fix: close modsdir snapshot file descriptors

.changes/v1.16/BUG FIXES-20260324-113149.yaml

@@ -0,0 +1,5 @@
+kind: BUG FIXES
+body: Close module manifest snapshot files after writing to avoid leaking file descriptors.
+time: 2026-03-24T11:31:49-07:00
+custom:
+    Issue: "38302"

internal/modsdir/manifest.go

@@ -5,6 +5,7 @@ package modsdir
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -172,12 +173,15 @@ func (m Manifest) WriteSnapshot(w io.Writer) error {
 	return err
 }
 
-func (m Manifest) WriteSnapshotToDir(dir string) error {
+func (m Manifest) WriteSnapshotToDir(dir string) (retErr error) {
 	fn := filepath.Join(dir, ManifestSnapshotFilename)
 	log.Printf("[TRACE] modsdir: writing modules manifest to %s", fn)
 	w, err := os.Create(fn)
 	if err != nil {
 		return err
 	}
+	defer func() {
+		retErr = errors.Join(retErr, w.Close())
+	}()
 	return m.WriteSnapshot(w)
 }

internal/modsdir/manifest_test.go

@@ -0,0 +1,73 @@
+//go:build darwin || linux
+
+// Copyright IBM Corp. 2014, 2026
+// SPDX-License-Identifier: BUSL-1.1
+
+package modsdir
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"runtime/debug"
+	"testing"
+
+	"golang.org/x/sys/unix"
+)
+
+func TestManifestWriteSnapshotToDirClosesFile(t *testing.T) {
+	oldGCPercent := debug.SetGCPercent(-1)
+	defer debug.SetGCPercent(oldGCPercent)
+
+	manifest := Manifest{
+		"root": {
+			Key: "root",
+			Dir: "modules/root",
+		},
+	}
+
+	baseDir := t.TempDir()
+	before := countOpenFileDescriptors(t)
+
+	const iterations = 32
+	for i := range iterations {
+		dir := filepath.Join(baseDir, fmt.Sprintf("manifest-%d", i))
+		if err := os.Mkdir(dir, 0o755); err != nil {
+			t.Fatalf("creating manifest dir %d: %s", i, err)
+		}
+
+		if err := manifest.WriteSnapshotToDir(dir); err != nil {
+			t.Fatalf("writing manifest %d: %s", i, err)
+		}
+	}
+
+	after := countOpenFileDescriptors(t)
+	if leaked := after - before; leaked > 2 {
+		t.Fatalf("expected WriteSnapshotToDir to close its file descriptor, but open descriptor count increased by %d", leaked)
+	}
+}
+
+func countOpenFileDescriptors(t *testing.T) int {
+	t.Helper()
+
+	var limit unix.Rlimit
+	if err := unix.Getrlimit(unix.RLIMIT_NOFILE, &limit); err != nil {
+		t.Fatalf("reading RLIMIT_NOFILE: %s", err)
+	}
+
+	maxFD := int(limit.Cur)
+	if maxFD > 4096 {
+		maxFD = 4096
+	}
+
+	openDescriptors := 0
+	for fd := range maxFD {
+		if _, err := unix.FcntlInt(uintptr(fd), unix.F_GETFD, 0); err == nil {
+			openDescriptors++
+		} else if err != unix.EBADF {
+			t.Fatalf("checking file descriptor %d: %s", fd, err)
+		}
+	}
+
+	return openDescriptors
+}