package backup

import "sync"

// InFlight is the host-wide "one heavy guest operation at a time" gate.
//
// R-85 (Scenario F). The operator's R-82 ruling was "one backup at a time per guest"; a restore-test
// must JOIN that single-flight rather than sit outside it. It is not a lock-contention concern —
// a restore-test uses a scratch VMID, so it never touches the live guest's vzdump lock. It is a
// LINK concern: an offsite restore PULLS a multi-GB archive while an offsite backup PUSHES one, over
// the same WireGuard tunnel. On the demo fleet that link runs at ~33 MB/min upstream; running both
// at once makes each slower and pushes both toward their timeouts, which is how a healthy tier ends
// up recorded as failed.
//
// It is deliberately host-wide and coarse rather than per-guest: these boxes carry one customer
// guest, and the resource being protected (the uplink) is shared by everything on the host anyway.
//
// The gate is ADVISORY in one direction only — it never cancels anything already running. A caller
// that cannot acquire DEFERS to its next cadence. Deferring a restore-test costs a few hours of
// coverage; cancelling a running backup costs the backup.
type InFlight struct {
	mu   sync.Mutex
	what string // "" = idle
}

// TryAcquire claims the gate for `what`. ok=false means something else holds it, and `busy` names
// it — the name matters, because "deferred" with no reason is indistinguishable from "broken".
func (g *InFlight) TryAcquire(what string) (release func(), busy string, ok bool) {
	if g == nil {
		// Not wired (older call sites, tests) → no gating, previous behaviour.
		return func() {}, "", true
	}
	g.mu.Lock()
	defer g.mu.Unlock()
	if g.what != "" {
		return nil, g.what, false
	}
	g.what = what
	var once sync.Once
	return func() {
		once.Do(func() {
			g.mu.Lock()
			g.what = ""
			g.mu.Unlock()
		})
	}, "", true
}

// Busy reports what currently holds the gate ("" = idle).
func (g *InFlight) Busy() string {
	if g == nil {
		return ""
	}
	g.mu.Lock()
	defer g.mu.Unlock()
	return g.what
}