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docs: correct the false Arc<Mutex> MakeWriter claim

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Both the spec and the plan asserted that Arc<Mutex<RotatingLogWriter>>
satisfies tracing-subscriber's MakeWriter via impl MakeWriter for Arc<W>.
That impl requires &'a W: io::Write, and &Mutex<W> does not implement
io::Write. The error reached the implementer and cost a fix round before
being caught; the shipped code uses a bare Mutex.

Corrections are marked inline so the mistake stays visible rather than
being silently erased.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EGntTHCW3sEPy1VBRopNNp
This commit is contained in:
logaritmisk
2026-08-01 00:13:54 +02:00
co-authored by Claude Opus 5
parent 41acc3e21a
commit fd842289e3
2 changed files with 18 additions and 10 deletions
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docs/superpowers/specs/2026-07-31-xy-start-on-login-design.md
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@@ -154,11 +154,17 @@ which is too late to open a log file for the logger itself.
For the `Cmd::Daemon` arm only, the subscriber writes to `stderr.and(file)` via
`MakeWriterExt`, where the file half is `log_dir/xy.log` backed by the existing
`xy_supervisor::logs::RotatingLogWriter` (10 MB × 5, the same rotation used for
per-server logs). No adapter type is needed: `tracing-subscriber` 0.3 already
implements `MakeWriter` for `Mutex<W> where W: io::Write` (`fmt/writer.rs:808`)
and for `Arc<W> where W: MakeWriter` (`fmt/writer.rs:694`), so
`Arc<Mutex<RotatingLogWriter>>` satisfies the bound once Task 1 lands the
`io::Write` impl.
per-server logs). No adapter type is needed: `tracing-subscriber` 0.3
implements `MakeWriter` for `Mutex<W> where W: io::Write`
(`fmt/writer.rs:808`), so a plain `Mutex<RotatingLogWriter>` satisfies
`with_writer` once the `io::Write` impl lands.
**Corrected 2026-08-01.** An earlier draft of this section claimed
`Arc<Mutex<RotatingLogWriter>>` also satisfied the bound, via
`impl MakeWriter for Arc<W>`. That is false: the `Arc` impl
(`fmt/writer.rs:694`) requires `&'a W: io::Write`, and `&Mutex<W>` does not
implement `io::Write`. The claim survived into the implementation plan and cost
a fix round before being caught. The shipped code uses a bare `Mutex`.
Every other subcommand keeps stderr-only logging; CLI output does not belong in
the daemon's log.