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fix(avr8js): write headless.mjs inside cache dir so ESM can resolve avr8js#292

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Merged
zackees merged 1 commit into from
May 30, 2026
Merged

fix(avr8js): write headless.mjs inside cache dir so ESM can resolve avr8js #292

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10 changes: 9 additions & 1 deletion crates/fbuild-daemon/src/handlers/emulator/avr8js_deploy.rs
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Original file line numberDiff line numberDiff line change
Expand Up@@ -208,7 +208,15 @@ pub async fn deploy_avr8js(
}
};

let script_path = session_dir.join("headless.mjs");
// headless.mjs uses `import ... from "avr8js"` (bare ESM specifier).
// Node's ESM resolver does NOT honor NODE_PATH — it only walks
// upward from the script's directory looking for node_modules.
// Stage the script inside the cache dir so node_modules/avr8js
// is on that walk-up path. Script content is a compile-time
// constant (`include_str!`), so concurrent writes are idempotent.
// The per-session firmware.hex and session.json continue to live
// under session_dir. See FastLED/fbuild#291.
let script_path = avr8js_cache.join("headless.mjs");
if let Err(e) = std::fs::write(&script_path, AVR8JS_HEADLESS_MJS) {
return (
StatusCode::INTERNAL_SERVER_ERROR,
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11 changes: 9 additions & 2 deletions crates/fbuild-daemon/src/handlers/emulator/runners.rs
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Expand Up@@ -173,8 +173,15 @@ impl EmulatorRunner for Avr8jsRunner {
let node_path = find_node()?;
let avr8js_cache = ensure_avr8js_npm()?;

let session_dir = tempfile::TempDir::new()?;
let script_path = session_dir.path().join("headless.mjs");
// headless.mjs uses `import ... from "avr8js"` (a bare ESM
// specifier). Node's ESM resolver does NOT honor NODE_PATH —
// it only walks upward from the script's directory looking
// for node_modules. Stage the script directly inside the
// cache dir so its node_modules/avr8js is on that walk-up
// path. Script content is a compile-time constant
// (`include_str!`), so concurrent writes are idempotent.
// See FastLED/fbuild#291.
let script_path = avr8js_cache.join("headless.mjs");
std::fs::write(&script_path, AVR8JS_HEADLESS_MJS)?;
Comment on lines +184 to 185

@coderabbitaicoderabbitaiBotMay 30, 2026

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⚠️ Potential issue | 🟡 Minor | ⚡ Quick win

Non-atomic write to the shared cache file can race under parallel runs.

The "idempotent" reasoning in the comment covers content equality, but std::fs::write truncates then writes, so it is not atomic. Since avr8js_cache is a process-global shared directory, a second concurrent run truncating <cache>/headless.mjs while a first run's node is still reading it during startup can expose an empty or partial script (transient SyntaxError/ERR_MODULE_NOT_FOUND). Writing to a sibling temp file and atomically renaming avoids the partial-read window.

🛡️ Proposed atomic write
- let script_path = avr8js_cache.join("headless.mjs");- std::fs::write(&script_path, AVR8JS_HEADLESS_MJS)?;+ let script_path = avr8js_cache.join("headless.mjs");+ let tmp_path = avr8js_cache.join(format!("headless.mjs.{}.tmp", std::process::id()));+ std::fs::write(&tmp_path, AVR8JS_HEADLESS_MJS)?;+ std::fs::rename(&tmp_path, &script_path)?;
📝 Committable suggestion

‼️IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change
let script_path = avr8js_cache.join("headless.mjs");
std::fs::write(&script_path,AVR8JS_HEADLESS_MJS)?;
let script_path = avr8js_cache.join("headless.mjs");
let tmp_path = avr8js_cache.join(format!("headless.mjs.{}.tmp", std::process::id()));
std::fs::write(&tmp_path,AVR8JS_HEADLESS_MJS)?;
std::fs::rename(&tmp_path,&script_path)?;
🤖 Prompt for AI Agents
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.
In `@crates/fbuild-daemon/src/handlers/emulator/runners.rs` around lines 184 -
185, The current use of std::fs::write(&script_path, AVR8JS_HEADLESS_MJS) is
non-atomic and can race when multiple processes share avr8js_cache; instead,
write AVR8JS_HEADLESS_MJS to a sibling temporary file inside avr8js_cache (e.g.,
script_path with a unique suffix using PID/timestamp or rand) and then
atomically rename (std::fs::rename) the temp file to script_path so readers
never see a truncated/partial file; update the code paths that create
script_path and replace std::fs::write with this temp-write-and-rename pattern.


let f_cpu_hz: u32 = self
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