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@@ -0,0 +1,118 @@
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+---
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+bug_id: BUG-2026-07-27T120657
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+status: open
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+severity: medium
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+scope: i18n
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+title: Spawned tasks fall back to English (thread-local set_locale)
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+---
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+
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+# BUG-2026-07-27T120657: Spawned tasks fall back to English (thread-local set_locale)
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+
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+## Problem
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+
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+**Actual:** When the user's locale is non-English (e.g. `LANG=ru_RU.UTF-8`),
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+messages emitted from inside `tokio::spawn` worker tasks render in English
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+(the fallback), not the user's locale.
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+
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+**Expected:** All user-facing messages — including those from spawned
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+parallel tasks (serve, before_serve, after_stop, power, health, etc.) —
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+render in the locale set via `LANG`.
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+
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+**Reproduce:**
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+```
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+LANG=ru_RU.UTF-8 cargo run -- -d serve --recipe test-composed
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+# → "[dry-run] would build composed model dir ... 3 sources, 1 file"
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+# (English; should be Russian: "3 источника, 1 файл")
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+```
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+
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+Main-thread `t!` calls DO render in the user's locale (verified: a
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+main-thread `t!("composed-dir-built", ...)` with `set_locale("ru-RU")`
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+prints "3 источника, 1 файл"). Only spawned tasks are affected.
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+
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+## Root Cause Analysis
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+
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+**Code path:** `main.rs` calls `fluent_i18n::set_locale(locale)` during
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+startup. `t!` macro calls read the current locale. Spawned tasks call
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+`t!` from worker threads.
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+
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+**Why it fails:** `fluent_i18n::set_locale` writes to a `thread_local!`
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+`RefCell<Option<LanguageIdentifier>>` (confirmed by reading the crate
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+source: `CURRENT_LOCALE: RefCell<Option<LanguageIdentifier>>`).
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+`tokio::spawn` schedules the task onto a worker thread that has its own,
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+independent thread-local slot — which was never set. The `t!` macro reads
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+`None` there, falls back to the static fallback locale (en-US).
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+
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+**Contributing factor:** The launcher uses `tokio::spawn` for parallel
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+per-node SSH/Docker work across 16 call sites. Every `t!` call inside
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+those spawned blocks is affected.
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+
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+**Risk level:** Medium. No data loss, no security boundary. User-facing
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+i18n is broken for non-English locales in the async paths; English
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+(the fallback and most common case) is unaffected. 16 spawn sites need
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+migration.
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+
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+**diagnose-root 4-phase RCA (performed earlier this session):**
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+- **Reproduce:** `LANG=ru_RU.UTF-8 ./sm121 -d serve --recipe test-composed`
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+ → English output from spawned composed-dir builder.
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+- **Isolate:** `set_locale("ru-RU")` succeeds on main;
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+ `get_locale()` returns `ru-RU` on main. A `t!` call on the main thread
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+ returns Russian. A `t!` call inside `tokio::spawn` returns English.
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+- **Hypothesize:** `fluent_i18n::set_locale` is thread-local; tokio
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+ worker threads don't inherit the main-thread locale.
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+- **Verify:** Read `fluent-i18n` source — `CURRENT_LOCALE` is declared
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+ `thread_local! { RefCell<Option<LanguageIdentifier>> }`. Confirmed.
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+
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+## TDD Fix Plan
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+
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+**Fix approach:** Wrap `tokio::spawn` with a locale-aware spawner that
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+captures the locale from a process-global store (set once on main) and
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+calls `set_locale` at the start of each spawned task. Migrate the 16
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+`tokio::spawn` call sites to the wrapper.
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+
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+### RED-GREEN cycles
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+
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+1. **RED:** Write a test that sets the global locale to `ru-RU`, spawns a
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+ task via the (not-yet-existing) `spawn_with_locale` helper, calls
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+ `t!("composed-dir-built", ...)`, and asserts the result contains a
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+ Russian word (e.g. `источник`). Test fails to compile (helper missing).
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+ **GREEN:** Create `src/locale.rs` with a `OnceLock<Option<String>>`
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+ global + `set_global_locale()` + `spawn_with_locale()` wrapper that
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+ calls `fluent_i18n::set_locale` before awaiting the future.
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+ **verify:** `cargo test -- spawn_with_locale_inherits_locale`
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+
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+2. **RED:** Add a characterization test proving a bare `tokio::spawn` (no
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+ wrapper) still returns English despite the global locale being set —
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+ documents the underlying crate behavior we're working around.
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+ **GREEN:** No code change (characterization test; pins the bug).
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+ **verify:** `cargo test -- bare_spawn_falls_back_to_english`
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+
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+3. **REFACTOR:** Migrate the 16 `tokio::spawn(async move { ... })` call
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+ sites to `locale::spawn_with_locale(async move { ... })`. Mechanical
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+ replacement; each site's `JoinHandle` return type is unchanged.
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+ **verify:** `make check` (fmt + clippy + 128+ tests + ≤500 lines)
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+
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+### Migration sites (16)
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+
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+```
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+src/tmux/ops.rs:203 src/tmux/serve.rs:108
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+src/before_serve.rs:51 src/cluster_state.rs:49
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+src/doctor.rs:74 src/power.rs:66, :301
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+src/after_stop.rs:42 src/health.rs:64
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+src/exec.rs:77 src/docker/logs.rs:77
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+src/docker/mod.rs:367 src/docker/deploy.rs:47
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+src/docker/health.rs:31, :77, :224
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+```
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+
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+## Acceptance Criteria
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+
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+- [ ] `cargo test -- spawn_with_locale_inherits_locale` passes
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+- [ ] `cargo test -- bare_spawn_falls_back_to_english` passes (characterization)
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+- [ ] `make check` green (fmt + clippy + all tests + ≤500 lines)
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+- [ ] `LANG=ru_RU.UTF-8 cargo run -- -d serve --recipe test-composed` prints
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+ Russian for the composed-dir dry-run message
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+- [ ] All 16 `tokio::spawn` sites migrated to `spawn_with_locale`
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+- [ ] No new `#[allow(...)]` attributes
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+
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+## Resolution
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+
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+<!-- filled in by validate-fix -->
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