannot find the file specified. (os error 2))

Environment

• fbuild 2.3.14 (latest, pinned in FastLED pyproject.toml)
• Windows 10 Pro 19045
• Invoked from C:\Users\niteris\dev\fastled (writable user dir)
• Toolchain: xtensa-esp-elf-gcc 14.2.0

Diagnosis

The compiler subprocess is being launched with cwd = C:\Windows\ (or with TMP / TEMP unset, so the compiler's default-tempfile path falls back to the cwd, which on the system PATH ends up being C:\Windows\). Either way, the compiler tries to write a temp object/preprocess file there, hits ACL denial, and bails before producing output.

PR #850 (fix(tempfile): root 4 production callers via fbuild_paths::temp_subdir) routed four production callers through fbuild_paths::temp_subdir. This particular path — the per-source compile subprocess spawn during ESP32 framework-library compilation — is evidently not one of the four covered callers.

Repro

git clone https://github.com/FastLED/FastLED
cd FastLED
# pyproject.toml already pins fbuild 2.3.14
uv sync
bash compile esp32s3 --examples Blink

Ask

1. Confirm whether the per-source compile spawn in the ESP32 orchestrator (crates/fbuild-build/src/esp32/orchestrator.rs or wherever the per-.cpp invocation lives) inherits a writable cwd / TEMP env.
2. Extend the fbuild_paths::temp_subdir rooting to that spawn site.
3. Set TMP/TEMP in the subprocess env explicitly to fbuild_paths::temp_subdir().as_os_str() so the compiler's default-tempfile path can't fall back to cwd.

This blocks all ESP32-family local compile on Windows for the FastLED dev pass, including hardware validation of the in-flight bus enum migration (#3439).

Workarounds tried

• Re-running with the daemon down (fbuild daemon stop) — same error.
• Setting TMP / TEMP in the parent shell — propagates one level but the compiler subprocess still fails the same way.

Related: forge-ci/soldr#1105 (MSVC LIB auto-injection) is a sister Windows-toolchain ergonomics issue but separate root cause.