perf: tighten thumbnail cache memory usage

internal/api/thumbnail_cache_test.go

@@ -70,3 +70,19 @@ func TestThumbnailCacheEvictsByBound(t *testing.T) {
 		t.Fatalf("cache size=%d/list=%d, want 2/2", len(c.items), c.order.Len())
 	}
 }
+
+func TestThumbnailCacheEvictsByBytes(t *testing.T) {
+	c := newThumbnailCache()
+	c.maxBytes = 3
+	for i := 0; i < 2; i++ {
+		key := thumbnailCacheKey{Path: string(rune('x' + i))}
+		if _, err := c.Do(key, func() ([]byte, error) { return []byte("xx"), nil }); err != nil {
+			t.Fatal(err)
+		}
+	}
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if c.bytes > c.maxBytes || len(c.items) != 1 {
+		t.Fatalf("cache bytes=%d items=%d, want <=%d/1", c.bytes, len(c.items), c.maxBytes)
+	}
+}