Module event_log 

Reducer event log: an in-memory ring buffer of recent Events for replay, plus an optional append-only JSON-lines disk stream for crash forensics.

This module used to exist twice — crates/launcher/src/event_log.rs (Phase D.2) and a byte-for-byte mirror in crates/srv/src/event_log.rs (Phase E.1b), 415 lines each, whose only code-level difference was which logging sink a warning went to. The srv copy’s own header carried the to-do: “Phase E.7 cleanup: lift the shared parts into agentmux-common and unify launcher + srv event logs. (reagent P2 #610.)” This is that lift (docs/reports/REPORT_DRY_AND_MODULARITY_AUDIT_2026_09_06.md §2.2).

The one real divergence is preserved as a parameter: run_disk_writer takes a WarnSink, because the launcher logs to its own rotating file (crate::log) and has no tracing subscriber installed, while srv logs via tracing::warn!. Hard-coding either would silently drop the other crate’s warnings.

Two roles, kept clean:

1. In-memory ring is the source of truth for replay during the owning process’s lifetime. GetEvents { since: u64 } reads from it. Bounded by MAX_RING_EVENTS to stop unbounded memory growth in long sessions; oldest events evict first. A subscriber that fell behind further than the ring’s coverage gets an EventList of whatever’s still in the ring + a flag indicating they may have missed some — it’s their job to recover by treating subsequent events as authoritative.

2. Disk file is purely forensic. Append-only JSON-lines. Survives a crash so an operator can post-mortem “what was it doing right before it died.” Fire-and-forget: write failures log a warning but never block the in-memory path. Rotated when it exceeds MAX_DISK_BYTES (renames current to .old, starts fresh).

Why both: in-memory satisfies D.3’s resync needs at zero I/O cost in the happy path. Disk satisfies the “what happened before the crash” debugging story without complicating the replay reader.

Structs

EventLog

Append-only ring + optional disk persistence.

Functions

event_version

Extract the version field from any Event variant. When a new variant is added that carries a version, add it here too — the exhaustive match catches future-variant compile errors.

run_disk_writer

Background task: write events to the disk file as they arrive on the broadcast bus. Rotates when the file exceeds MAX_DISK_BYTES.

Type Aliases

WarnSink

Where run_disk_writer sends its warnings. Each owning crate binds its own logging facility — see the module doc for why this is a parameter rather than a hard-coded tracing::warn!.