package runner

import (
	"encoding/json"
	"errors"
	"fmt"
	"io/fs"
	"os"
	"path/filepath"
	"time"
)

// Marker is what systemd saw when the unit ended, recorded by ExecStopPost.
//
// It exists because the platform is deliberately not watching: a run outlives a deploy, so the
// process that started it is usually not the process that learns it is over. A D-Bus signal answers
// only the case where the platform happened to be up at that instant — which is the case the design
// says will often be false, since the control plane is restarted several times a week ON PURPOSE.
// A file in the platform's state directory answers both.
type Marker struct {
	Unit string `json:"unit"`
	// Result is systemd's $SERVICE_RESULT: success · exit-code · signal · oom-kill · timeout · … .
	// It is the discriminator the exit code cannot give: "the engine exited 1" and "the kernel took
	// it away at the memory limit" are the same 1 to a caller reading only the code.
	Result string `json:"result"`
	// Code is $EXIT_CODE — "exited" or "killed" — and Status is $EXIT_STATUS, which is the process's
	// exit status for the first and a signal NAME for the second. Kept as text because that is what
	// systemd hands over, and inventing a numeric union here would lose the signal names.
	Code   string    `json:"code"`
	Status string    `json:"status"`
	At     time.Time `json:"at"`
}

// Exited reports the engine's own exit status, which is the only thing the ratified exit contract
// (0 clean · 2 flagged · 3 bank stop · 1 anything else) is defined over. ok is false when the
// process did not exit on its own — killed, timed out, OOM — because then there is no exit code to
// map and the run has to be judged by Result instead.
func (m Marker) Exited() (code int, ok bool) {
	if m.Code != "exited" {
		return 0, false
	}
	var n int
	if _, err := fmt.Sscanf(m.Status, "%d", &n); err != nil {
		return 0, false
	}
	return n, true
}

// WriteMarker records the end of a unit. Called by the marker command that runs as ExecStopPost.
//
// The write is atomic (temp file plus rename in the same directory), because the reader is the
// reconciler and it polls: a half-written marker read as "this run ended" would settle money
// against a result nobody produced.
func WriteMarker(path string, m Marker) error {
	if m.At.IsZero() {
		m.At = time.Now().UTC()
	}
	body, err := json.Marshal(m)
	if err != nil {
		return fmt.Errorf("runner: encode marker: %w", err)
	}
	body = append(body, '\n')
	if err := os.MkdirAll(filepath.Dir(path), 0o750); err != nil {
		return fmt.Errorf("runner: marker directory: %w", err)
	}
	tmp, err := os.CreateTemp(filepath.Dir(path), ".marker-*")
	if err != nil {
		return fmt.Errorf("runner: marker temp file: %w", err)
	}
	defer func() { _ = os.Remove(tmp.Name()) }() // a no-op once the rename succeeded
	if _, err := tmp.Write(body); err != nil {
		tmp.Close()
		return fmt.Errorf("runner: write marker: %w", err)
	}
	// Durability before visibility: the marker outlives a machine that loses power, and the reader
	// is allowed to conclude "the run is over" from its mere presence.
	if err := tmp.Sync(); err != nil {
		tmp.Close()
		return fmt.Errorf("runner: sync marker: %w", err)
	}
	if err := tmp.Close(); err != nil {
		return fmt.Errorf("runner: close marker: %w", err)
	}
	if err := os.Rename(tmp.Name(), path); err != nil {
		return fmt.Errorf("runner: publish marker: %w", err)
	}
	return nil
}

// ErrNoMarker is a unit that has not ended, or ended without being able to say so (a reboot takes
// the whole manager, and with it any ExecStopPost that had not run yet).
var ErrNoMarker = errors.New("runner: no exit marker")

// ErrBadMarker is a marker that EXISTS and does not parse.
//
// It is told apart from an I/O error deliberately, and the distinction is the same one the journal's
// reader makes: a moment in which the file could not be read heals on the next sweep, whereas a file
// whose CONTENT is not ours never does. WriteMarker is atomic — temp file, fsync, rename — so no
// crash of this platform can produce a half-written one; what can is something outside it, an
// operator editing the file or a volume that came back damaged. Retrying such a marker wedged the
// reconciliation of that run for good, on every pass (register row PD-164).
var ErrBadMarker = errors.New("runner: the exit marker cannot be read")

// UnitVanishedResult is the exit_result of an attempt whose unit went away without writing a marker
// at all — a reboot takes the manager and every ExecStopPost with it. Like the two values around it,
// it is deliberately one systemd cannot produce.
const UnitVanishedResult = "unit-vanished"

// UnreadableMarkerResult is the exit_result of an attempt whose marker could not be parsed.
// Deliberately a value systemd cannot produce, like StopRequestedResult: an operator reading the
// column has to be able to tell what the machine said from what this platform concluded.
const UnreadableMarkerResult = "marker-unreadable"

// ReadMarker reads the end of a unit.
func ReadMarker(path string) (Marker, error) {
	b, err := os.ReadFile(path)
	if errors.Is(err, fs.ErrNotExist) {
		return Marker{}, ErrNoMarker
	}
	if err != nil {
		return Marker{}, fmt.Errorf("runner: read marker: %w", err)
	}
	var m Marker
	if err := json.Unmarshal(b, &m); err != nil {
		return Marker{}, fmt.Errorf("%w: decode %s: %w", ErrBadMarker, path, err)
	}
	return m, nil
}

// MarkerFromEnv builds a marker from the variables systemd sets for ExecStopPost=.
func MarkerFromEnv(unit string, look func(string) string) Marker {
	return Marker{
		Unit:   unit,
		Result: look("SERVICE_RESULT"),
		Code:   look("EXIT_CODE"),
		Status: look("EXIT_STATUS"),
		At:     time.Now().UTC(),
	}
}