http3: fix capsule parsing (#4683)

The current code returns an io.ErrUnexpectedEOF on partial reads of the
capsule content.

http3/capsule.go

@@ -10,12 +10,12 @@ import (
 type CapsuleType uint64
 
 type exactReader struct {
-	R *io.LimitedReader
+	R io.LimitedReader
 }
 
 func (r *exactReader) Read(b []byte) (int, error) {
 	n, err := r.R.Read(b)
-	if r.R.N > 0 {
+	if err == io.EOF && r.R.N > 0 {
 		return n, io.ErrUnexpectedEOF
 	}
 	return n, err
@@ -35,7 +35,7 @@ func (r *countingByteReader) ReadByte() (byte, error) {
 }
 
 // ParseCapsule parses the header of a Capsule.
-// It returns an io.LimitedReader that can be used to read the Capsule value.
+// It returns an io.Reader that can be used to read the Capsule value.
 // The Capsule value must be read entirely (i.e. until the io.EOF) before using r again.
 func ParseCapsule(r quicvarint.Reader) (CapsuleType, io.Reader, error) {
 	cbr := countingByteReader{ByteReader: r}
@@ -55,7 +55,7 @@ func ParseCapsule(r quicvarint.Reader) (CapsuleType, io.Reader, error) {
 		}
 		return 0, nil, err
 	}
-	return CapsuleType(ct), &exactReader{R: io.LimitReader(r, int64(l)).(*io.LimitedReader)}, nil
+	return CapsuleType(ct), &exactReader{R: io.LimitedReader{R: r, N: int64(l)}}, nil
 }
 
 // WriteCapsule writes a capsule

http3/capsule_test.go

@@ -19,9 +19,14 @@ var _ = Describe("Capsule", func() {
 		ct, r, err := ParseCapsule(bytes.NewReader(b))
 		Expect(err).ToNot(HaveOccurred())
 		Expect(ct).To(BeEquivalentTo(1337))
-		val, err := io.ReadAll(r)
+		buf := make([]byte, 3)
+		n, err := r.Read(buf)
 		Expect(err).ToNot(HaveOccurred())
-		Expect(string(val)).To(Equal("foobar"))
+		Expect(n).To(Equal(3))
+		Expect(buf).To(Equal([]byte("foo")))
+		data, err := io.ReadAll(r)
+		Expect(err).ToNot(HaveOccurred())
+		Expect(data).To(Equal([]byte("bar")))
 	})
 
 	It("writes capsules", func() {