package apply

import (
	"context"
	"errors"
	"fmt"
	"os"
	"strings"
	"time"

	"github.com/novox/mesh-host/internal/declaration"
	"github.com/novox/mesh-host/internal/store"
	"github.com/novox/mesh-host/internal/system"
	"github.com/novox/mesh-host/internal/tunnel"
)

// The private network takes over the tunnel it found (novox/hq ADR 0105).
//
// The controller says so on the interface's service: `takes-over` names the found interface, the
// unit that raised it and its configuration file. Before the mesh's unit is started, the host keeps
// that file like any held file — the original recorded before anything else happens to it — and
// stops and disables the found unit. Never a flush: `wg set … peer … remove` is never run, the
// file is never written, and the found interface goes down the way its own unit takes it down.
// Then the mesh's interface comes up, with the found key the node took at enrolment, on the found
// port, with the found peers in its list — and a peer of the tunnel cannot tell it changed hands.
//
// Every apply, not once: a found unit somebody starts again would take the port back from the
// mesh's interface, so it is stopped again and said so. That is the one place an adopted node
// undoes something done by hand, and it is because the tunnel is the mesh's now.

// TakenTunnel is what an apply says about a tunnel it took over, for the node's report.
type TakenTunnel struct {
	Interface string
	Port      int
	Range     string
	Peers     int
	// Taken is whether the found interface is down and disabled and the mesh's up in its place.
	Taken bool
	Kept  string
}

// takeOverID is the held record's id for the found configuration: the service's own with a suffix,
// so it is declared for as long as the service is and never mistaken for the service itself.
func takeOverID(svc *declaration.Service) string { return svc.ID + ".takes-over" }

// takeOver keeps the found tunnel's configuration and stops its unit, ahead of the service that
// replaces it. Returned is the hold's outcome, and what was found for the report.
func takeOver(ctx context.Context, sys system.System, svc *declaration.Service, d *declaration.Declaration,
	known *store.State, run Runner, keep Keep, now time.Time) (Outcome, TakenTunnel, error) {
	t := svc.TakesOver
	id := takeOverID(svc)
	module, _ := d.Adoption.UntakenModuleOf(svc.ID)
	if module == "" {
		module = "the private network"
	}
	facts := TakenTunnel{Interface: t.Interface}

	// 1. The configuration, kept like any held file. A synthetic file resource stands for it, so
	//    the same code keeps its original, digests it and notices it changing.
	file := &declaration.File{ID: id, Type: declaration.TypeFile, Path: t.Config}
	was, already := known.HeldAt(id)
	out, held, err := hold(ctx, sys, file, module, was, already,
		"the configuration of the tunnel "+t.Interface+", taken over by "+svc.Unit, run, keep, now)
	if err != nil {
		return begin(file), facts, fmt.Errorf("keeping the found tunnel's configuration: %w", err)
	}
	known.RecordHeld(held)
	facts.Kept = held.Kept
	// What the file says, for the report: read from the machine, or from the kept original when
	// the machine's copy is gone. The private key stays in the file; nothing here keeps it.
	unread := ""
	raw, err := os.ReadFile(t.Config)
	if err != nil && held.Kept != "" {
		raw, err = os.ReadFile(held.Kept)
	}
	if err == nil {
		if found, perr := tunnel.Parse(raw); perr == nil {
			facts.Port, facts.Range, facts.Peers = found.Port, found.Range, len(found.Peers)
		} else {
			unread = perr.Error()
		}
	} else {
		unread = err.Error()
	}

	// 2. The found unit: stopped if it runs, disabled if it starts at boot. A unit that is not
	//    there is not an error — the interface may have been raised another way, which the check
	//    below catches — and neither is one already down.
	var did []string
	state, err := sys.ServiceState(ctx, run, t.Unit)
	switch {
	case err != nil:
		did = append(did, t.Unit+" is not a unit here")
	case state == "running":
		if err := sys.SetServiceState(ctx, run, t.Unit, "stopped"); err != nil {
			return out, facts, fmt.Errorf("stopping the found %s: %w", t.Unit, err)
		}
		after, err := sys.ServiceState(ctx, run, t.Unit)
		if err != nil {
			return out, facts, err
		}
		if after != "stopped" {
			return out, facts, fmt.Errorf("%s was asked to stop and is %s", t.Unit, after)
		}
		did = append(did, "stopped "+t.Unit)
	}
	if err == nil {
		if boot, err := sys.ServiceBoot(ctx, run, t.Unit); err == nil && boot == "enabled" {
			if err := sys.SetServiceBoot(ctx, run, t.Unit, "disabled"); err != nil {
				return out, facts, fmt.Errorf("disabling the found %s at boot: %w", t.Unit, err)
			}
			did = append(did, "disabled it at boot")
		}
	}

	// 3. The interface is gone. If it is still up, something other than its unit raised it, and
	//    starting the mesh's on the same port and address would fail or, worse, half work.
	if up, err := run(ctx, "wg", "show", "interfaces"); err == nil {
		for _, iface := range strings.Fields(up) {
			if iface == t.Interface {
				return out, facts, fmt.Errorf("%s is still up after its unit %s was stopped: something other "+
					"than that unit raises it, and the mesh's interface cannot take its port and address "+
					"while it does. Nothing was flushed", t.Interface, t.Unit)
			}
		}
	}

	out.Detail = "the tunnel " + t.Interface + "'s configuration, kept as found"
	if held.Kept != "" {
		out.Detail += " (original at " + held.Kept + ")"
	}
	if len(did) > 0 {
		out.Detail += "; " + strings.Join(did, ", ") + " — never flushed"
	}
	if held.Changed != "" {
		out.Detail += "; " + held.Changed + " by something other than the mesh since it was found"
	}
	if unread != "" {
		// Said, not swallowed: the report would otherwise say a tunnel with no port and no
		// peers was carried, which reads as a tunnel that was not one.
		out.Detail += "; what it says could not be read as a tunnel's: " + unread
	}
	return out, facts, nil
}

// takesOver is the service in a declaration that takes over a tunnel, if any: one per node, since
// a machine has one private network.
func takesOver(d *declaration.Declaration) *declaration.Service {
	for _, r := range d.Resources {
		if svc, ok := r.(*declaration.Service); ok && svc.TakesOver != nil {
			return svc
		}
	}
	return nil
}

// errNotAdopted is a takeover on a declaration that does not say the node is adopted, which the
// parser refuses already; kept as a second line of defence at the point of acting.
var errNotAdopted = errors.New("a tunnel is taken over on an adopted node only")