fm-rds-tx/internal/output/frame_queue.go

package output

import (
	"context"
	"errors"
	"sync"
)

// ErrFrameQueueClosed is returned when a queue operation is attempted after the queue
// has been closed.
var ErrFrameQueueClosed = errors.New("frame queue closed")

// QueueStats exposes the runtime state of a frame queue.
type QueueStats struct {
	Capacity       int         `json:"capacity"`
	Depth          int         `json:"depth"`
	FillLevel      float64     `json:"fillLevel"`
	Health         QueueHealth `json:"health"`
	HighWaterMark  int         `json:"highWaterMark"`
	LowWaterMark   int         `json:"lowWaterMark"`
	PushTimeouts   uint64      `json:"pushTimeouts"`
	PopTimeouts    uint64      `json:"popTimeouts"`
	DroppedFrames  uint64      `json:"droppedFrames"`
	RepeatedFrames uint64      `json:"repeatedFrames"`
	MutedFrames    uint64      `json:"mutedFrames"`
}

type QueueHealth string

const (
	QueueHealthCritical QueueHealth = "critical"
	QueueHealthLow      QueueHealth = "low"
	QueueHealthNormal   QueueHealth = "normal"
)

const (
	queueHealthCriticalThreshold = 0.2
	queueHealthLowThreshold      = 0.5
)

// FrameQueue is a bounded ring that holds CompositeFrame instances between the
// generator and the writer. Push blocks when the queue is full until space
// becomes available or the provided context is cancelled. Pop blocks when the
// queue is empty until a new frame arrives or the context is cancelled.
type FrameQueue struct {
	capacity int
	ch       chan *CompositeFrame
	// closeCh is closed by Close() and used in Push/Pop selects so that
	// a concurrent Close() can never race with a channel send.
	closeCh  chan struct{}

	mu            sync.Mutex
	depth         int
	highWaterMark int
	lowWaterMark  int
	pushTimeouts  uint64
	popTimeouts   uint64
	dropped       uint64
	repeated      uint64
	muted         uint64
	closed        bool

	closeOnce sync.Once
}

// NewFrameQueue builds a bounded queue that holds up to capacity frames.
func NewFrameQueue(capacity int) *FrameQueue {
	if capacity <= 0 {
		capacity = 1
	}
	fq := &FrameQueue{
		capacity:     capacity,
		ch:           make(chan *CompositeFrame, capacity),
		closeCh:      make(chan struct{}),
		lowWaterMark: capacity,
	}
	fq.trackDepth(0)
	return fq
}

// Capacity returns the fixed frame capacity of the queue.
func (q *FrameQueue) Capacity() int {
	return q.capacity
}

// FillLevel reports the current occupancy as a fraction of capacity.
// Uses len(ch) directly for accuracy: updateDepth() is called after the
// channel operation, so q.depth can lag by one frame transiently.
func (q *FrameQueue) FillLevel() float64 {
	if q.capacity == 0 {
		return 0
	}
	return float64(len(q.ch)) / float64(q.capacity)
}

// Depth returns the current number of frames in the queue.
// Uses len(ch) directly for accuracy (see FillLevel).
func (q *FrameQueue) Depth() int {
	return len(q.ch)
}

// Stats returns a snapshot of the queue metrics.
func (q *FrameQueue) Stats() QueueStats {
	q.mu.Lock()
	fill := q.fillLevelLocked()
	stats := QueueStats{
		Capacity:       q.capacity,
		Depth:          len(q.ch),
		FillLevel:      fill,
		Health:         queueHealthFromFill(fill),
		HighWaterMark:  q.highWaterMark,
		LowWaterMark:   q.lowWaterMark,
		PushTimeouts:   q.pushTimeouts,
		PopTimeouts:    q.popTimeouts,
		DroppedFrames:  q.dropped,
		RepeatedFrames: q.repeated,
		MutedFrames:    q.muted,
	}
	q.mu.Unlock()
	return stats
}

