fix: fail early when --enable-feature=use-uncached-io is unsupported

Signed-off-by: machine424 <ayoubmrini424@gmail.com>

cmd/prometheus/main.go

@@ -77,6 +77,7 @@ import (
 	"github.com/prometheus/prometheus/tracing"
 	"github.com/prometheus/prometheus/tsdb"
 	"github.com/prometheus/prometheus/tsdb/agent"
+	"github.com/prometheus/prometheus/tsdb/fileutil"
 	"github.com/prometheus/prometheus/util/compression"
 	"github.com/prometheus/prometheus/util/documentcli"
 	"github.com/prometheus/prometheus/util/features"
@@ -319,6 +320,9 @@ func (c *flagConfig) setFeatureListOptions(logger *slog.Logger) error {
 				c.web.EnableTypeAndUnitLabels = true
 				logger.Info("Experimental type and unit labels enabled")
 			case "use-uncached-io":
+				if !fileutil.UncachedIOSupported() {
+					return errors.New("experimental Uncached IO is not supported")
+				}
 				c.tsdb.UseUncachedIO = true
 				logger.Info("Experimental Uncached IO is enabled.")
 			default:

tsdb/fileutil/direct_io_force.go

@@ -26,3 +26,7 @@ func NewDirectIOWriter(f *os.File, size int) (BufWriter, error) {
 func NewBufioWriterWithSize(f *os.File, size int) (BufWriter, error) {
 	return NewDirectIOWriter(f, size)
 }
+
+func UncachedIOSupported() bool {
+	return true
+}

tsdb/fileutil/direct_io_linux.go

@@ -27,3 +27,7 @@ func NewBufioWriterWithSize(f *os.File, size int) (BufWriter, error) {
 func NewDirectIOWriter(f *os.File, size int) (BufWriter, error) {
 	return newDirectIOWriter(f, size)
 }
+
+func UncachedIOSupported() bool {
+	return true
+}

tsdb/fileutil/direct_io_unsupported.go

@@ -27,3 +27,7 @@ func NewBufioWriterWithSize(f *os.File, size int) (BufWriter, error) {
 func NewDirectIOWriter(*os.File, int) (BufWriter, error) {
 	return nil, errDirectIOUnsupported
 }
+
+func UncachedIOSupported() bool {
+	return false
+}