// Package crashboot decides how long the controller's app mails wait after the controller starts
// (R-856, `09` §3 decision 143, extending decision 129).
//
// After a NORMAL start the dead-app check waits NormalGrace (90 s) — apps legitimately take 30–60 s
// to come up. After a CRASH boot of the host (a kernel crash, a power cut or a hard reset — the crash
// guard cannot tell them apart, `11` §5.9) the apps come up slower and the hub already tells the
// household „restarted after an unexpected stop"; the 2026-10-04 crash-guard test on demo-hp then
// mailed `app_start_failed` and `app_stopped_unhealthy` 3.5 and 9 minutes after the boot, for apps
// that were still coming up. So after a crash boot the wait is CrashGrace (about 15 minutes).
//
// THE FACT comes from the host's crash guard (`felhom-crash-guard`, state.json `last_boot_unclean` +
// `last_boot_at`), through the agent's local API. The guest cannot see the host's state file, so it
// is a Probe seam.
//
// UNKNOWN IS A NORMAL BOOT. An agent that predates the route (404), an unreachable agent, a box with
// no crash guard, an unparseable time: all keep today's 90 s. A longer silence must never come from
// a guess — it would hide a real outage for 15 minutes on every box whose fact we cannot read.
package crashboot

import (
	"context"
	"log"
	"sync"
	"time"

	"gitea.dooplex.hu/admin/felhom-controller/internal/agentapi"
)

const (
	// NormalGrace is the dead-app boot grace after an ordinary start (the pre-R-856 deadAppBootGrace).
	NormalGrace = 90 * time.Second
	// CrashGrace is the wait after a crash boot — „about 15 minutes" (decision 143). The 2026-10-04
	// mails came at 3.5 and 9 minutes; 15 covers both with room.
	CrashGrace = 15 * time.Minute
	// BootWindow bounds how recent the host's unclean boot must be for THIS controller start to be the
	// one that followed it. A controller restarted days after a crash (a self-update, a kill) is a
	// normal start; a guest starts its controller well within this window after a host boot.
	BootWindow   = 30 * time.Minute
	probeTimeout = 5 * time.Second
)

// Fact is what the host's crash guard says about its most recent boot.
type Fact struct {
	Known   bool      // false: no crash guard, no state, or an agent that cannot say
	Unclean bool      // the most recent host boot followed an unclean stop
	BootAt  time.Time // when that boot happened
}

// Probe asks the host for the fact. An error means unknown.
type Probe func(ctx context.Context) (Fact, error)

// Grace answers "must the app mails still wait?" for one controller run.
type Grace struct {
	start  time.Time
	probe  Probe
	logger *log.Logger

	mu        sync.Mutex
	resolved  bool
	crashBoot bool
}

// New builds the grace for a controller that started at start. probe may be nil (no agent): unknown.
func New(start time.Time, probe Probe, logger *log.Logger) *Grace {
	return &Grace{start: start, probe: probe, logger: logger}
}

// Within reports whether now is still inside the boot grace, i.e. the app mails must wait.
//
// The fact is asked for while the normal grace runs (the agent may come up a few seconds after the
// controller) and once more at its end; whatever is known then decides, and unknown resolves to a
// normal boot. Once resolved it is never asked again.
func (g *Grace) Within(now time.Time) bool {
	elapsed := now.Sub(g.start)
	return elapsed < g.duration(elapsed)
}

// Duration is the grace this run uses (resolving it if it can). For logs and tests.
func (g *Grace) Duration(now time.Time) time.Duration {
	return g.duration(now.Sub(g.start))
}

func (g *Grace) duration(elapsed time.Duration) time.Duration {
	g.mu.Lock()
	defer g.mu.Unlock()
	if !g.resolved {
		g.tryResolve(elapsed >= NormalGrace)
	}
	if g.crashBoot {
		return CrashGrace
	}
	return NormalGrace
}

// tryResolve asks the probe once. final: the normal grace is over, so an unknown answer is final too.
func (g *Grace) tryResolve(final bool) {
	if g.probe == nil {
		g.resolve(false, "no agent to ask — a normal boot")
		return
	}
	ctx, cancel := context.WithTimeout(context.Background(), probeTimeout)
	f, err := g.probe(ctx)
	cancel()
	switch {
	case err != nil || !f.Known:
		if final {
			why := "the host's crash guard has no record"
			if err != nil {
				why = "the host's crash guard could not be read (" + err.Error() + ")"
			}
			g.resolve(false, why+" — treated as a normal boot")
		}
	case !f.Unclean:
		g.resolve(false, "the host's last boot was clean")
	case f.BootAt.IsZero() || g.start.Sub(f.BootAt) > BootWindow:
		g.resolve(false, "the host's last unclean boot ("+f.BootAt.UTC().Format(time.RFC3339)+") is not the one this start followed")
	default:
		g.crashBoot = true
		g.resolve(true, "the host's last boot ("+f.BootAt.UTC().Format(time.RFC3339)+") followed an UNCLEAN stop")
	}
}

func (g *Grace) resolve(crash bool, why string) {
	g.resolved = true
	if g.logger == nil {
		return
	}
	d := NormalGrace
	if crash {
		d = CrashGrace
	}
	g.logger.Printf("[INFO] [deadapp] boot grace %s: %s (R-856)", d, why)
}

// AgentProbe adapts the agent's GET /host/crash-guard (agentapi.Client.CrashGuard) to a Probe. A
// state the host has not written (Present=false) is unknown; an unparseable boot time is a known
// unclean boot with no time, which New's window check reads as NOT this start's — a normal boot.
func AgentProbe(get func(ctx context.Context) (agentapi.CrashGuardState, error)) Probe {
	return func(ctx context.Context) (Fact, error) {
		st, err := get(ctx)
		if err != nil {
			return Fact{}, err
		}
		if !st.Present {
			return Fact{}, nil
		}
		f := Fact{Known: true, Unclean: st.LastBootUnclean}
		if t, perr := time.Parse(time.RFC3339, st.LastBootAt); perr == nil {
			f.BootAt = t
		}
		return f, nil
	}
}