p2p/enr: fix decoding of incomplete lists (#22484)

Given a list of less than two elements DecodeRLP returned rlp.EOL,
leading to issues in outer decoders.

p2p/enr/enr.go

@@ -50,6 +50,7 @@ var (
 	errNotSorted      = errors.New("record key/value pairs are not sorted by key")
 	errDuplicateKey   = errors.New("record contains duplicate key")
 	errIncompletePair = errors.New("record contains incomplete k/v pair")
+	errIncompleteList = errors.New("record contains less than two list elements")
 	errTooBig         = fmt.Errorf("record bigger than %d bytes", SizeLimit)
 	errEncodeUnsigned = errors.New("can't encode unsigned record")
 	errNotFound       = errors.New("no such key in record")
@@ -209,9 +210,15 @@ func decodeRecord(s *rlp.Stream) (dec Record, raw []byte, err error) {
 		return dec, raw, err
 	}
 	if err = s.Decode(&dec.signature); err != nil {
+		if err == rlp.EOL {
+			err = errIncompleteList
+		}
 		return dec, raw, err
 	}
 	if err = s.Decode(&dec.seq); err != nil {
+		if err == rlp.EOL {
+			err = errIncompleteList
+		}
 		return dec, raw, err
 	}
 	// The rest of the record contains sorted k/v pairs.

p2p/enr/enr_test.go

@@ -231,6 +231,29 @@ func TestRecordTooBig(t *testing.T) {
 	require.NoError(t, signTest([]byte{5}, &r))
 }
 
+// This checks that incomplete RLP inputs are handled correctly.
+func TestDecodeIncomplete(t *testing.T) {
+	type decTest struct {
+		input []byte
+		err   error
+	}
+	tests := []decTest{
+		{[]byte{0xC0}, errIncompleteList},
+		{[]byte{0xC1, 0x1}, errIncompleteList},
+		{[]byte{0xC2, 0x1, 0x2}, nil},
+		{[]byte{0xC3, 0x1, 0x2, 0x3}, errIncompletePair},
+		{[]byte{0xC4, 0x1, 0x2, 0x3, 0x4}, nil},
+		{[]byte{0xC5, 0x1, 0x2, 0x3, 0x4, 0x5}, errIncompletePair},
+	}
+	for _, test := range tests {
+		var r Record
+		err := rlp.DecodeBytes(test.input, &r)
+		if err != test.err {
+			t.Errorf("wrong error for %X: %v", test.input, err)
+		}
+	}
+}
+
 // TestSignEncodeAndDecodeRandom tests encoding/decoding of records containing random key/value pairs.
 func TestSignEncodeAndDecodeRandom(t *testing.T) {
 	var r Record