package imgcache

import (
	"sync"
	"sync/atomic"
	"testing"
	"time"
)

// TestContentLockExcludesSameKey verifies that two goroutines locking
// the same key never run their critical sections concurrently.
func TestContentLockExcludesSameKey(t *testing.T) {
	t.Parallel()

	lock := newContentLock()

	var (
		active  atomic.Int32
		maxSeen int32
		wg      sync.WaitGroup
	)

	const goroutines = 20

	wg.Add(goroutines)

	for range goroutines {
		go func() {
			defer wg.Done()

			unlock := lock.Lock("same-key")
			defer unlock()

			n := active.Add(1)

			for {
				seen := atomic.LoadInt32(&maxSeen)
				if n <= seen || atomic.CompareAndSwapInt32(&maxSeen, seen, n) {
					break
				}
			}

			time.Sleep(time.Millisecond)

			active.Add(-1)
		}()
	}

	wg.Wait()

	if maxSeen != 1 {
		t.Errorf("max concurrent holders of the same key = %d, want 1", maxSeen)
	}
}

// TestContentLockAllowsDifferentKeys verifies that locking distinct
// keys does not serialize unrelated work: all goroutines must be able
// to enter their critical sections at once, proven by every one of
// them reaching the rendezvous point before any is allowed to
// proceed.
func TestContentLockAllowsDifferentKeys(t *testing.T) {
	t.Parallel()

	lock := newContentLock()

	const goroutines = 20

	var (
		wg      sync.WaitGroup
		inside  atomic.Int32
		reached = make(chan struct{}, goroutines)
	)

	wg.Add(goroutines)

	release := make(chan struct{})

	for i := range goroutines {
		key := string(rune('a' + i))

		go func() {
			defer wg.Done()

			unlock := lock.Lock(key)
			defer unlock()

			inside.Add(1)

			reached <- struct{}{}

			<-release
		}()
	}

	// Every goroutine must reach the rendezvous point (i.e. acquire its
	// own key's lock) without needing any other to release first. If
	// keys were incorrectly serialized onto one underlying lock, only
	// one would get here and this would time out.
	for i := range goroutines {
		select {
		case <-reached:
		case <-time.After(2 * time.Second):
			t.Fatalf("only %d/%d goroutines locking distinct keys made progress; "+
				"keys may be incorrectly serialized", i, goroutines)
		}
	}

	if n := inside.Load(); n != goroutines {
		t.Errorf("goroutines inside their critical section = %d, want %d",
			n, goroutines)
	}

	close(release)
	wg.Wait()
}

// TestContentLockRemovesEntryAfterUnlock verifies that the internal
// entries map does not grow without bound: once no goroutine holds or
// awaits a key, its entry is removed.
func TestContentLockRemovesEntryAfterUnlock(t *testing.T) {
	t.Parallel()

	lock := newContentLock()

	unlock := lock.Lock("k")

	lock.mu.Lock()
	if _, ok := lock.entries["k"]; !ok {
		lock.mu.Unlock()
		t.Fatal("entry missing while lock is held")
	}
	lock.mu.Unlock()

	unlock()

	lock.mu.Lock()
	defer lock.mu.Unlock()

	if _, ok := lock.entries["k"]; ok {
		t.Error("entry for key still present after the last holder unlocked")
	}
}