// Push enqueues a frame, blocking until space is available or ctx is done.
func (q *FrameQueue) Push(ctx context.Context, frame *CompositeFrame) error {
	if frame == nil {
		return errors.New("frame required")
	}

	// BUG-A fix: use closeCh in the select — no TOCTOU gap.
	// BUG-3 fix: updateDepth(+1) and trackDepth (highWaterMark) only on
	// successful send. Pre-incrementing before the select caused the
	// high-watermark to count frames that were never actually queued
	// (cancelled or closed path), potentially reporting capacity+1.
	select {
	case q.ch <- frame:
		q.updateDepth(+1)
		return nil
	case <-q.closeCh:
		return ErrFrameQueueClosed
	case <-ctx.Done():
		q.recordPushTimeout()
		return ctx.Err()
	}
}

// Pop removes a frame, blocking until one is available or ctx signals done.
func (q *FrameQueue) Pop(ctx context.Context) (*CompositeFrame, error) {
	select {
	case frame, ok := <-q.ch:
		if !ok {
			return nil, ErrFrameQueueClosed
		}
		q.updateDepth(-1)
		return frame, nil
	case <-ctx.Done():
		q.recordPopTimeout()
		return nil, ctx.Err()
	}
}

// Drain removes and discards all frames currently in the queue.
// Call before restarting a stopped engine to avoid replaying stale frames.
func (q *FrameQueue) Drain() {
	for {
		select {
		case <-q.ch:
			q.updateDepth(-1)
		default:
			return
		}
	}
}

// Close marks the queue as closed and wakes up blocked callers.
func (q *FrameQueue) Close() {
	q.closeOnce.Do(func() {
		q.mu.Lock()
		q.closed = true
		q.mu.Unlock()
		// Close closeCh first so blocked Push() calls unblock safely
		// before close(ch) removes the destination.
		close(q.closeCh)
		close(q.ch)
	})
}

// RecordDrop increments the drop counter for instrumentation.
func (q *FrameQueue) RecordDrop() {
	q.mu.Lock()
	q.dropped++
	q.mu.Unlock()
}

// RecordRepeat increments the repeat counter for instrumentation.
func (q *FrameQueue) RecordRepeat() {
	q.mu.Lock()
	q.repeated++
	q.mu.Unlock()
}

// RecordMute increments the mute counter for instrumentation.
func (q *FrameQueue) RecordMute() {
	q.mu.Lock()
	q.muted++
	q.mu.Unlock()
}

func (q *FrameQueue) isClosed() bool {
	q.mu.Lock()
	closed := q.closed
	q.mu.Unlock()
	return closed
}

func (q *FrameQueue) updateDepth(delta int) {
	q.mu.Lock()
	q.depth += delta
	q.trackDepth(q.depth)
	q.mu.Unlock()
}

func (q *FrameQueue) trackDepth(depth int) {
	if depth > q.highWaterMark {
		q.highWaterMark = depth
	}
	if depth < q.lowWaterMark {
		q.lowWaterMark = depth
	}
}

func (q *FrameQueue) fillLevelLocked() float64 {
	if q.capacity == 0 {
		return 0
	}
	// Use len(ch) rather than q.depth: depth is updated after the channel
	// operation, so it can be off by one during the Push/Pop window.
	return float64(len(q.ch)) / float64(q.capacity)
}

func (q *FrameQueue) recordPushTimeout() {
	q.mu.Lock()
	q.pushTimeouts++
	q.mu.Unlock()
}

func (q *FrameQueue) recordPopTimeout() {
	q.mu.Lock()
	q.popTimeouts++
	q.mu.Unlock()
}

func queueHealthFromFill(fill float64) QueueHealth {
	switch {
	case fill <= queueHealthCriticalThreshold:
		return QueueHealthCritical
	case fill <= queueHealthLowThreshold:
		return QueueHealthLow
	default:
		return QueueHealthNormal
	}
}