# MCP restart coordinator and impact analysis (#658)

Before any sanctioned MCP restart, a central coordinator evaluates the live
control-plane state and produces an **impact preview** so operators and the web
console (#642 / #652) can see the blast radius *before* concurrent LLM work is
disrupted. Uncoordinated restarts destroy in-flight author/reviewer/merger work
and give operators no way to see what they are about to break.

This lands the coordinator + impact DTO + a dry-run MCP tool. It is the single
sanctioned entry point for restart evaluation post-#657 (which inventoried the
restart/reload/kill paths). The **mutative apply** path — actually performing a
restart — is a later child gated by a drain proof and is explicitly out of
scope here.

## Components

| Piece | Where | Responsibility |
|-------|-------|----------------|
| `restart_coordinator.evaluate_restart_impact` | `restart_coordinator.py` | Pure classification: inventory → impact report DTO. No I/O, no restart. |
| `RestartImpactReport` / `SessionImpact` / `LeaseImpact` | `restart_coordinator.py` | Console-facing DTO (`.as_dict()` is JSON-serializable). |
| `ControlPlaneDB.list_sessions` | `control_plane_db.py` | Read-only session inventory (the process-level unit a restart kills). |
| `gitea_request_mcp_restart` | `gitea_mcp_server.py` | MCP tool: gathers inventory from the #613 DB, calls the coordinator, returns the report. Dry-run only. |

## Dimensions evaluated

The coordinator classifies the inventory across the dimensions #658 requires:

- **Sessions** — every active MCP session; a restart terminates all of them.
  Liveness = `status == active` **and** the owner pid is alive **and** the
  heartbeat is fresh (default window 15 min). Dead/stale sessions do not count
  toward blast radius.
- **Leases / locks** — control-plane leases joined with work items and their
  freshness (`lease_lifecycle.classify_lease_freshness`). Only `active` (live
  owner) leases are *disruptive*; expired / released / dead-process leases never
  withhold a restart.
- **Issue / PR work** — the issues and PRs behind disruptive leases.
- **Mutations / critical sections** — a live lease carrying an author worktree
  or a mutating phase (`implementing`, `publishing`, `merging`, …) is a
  critical section a restart must not sever.
- **Terminal (merge) lock** — an active terminal lock always makes a restart
  unsafe.
- **Prior recovery attempts** — narrower recovery already tried (e.g. sanctioned
  client reconnects) is echoed so the operator sees the escalation history.

## Verdict

Exactly three verdicts, matching the acceptance criteria:

| Verdict | `allow_restart` | Meaning |
|---------|-----------------|---------|
| `safe` | `true` | No other live sessions, no live leases, no terminal lock. |
| `unsafe` | `false` | Live work would be disrupted and no operator override is present — **or** the inventory could not be completed (fail closed). |
| `override` | `true` | Live work present, but an operator override accepts the blast radius. |

`override_would_allow` tells the console whether an override path exists for the
current state. `blast_radius` is a `none` / `low` / `medium` / `high` severity
band derived from the affected session and work counts.

### Fail closed

If the control-plane inventory cannot be completed (DB unavailable, a listing
failed), `inventory_complete` is `false` and the verdict is `unsafe` / deny. An
incomplete evaluation must never green-light a restart.

### Operator override authority

Override authority is read from the environment variable
`GITEA_OPERATOR_RESTART_OVERRIDE_AUTHORIZATION` and **never** from a tool
argument. A worker session cannot set an environment variable on an
already-running daemon, so override cannot be self-asserted (same pattern as the
#630 daemon-maintenance authorization). The `request_override` tool argument only
expresses caller intent; it takes effect solely when the environment
authorization is present.

## The tool

```text
gitea_request_mcp_restart(remote, host, org, repo,
                          dry_run=True, request_override=False,
                          session_id=None, limit=200)
```

Read-only, dry-run, and it **never restarts anything**. `apply_supported` is
always `false`; passing `dry_run=False` performs no restart and reports that
apply is gated by a drain proof (a separate child).

## Audit

Every evaluation carries an `audit_record` (event, coordinator version, verdict,
allow decision, blast radius, counts, timestamp) so restart decisions are
auditable. No secrets flow through the coordinator — session ids, pids, and
profiles are operational metadata only.

A representative dry-run report is in
[`mcp-restart-impact-sample.json`](./mcp-restart-impact-sample.json).