package main

import (
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"strings"
	"sync"
	"sync/atomic"
	"time"
)

// **The build agent says whether it is busy, and being asked drains it** (novox/hq ADR 0335 §1, §5). Its
// container declares `busy` as `mesh-builder busy`: the node-engine runs it inside the container before it
// replaces it. It exits 0 while a build is in hand and 1 when none is; and from the first ask the agent takes
// no new build request, so its answer ends once the build in hand ends — a replaced builder finishes the check
// it runs, instead of the build queue handing it out again from the start (issue 594).
//
// Two marks in the container's own temporary directory, as the ready mark (ready.go): a new container starts
// without them, and a restarted one removes them before it takes work again.
//
//   - in hand: written by the agent when it claims a request, before it reads whether it is drained; removed
//     when the build ends.
//   - drained: written by the ask, before it reads whether a build is in hand.
//
// Each side writes its own mark before it reads the other's, so an ask racing a fetch sees the build in hand,
// or the agent sees the drain and hands the request back; never neither.

func inHandMark() string { return filepath.Join(os.TempDir(), "mesh-builder.in-hand") }
func drainMark() string  { return filepath.Join(os.TempDir(), "mesh-builder.drained") }

// The drain lapses when no ask has come for drainLapse: the node-engine asks every 20 seconds for as long as it
// really waits, so an agent whose replacement was withdrawn — the declaration put back, the walk stopped —
// takes work again rather than staying drained for ever (review of ADR 0335). And an agent drained for longer
// than the node-engine's bound and the slack says it is not ready: the replacement it was drained for never
// came, which is a fault to see.
const (
	drainLapse   = 2 * time.Minute
	drainTooLong = 15*time.Minute + 5*time.Minute
)

// drainedAt is when the drain began and when it was last asked, from its mark; false when there is no mark.
func drainedAt() (began, asked time.Time, ok bool) {
	info, err := os.Stat(drainMark())
	if err != nil {
		return time.Time{}, time.Time{}, false
	}
	asked = info.ModTime()
	began = asked
	if raw, err := os.ReadFile(drainMark()); err == nil {
		if t, err := time.Parse(time.RFC3339, strings.TrimSpace(string(raw))); err == nil && !t.After(asked) {
			began = t
		}
	}
	return began, asked, true
}

// drained says the agent was asked whether it is busy within drainLapse: it takes no new build request.
func drained() bool { return drainedSince(time.Now()) }

func drainedSince(now time.Time) bool {
	_, asked, ok := drainedAt()
	return ok && now.Sub(asked) < drainLapse
}

// drainedTooLong is the health check's second half: nil unless the agent has been drained, asked all along, for
// longer than its node-engine's bound and the slack.
func drainedTooLong(now time.Time) error {
	began, asked, ok := drainedAt()
	if !ok || now.Sub(asked) >= drainLapse || now.Sub(began) <= drainTooLong {
		return nil
	}
	return fmt.Errorf("drained since %s and still asked: the replacement its node-engine waits for has not come "+
		"within %s (novox/hq ADR 0335)", began.UTC().Format(time.RFC3339), drainTooLong)
}

// inHand is true while this process has claimed a build and not yet released it. The mark in the temporary directory
// is only believed while it is: a mark with no build of this process behind it is stale (a crash, a fault in an
// earlier path) and is removed, because left alone it hands back every ask for ever (issue 1768).
var inHand atomic.Bool

// markRetry is how long an agent that cannot mark a build in hand takes no work, and markFault when it last failed to.
// Taking nothing is the point: an ask fetched and handed back at once, many times a second, spends all its deliveries.
const markRetry = 30 * time.Second

var (
	markMu    sync.Mutex
	markFault time.Time
	markSaid  string
)

