Missing data read in super block extra parsing (#8392)

* storage: fail on short super block extra read even without an error

Some backends (e.g. s3_backend) clear io.EOF after a short read, returning
n < len(p) with a nil error. Checking n only when the error is non-nil
lets truncated extra data reach proto.Unmarshal with a zero-filled tail.

* storage: apply the same short-read check to the super block header

A backend that returns n < SuperBlockSize with a nil error previously
slipped through and parsed a partially zero-filled header.

* storage: test super block extra reads

Covers a round trip through ReadSuperBlock, a truncated extra region, a
backend returning io.EOF alongside a full read, and a backend returning
a short read with a nil error.

---------

Co-authored-by: Chris Lu <chrislusf@users.noreply.github.com>

weed/storage/super_block/super_block_read.go.go

@@ -2,6 +2,7 @@ package super_block
 
 import (
 	"fmt"
+	"io"
 
 	"google.golang.org/protobuf/proto"
 
@@ -15,11 +16,12 @@ import (
 func ReadSuperBlock(datBackend backend.BackendStorageFile) (superBlock SuperBlock, err error) {
 
 	header := make([]byte, SuperBlockSize)
-	if n, e := datBackend.ReadAt(header, 0); e != nil {
-		if n != SuperBlockSize {
-			err = fmt.Errorf("cannot read volume %s super block: %v", datBackend.Name(), e)
-			return
+	if n, e := datBackend.ReadAt(header, 0); n != SuperBlockSize {
+		if e == nil {
+			e = io.ErrUnexpectedEOF
 		}
+		err = fmt.Errorf("cannot read volume %s super block: %v", datBackend.Name(), e)
+		return
 	}
 
 	superBlock.Version = needle.Version(header[0])
@@ -34,11 +36,12 @@ func ReadSuperBlock(datBackend backend.BackendStorageFile) (superBlock SuperBloc
 	if superBlock.ExtraSize > 0 {
 		// read more
 		extraData := make([]byte, int(superBlock.ExtraSize))
-		if n, e := datBackend.ReadAt(extraData, SuperBlockSize); e != nil {
-			if n != len(extraData) {
-				err = fmt.Errorf("cannot read volume %s super block extra: %v", datBackend.Name(), e)
-				return
+		if n, e := datBackend.ReadAt(extraData, SuperBlockSize); n != len(extraData) {
+			if e == nil {
+				e = io.ErrUnexpectedEOF
 			}
+			err = fmt.Errorf("cannot read volume %s super block extra: %v", datBackend.Name(), e)
+			return
 		}
 		superBlock.Extra = &master_pb.SuperBlockExtra{}
 		err = proto.Unmarshal(extraData, superBlock.Extra)

weed/storage/super_block/super_block_test.go

@@ -142,3 +142,36 @@ func TestReadSuperBlockTruncatedExtra(t *testing.T) {
 		t.Fatalf("expected read error for truncated extra, got %v", err)
 	}
 }
+
+// s3_backend.ReadAt clears io.EOF even when it returns fewer bytes than requested.
+type shortReadBackendStorageFile struct {
+	backend.DiskFile
+	data []byte
+}
+
+func (f *shortReadBackendStorageFile) ReadAt(p []byte, off int64) (n int, err error) {
+	return copy(p, f.data[off:]), nil
+}
+
+func TestReadSuperBlockShortReadWithoutError(t *testing.T) {
+	rp, _ := NewReplicaPlacementFromByte(byte(001))
+	ttl, _ := needle.ReadTTL("15d")
+	s := &SuperBlock{
+		Version:          needle.GetCurrentVersion(),
+		ReplicaPlacement: rp,
+		Ttl:              ttl,
+		Extra: &master_pb.SuperBlockExtra{
+			ErasureCoding: &master_pb.SuperBlockExtra_ErasureCoding{
+				Data:      10,
+				Parity:    4,
+				VolumeIds: []uint32{1, 2, 3},
+			},
+		},
+	}
+
+	data := s.Bytes()
+	_, err := ReadSuperBlock(&shortReadBackendStorageFile{data: data[:len(data)-1]})
+	if err == nil || !strings.Contains(err.Error(), "cannot read") {
+		t.Fatalf("expected read error for short extra read, got %v", err)
+	}
+}