// Package upgrade is how the host survives replacing itself.
//
// novox/hq ADR 0005. Two facts, and neither is the host judging its own health:
//
//   - whether the executable this process started from has been replaced on disk, which is how
//     it knows to stand aside for a new one;
//   - which version last got as far as a completed reconcile, which is what a rollback outside
//     this binary reads when this binary will not start.
//
// The second is written for a reader that is not the host. A binary that cannot start cannot be
// its own recovery, so what it leaves behind has to be plain enough for a shell script.
package upgrade

import (
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"strings"
)

// Files the launcher reads and this binary writes. Next to the store, because they are node
// state of exactly the same kind.
const (
	KnownGoodName = "known-good"
	AttemptsName  = "start-attempts"
)

// Self is the executable this process started from, remembered.
//
// Identity is taken once, at start, and compared later. The obvious alternative — asking
// /proc/self/exe whether it is marked deleted — was tried and is worse in two ways: it is Linux
// procfs behaviour rather than a fact about files, and it catches only *unlink*, so a binary
// swapped by rename onto the same path reads as untouched. Remembering what we started from
// needs no special filesystem and misses neither case.
type Self struct {
	path string
	info os.FileInfo
}

// Current captures the running executable's identity.
//
// path is what os.Executable() returned; a test passes one it can manipulate, because the
// boundary being tested is the filesystem and a fake would assert that the fake behaves as
// expected (novox/hq ADR 0017).
func Current(path string) (Self, error) {
	info, err := os.Stat(path)
	if err != nil {
		return Self{}, fmt.Errorf(
			"cannot stat %s, so this host cannot tell whether it is later replaced: %w", path, err)
	}
	return Self{path: path, info: info}, nil
}

// Path is where the executable was when this process started.
func (s Self) Path() string { return s.path }

// Replaced reports whether a different file is at that path now, or none.
//
// Never a silent false: a host that cannot read its own image says so rather than assuming it is
// current, which is the shape of every fault this repository catalogues.
func (s Self) Replaced() (bool, error) {
	if s.info == nil {
		return false, errors.New("this host never captured its own identity, so it cannot tell " +
			"whether it has been replaced")
	}

	now, err := os.Stat(s.path)
	if errors.Is(err, os.ErrNotExist) {
		// Removed rather than upgraded. Still not what is running, and saying "unchanged"
		// would leave the host claiming a version that is no longer installed.
		return true, nil
	}
	if err != nil {
		return false, err
	}

	return !os.SameFile(s.info, now), nil
}

// KnownGoodPath is where the marker lives, given where the store lives.
func KnownGoodPath(statePath string) string {
	return filepath.Join(filepath.Dir(statePath), KnownGoodName)
}

// AttemptsPath is where the launcher counts starts that have not yet worked.
func AttemptsPath(statePath string) string {
	return filepath.Join(filepath.Dir(statePath), AttemptsName)
}

// ClearAttempts tells the launcher this start worked.
//
// Written at the same moment as known-good and for the same reason: a completed reconcile is
// the evidence, and it is the only evidence either of them has. Without this the counter only
// ever climbs, so a node that has been up for months rolls itself back on its third ordinary
// restart — a healthy machine undone by its own recovery.
func ClearAttempts(path string) error {
	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
		return err
	}
	return os.WriteFile(path, []byte("0\n"), 0o644)
}

// RecordKnownGood marks a version as one that started and completed a reconcile.
//
// Written atomically and as one bare line. The reader is a shell script running on a machine
// where the host is failing to start, so the format is the least it can be: no JSON, no
// escaping, nothing that needs a parser to be present and working.
func RecordKnownGood(path, version string) error {
	if strings.TrimSpace(version) == "" {
		return errors.New("refusing to record an empty version as known-good: a rollback " +
			"reading it would install nothing and report success")
	}
	if strings.ContainsAny(version, "\n\r") {
		return fmt.Errorf("refusing to record %q as known-good: it must be one line", version)
	}
	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
		return err
	}

	tmp, err := os.CreateTemp(filepath.Dir(path), ".known-good-*")
	if err != nil {
		return err
	}
	defer os.Remove(tmp.Name())

	if _, err := fmt.Fprintln(tmp, version); err != nil {
		tmp.Close()
		return err
	}
	if err := tmp.Sync(); err != nil {
		tmp.Close()
		return err
	}
	if err := tmp.Close(); err != nil {
		return err
	}
	if err := os.Chmod(tmp.Name(), 0o644); err != nil {
		return err
	}
	return os.Rename(tmp.Name(), path)
}

// ReadKnownGood returns the recorded version, or "" if there has never been one.
//
// Absence is not an error. A machine whose host has never completed a reconcile has no version
// to go back to, and that is a real state rather than a fault: the node was never working, so
// the failure belongs to the installation and not to an upgrade. A rollback that guessed here
// would become a second fault.
func ReadKnownGood(path string) (string, error) {
	raw, err := os.ReadFile(path)
	if errors.Is(err, os.ErrNotExist) {
		return "", nil
	}
	if err != nil {
		return "", err
	}
	return strings.TrimSpace(string(raw)), nil
}