// markFaulty says the agent could not mark a build in hand and has not yet been able to since: it takes no work. Once
// markRetry has passed it tries the mark itself, with no ask in hand, so that an ask is fetched only when marking works.
func markFaulty(now time.Time) bool {
	markMu.Lock()
	failed := markFault
	markMu.Unlock()
	if failed.IsZero() {
		return false
	}
	if now.Sub(failed) < markRetry {
		return true
	}
	f, err := os.OpenFile(inHandMark(), os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o644)
	if err != nil && errors.Is(err, os.ErrExist) && !inHand.Load() {
		_ = os.Remove(inHandMark())
		f, err = os.OpenFile(inHandMark(), os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o644)
	}
	if err != nil {
		markFailed(err)
		return true
	}
	_ = f.Close()
	_ = os.Remove(inHandMark())
	markMu.Lock()
	markFault, markSaid = time.Time{}, ""
	markMu.Unlock()
	fmt.Fprintf(os.Stderr, "the mark of a build in hand can be made again (%s): taking builds\n", inHandMark())
	return false
}

// markFailed notes that a build could not be marked in hand, and says it when it is a new reason.
func markFailed(err error) {
	markMu.Lock()
	defer markMu.Unlock()
	markFault = time.Now()
	if said := err.Error(); said != markSaid {
		markSaid = said
		fmt.Fprintf(os.Stderr, "cannot mark a build in hand (%s), so this agent takes no build for %s and tries again: %v\n",
			inHandMark(), markRetry, err)
	}
}

// claim marks a request in hand, unless the agent is drained: then nothing is in hand, and the request goes
// back to the queue.
func claim() bool {
	if !inHand.Load() {
		// Nothing of this process is in hand, so a mark already there is stale.
		if err := os.Remove(inHandMark()); err == nil {
			fmt.Fprintf(os.Stderr, "removed the mark of a build in hand (%s): no build of this agent is in hand\n", inHandMark())
		}
	}
	// Made, never written over: one build at a time, and a mark already there is a build in hand.
	f, err := os.OpenFile(inHandMark(), os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o644)
	if err != nil {
		// Unmarked, an ask would say idle while this build runs: the build is not taken.
		markFailed(err)
		return false
	}
	_, err = f.WriteString(time.Now().UTC().Format(time.RFC3339) + "\n")
	if closeErr := f.Close(); err == nil {
		err = closeErr
	}
	if err != nil {
		_ = os.Remove(inHandMark())
		markFailed(err)
		return false
	}
	if drained() {
		_ = os.Remove(inHandMark())
		fmt.Fprintln(os.Stderr, "asked whether busy, so this agent takes no new build: handed back")
		return false
	}
	inHand.Store(true)
	return true
}

// holding says what is in hand, for the ask to print: written once the request is read.
func holding(what string) {
	_ = os.WriteFile(inHandMark(), []byte(time.Now().UTC().Format(time.RFC3339)+" "+what+"\n"), 0o644)
}

// released ends the build in hand.
func released() {
	inHand.Store(false)
	if err := os.Remove(inHandMark()); err != nil && !errors.Is(err, os.ErrNotExist) {
		fmt.Fprintf(os.Stderr, "cannot remove the mark of the build in hand (%s): %v\n", inHandMark(), err)
	}
}

// undrained clears both marks at the start of a holder: a container started again is asked afresh.
func undrained() error {
	for _, mark := range []string{drainMark(), inHandMark()} {
		if err := os.Remove(mark); err != nil && !errors.Is(err, os.ErrNotExist) {
			return fmt.Errorf("cannot remove %s: %w", mark, err)
		}
	}
	return nil
}

// busy is the ask: it drains the agent, then says whether a build is in hand. Nil — exit 0 — while one is.
func busy() (string, error) {
	// The mark keeps when the drain began while it is asked within drainLapse, and is written again on every
	// ask: its time is the last ask.
	began := time.Now().UTC()
	if was, _, ok := drainedAt(); ok && drained() {
		began = was.UTC()
	}
	if err := os.WriteFile(drainMark(), []byte(began.Format(time.RFC3339)+"\n"), 0o644); err != nil {
		// Not drained, it could take the next build: said, and answered busy only for what is in hand now.
		fmt.Fprintf(os.Stderr, "cannot mark this agent drained (%s): %v\n", drainMark(), err)
	}
	held, err := os.ReadFile(inHandMark())
	if err != nil {
		return "", errors.New("idle: no build in hand, and this agent takes no new one")
	}
	since, what, _ := strings.Cut(strings.TrimSpace(string(held)), " ")
	if what == "" {
		what = "a build"
	}
	return fmt.Sprintf("busy: %s, in hand since %s; taking no new build", what, since), nil
}