add request buffer for nmap data store

management/internals/shared/requestbuffer/buffer.go

@@ -0,0 +1,102 @@
+// Package requestbuffer coalesces concurrent reads of the same expensive
+// resource into a single fetch.
+package requestbuffer
+
+import (
+	"context"
+	"os"
+	"sync"
+	"time"
+
+	log "github.com/sirupsen/logrus"
+)
+
+// FetchFunc reads the resource identified by key.
+type FetchFunc[T any] func(ctx context.Context, key string) (T, error)
+
+// Buffer batches requests per key: the first request opens a window, every
+// request arriving within it joins the batch, and a single fetch serves them
+// all. The fetch starts only after the window closed, so a caller never
+// observes data read before its own request.
+type Buffer[T any] struct {
+	ctx      context.Context
+	name     string
+	fetch    FetchFunc[T]
+	interval time.Duration
+
+	mu      sync.Mutex
+	waiting map[string][]chan result[T]
+}
+
+type result[T any] struct {
+	value T
+	err   error
+}
+
+// New returns a Buffer serving batched requests through fetch. ctx bounds the
+// fetches, not the callers, and must outlive them.
+func New[T any](ctx context.Context, name string, interval time.Duration, fetch FetchFunc[T]) *Buffer[T] {
+	return &Buffer[T]{
+		ctx:      ctx,
+		name:     name,
+		fetch:    fetch,
+		interval: interval,
+		waiting:  make(map[string][]chan result[T]),
+	}
+}
+
+// Get returns the value for key, sharing one fetch with the other callers of
+// the current batch. The value is shared as is, so callers must treat it as
+// read-only unless the fetch hands out copies.
+func (b *Buffer[T]) Get(ctx context.Context, key string) (T, error) {
+	ch := make(chan result[T], 1)
+
+	b.mu.Lock()
+	b.waiting[key] = append(b.waiting[key], ch)
+	first := len(b.waiting[key]) == 1
+	b.mu.Unlock()
+
+	if first {
+		time.AfterFunc(b.interval, func() { b.flush(key) })
+	}
+
+	select {
+	case res := <-ch:
+		return res.value, res.err
+	case <-ctx.Done():
+		var zero T
+		return zero, ctx.Err()
+	}
+}
+
+func (b *Buffer[T]) flush(key string) {
+	b.mu.Lock()
+	waiting := b.waiting[key]
+	delete(b.waiting, key)
+	b.mu.Unlock()
+
+	if len(waiting) == 0 {
+		return
+	}
+
+	start := time.Now()
+	value, err := b.fetch(b.ctx, key)
+	log.WithContext(b.ctx).Tracef("%s: fetched %s for %d waiters in %s", b.name, key, len(waiting), time.Since(start))
+
+	for _, ch := range waiting {
+		ch <- result[T]{value: value, err: err}
+	}
+}
+
+// Interval reads a buffer interval from envVar, falling back to def.
+func Interval(ctx context.Context, envVar string, def time.Duration) time.Duration {
+	value := os.Getenv(envVar)
+	interval, err := time.ParseDuration(value)
+	if err != nil {
+		if value != "" {
+			log.WithContext(ctx).Warnf("failed to parse %s: %s", envVar, err)
+		}
+		return def
+	}
+	return interval
+}

management/internals/shared/requestbuffer/buffer_test.go

@@ -0,0 +1,106 @@
+package requestbuffer
+
+import (
+	"context"
+	"errors"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestBufferCoalescesConcurrentRequests(t *testing.T) {
+	var fetches atomic.Int32
+	buffer := New(context.Background(), "test", 50*time.Millisecond,
+		func(ctx context.Context, key string) (string, error) {
+			fetches.Add(1)
+			return key, nil
+		})
+
+	var wg sync.WaitGroup
+	for range 10 {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			value, err := buffer.Get(context.Background(), "account")
+			assert.NoError(t, err)
+			assert.Equal(t, "account", value)
+		}()
+	}
+	wg.Wait()
+
+	assert.Equal(t, int32(1), fetches.Load())
+}
+
+func TestBufferSeparatesKeys(t *testing.T) {
+	keys := make(chan string, 2)
+	buffer := New(context.Background(), "test", 10*time.Millisecond,
+		func(ctx context.Context, key string) (string, error) {
+			keys <- key
+			return key, nil
+		})
+
+	var wg sync.WaitGroup
+	for _, key := range []string{"a", "b"} {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			_, err := buffer.Get(context.Background(), key)
+			assert.NoError(t, err)
+		}()
+	}
+	wg.Wait()
+	close(keys)
+
+	var fetched []string
+	for key := range keys {
+		fetched = append(fetched, key)
+	}
+	assert.ElementsMatch(t, []string{"a", "b"}, fetched)
+}
+
+func TestBufferFetchesAfterRequest(t *testing.T) {
+	var version atomic.Int32
+	buffer := New(context.Background(), "test", 10*time.Millisecond,
+		func(ctx context.Context, key string) (int32, error) {
+			return version.Load(), nil
+		})
+
+	first, err := buffer.Get(context.Background(), "account")
+	require.NoError(t, err)
+	assert.Equal(t, int32(0), first)
+
+	version.Store(1)
+
+	second, err := buffer.Get(context.Background(), "account")
+	require.NoError(t, err)
+	assert.Equal(t, int32(1), second)
+}
+
+func TestBufferPropagatesError(t *testing.T) {
+	fetchErr := errors.New("fetch failed")
+	buffer := New(context.Background(), "test", 10*time.Millisecond,
+		func(ctx context.Context, key string) (*int, error) {
+			return nil, fetchErr
+		})
+
+	value, err := buffer.Get(context.Background(), "account")
+	assert.ErrorIs(t, err, fetchErr)
+	assert.Nil(t, value)
+}
+
+func TestBufferHonorsCallerContext(t *testing.T) {
+	buffer := New(context.Background(), "test", time.Minute,
+		func(ctx context.Context, key string) (string, error) {
+			return key, nil
+		})
+
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Millisecond)
+	defer cancel()
+
+	_, err := buffer.Get(ctx, "account")
+	assert.ErrorIs(t, err, context.DeadlineExceeded)
+}