libindex: add fallback path for O_TMPFILE not being supported

Signed-off-by: Hank Donnay <hdonnay@redhat.com>

libindex/fetcher.go

@@ -34,12 +34,6 @@ var (
 	_ indexer.DescriptionRealizer = (*FetchProxy)(nil)
 )
 
-// BUG(hank) On Linux, the [RemoteFetchArena] makes use of the O_TMPFILE flag to
-// [open(2)], which requires version 3.11 or newer. There is no fallback for
-// older kernels.
-//
-// [open(2)]: https://man7.org/linux/man-pages/man2/open.2.html
-
 // RemoteFetchArena uses disk space to track fetched layers, removing them once
 // all users are done with the layers.
 type RemoteFetchArena struct {

libindex/tempfile_linux.go

@@ -1,24 +1,63 @@
 package libindex
 
 import (
+	"errors"
 	"fmt"
 	"os"
+	"sync"
 
 	"golang.org/x/sys/unix"
 )
 
+var tmpMap sync.Map
+
+func canTmp(dir string) (ok, loaded bool) {
+	v, loaded := tmpMap.Load(dir)
+	if v == nil {
+		return false, loaded
+	}
+	return v.(bool), loaded
+}
+
+func setTmp(dir string, ok bool) {
+	tmpMap.Store(dir, ok)
+}
+
 type tempFile struct {
 	*os.File
 }
 
 func openTemp(dir string) (*tempFile, error) {
-	f, err := os.OpenFile(dir, os.O_WRONLY|unix.O_TMPFILE, 0644)
+	var f *os.File
+	var err error
+
+	ok, loaded := canTmp(dir)
+	switch {
+	case loaded && ok:
+		f, err = os.OpenFile(dir, os.O_WRONLY|unix.O_TMPFILE, 0644)
+	case loaded && !ok:
+		f, err = os.CreateTemp(dir, "fetcher.*")
+	case !loaded:
+		f, err = os.OpenFile(dir, os.O_WRONLY|unix.O_TMPFILE, 0644)
+		if err == nil || !errors.Is(err, unix.ENOTSUP) {
+			ok = true
+			break
+		}
+		f, err = os.CreateTemp(dir, "fetcher.*")
+	default:
+		panic("unreachable")
+	}
+	if !loaded {
+		setTmp(dir, ok)
+	}
+	if !ok && err == nil {
+		f.Close()
+	}
+
 	if err != nil {
 		return nil, err
 	}
-	return &tempFile{
-		File: f,
-	}, nil
+	return &tempFile{File: f}, nil
 }
 
 func (t *tempFile) Reopen() (*os.File, error) {