correctly determine the length of STREAM frames for IETF QUIC

The length of the data has an influence on the length of the frame (if
it contains the data length), and the length can either consume 1 or 2
bytes due to variable length encoding.

internal/wire/stream_frame_legacy_test.go

@@ -394,7 +394,6 @@ var _ = Describe("STREAM frame (for gQUIC)", func() {
 					StreamID: 0xdecafbad,
 					Data:     []byte("foobar"),
 				}
-				frame.MinLength(0)
 				err := frame.Write(b, versionBigEndian)
 				Expect(err).ToNot(HaveOccurred())
 				Expect(b.Bytes()[0] & 0x3).To(Equal(uint8(0x3)))
@@ -480,4 +479,35 @@ var _ = Describe("STREAM frame (for gQUIC)", func() {
 			Expect(frame.DataLen()).To(Equal(protocol.ByteCount(6)))
 		})
 	})
+
+	Context("max data length", func() {
+		It("always returns a data length such that the resulting frame has the right size", func() {
+			const maxSize = 3000
+
+			data := make([]byte, maxSize)
+			f := &StreamFrame{
+				StreamID:       0x1337,
+				Offset:         0xdeadbeef,
+				DataLenPresent: true,
+			}
+			b := &bytes.Buffer{}
+			for i := 1; i < 3000; i++ {
+				b.Reset()
+				f.Data = nil
+				maxDataLen := f.MaxDataLen(protocol.ByteCount(i), versionBigEndian)
+				if maxDataLen == 0 { // 0 means that no valid STREAM_FRAME can be written
+					// check that writing a minimal size STREAM_FRAME (i.e. with 1 byte data) is actually larger than the desired size
+					f.Data = []byte{0}
+					err := f.Write(b, versionBigEndian)
+					Expect(err).ToNot(HaveOccurred())
+					Expect(b.Len()).To(BeNumerically(">", i))
+					continue
+				}
+				f.Data = data[:int(maxDataLen)]
+				err := f.Write(b, versionBigEndian)
+				Expect(err).ToNot(HaveOccurred())
+				Expect(b.Len()).To(Equal(i))
+			}
+		})
+	})
 })