Fix misuse of runtime.AddCleanup

This prevented the file from being unmapped when the `Reader` was
garbage collected.

Author: oschwald

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Changes
 
+## 2.1.1 - 2025-11-26
+
+- Fixed `runtime.AddCleanup` misuse that prevented the memory-mapped file from
+  being unmapped when the `Reader` was garbage collected.
+
 ## 2.1.0 - 2025-11-04
 
 - Updated `Offset` method on `Decoder` to return the resolved data offset

reader.go

@@ -141,7 +141,7 @@ type Reader struct {
 	ipv4Start         uint
 	ipv4StartBitDepth int
 	nodeOffsetMult    uint
-	hasMappedFile     atomic.Bool
+	hasMappedFile     *atomic.Bool
 }
 
 // Metadata holds the metadata decoded from the MaxMind DB file.
@@ -262,7 +262,7 @@ func Open(file string, options ...ReaderOption) (*Reader, error) {
 	reader.hasMappedFile.Store(true)
 	cleanup := &mmapCleanup{
 		data:      data,
-		hasMapped: &reader.hasMappedFile,
+		hasMapped: reader.hasMappedFile,
 	}
 	runtime.AddCleanup(reader, func(mc *mmapCleanup) {
 		if mc.hasMapped.CompareAndSwap(true, false) {
@@ -356,6 +356,7 @@ func OpenBytes(buffer []byte, options ...ReaderOption) (*Reader, error) {
 		Metadata:       metadata,
 		ipv4Start:      0,
 		nodeOffsetMult: metadata.RecordSize / 4,
+		hasMappedFile:  &atomic.Bool{},
 	}
 
 	err = reader.setIPv4Start()

reader_test.go

@@ -9,6 +9,7 @@ import (
 	"net/netip"
 	"os"
 	"path/filepath"
+	"runtime"
 	"sync"
 	"testing"
 	"time"
@@ -67,6 +68,32 @@ func TestReaderBytes(t *testing.T) {
 	}
 }
 
+func TestReaderLeaks(t *testing.T) {
+	collected := make(chan struct{})
+
+	func() {
+		r, err := Open(testFile("GeoLite2-City-Test.mmdb"))
+		require.NoError(t, err)
+
+		// We intentionally do NOT call Close() to test if GC picks it up
+		// and if AddCleanup doesn't prevent it.
+
+		runtime.SetFinalizer(r, func(obj *Reader) {
+			close(collected)
+		})
+	}()
+
+	require.Eventually(t, func() bool {
+		runtime.GC()
+		select {
+		case <-collected:
+			return true
+		default:
+			return false
+		}
+	}, 1*time.Second, 50*time.Millisecond, "Reader was NOT collected (leak detected)")
+}
+
 func TestLookupNetwork(t *testing.T) {
 	bigInt := new(big.Int)
 	bigInt.SetString("1329227995784915872903807060280344576", 10)