introduce a StreamLimitReachedError for Connection.Open{Uni}Stream (#4579)

Using a concrete type is preferable to relying on interpreting the
return values from net.Error.Timeout and net.Error.Temporary, especially
since the latter has been deprecated since Go 1.18.

interface.go

@@ -150,7 +150,7 @@ type SendStream interface {
 // * TransportError: for errors triggered by the QUIC transport (in many cases a misbehaving peer)
 // * IdleTimeoutError: when the peer goes away unexpectedly (this is a net.Error timeout error)
 // * HandshakeTimeoutError: when the cryptographic handshake takes too long (this is a net.Error timeout error)
-// * StatelessResetError: when we receive a stateless reset (this is a net.Error temporary error)
+// * StatelessResetError: when we receive a stateless reset
 // * VersionNegotiationError: returned by the client, when there's no version overlap between the peers
 type Connection interface {
 	// AcceptStream returns the next stream opened by the peer, blocking until one is available.
@@ -163,28 +163,29 @@ type Connection interface {
 	AcceptUniStream(context.Context) (ReceiveStream, error)
 	// OpenStream opens a new bidirectional QUIC stream.
 	// There is no signaling to the peer about new streams:
-	// The peer can only accept the stream after data has been sent on the stream.
-	// If the error is non-nil, it satisfies the net.Error interface.
-	// When reaching the peer's stream limit, err.Temporary() will be true.
-	// If the connection was closed due to a timeout, Timeout() will be true.
+	// The peer can only accept the stream after data has been sent on the stream,
+	// or the stream has been reset or closed.
+	// When reaching the peer's stream limit, it is not possible to open a new stream until the
+	// peer raises the stream limit. In that case, a StreamLimitReachedError is returned.
 	OpenStream() (Stream, error)
 	// OpenStreamSync opens a new bidirectional QUIC stream.
 	// It blocks until a new stream can be opened.
 	// There is no signaling to the peer about new streams:
 	// The peer can only accept the stream after data has been sent on the stream,
 	// or the stream has been reset or closed.
-	// If the error is non-nil, it satisfies the net.Error interface.
-	// If the connection was closed due to a timeout, Timeout() will be true.
 	OpenStreamSync(context.Context) (Stream, error)
 	// OpenUniStream opens a new outgoing unidirectional QUIC stream.
-	// If the error is non-nil, it satisfies the net.Error interface.
-	// When reaching the peer's stream limit, Temporary() will be true.
-	// If the connection was closed due to a timeout, Timeout() will be true.
+	// There is no signaling to the peer about new streams:
+	// The peer can only accept the stream after data has been sent on the stream,
+	// or the stream has been reset or closed.
+	// When reaching the peer's stream limit, it is not possible to open a new stream until the
+	// peer raises the stream limit. In that case, a StreamLimitReachedError is returned.
 	OpenUniStream() (SendStream, error)
 	// OpenUniStreamSync opens a new outgoing unidirectional QUIC stream.
 	// It blocks until a new stream can be opened.
-	// If the error is non-nil, it satisfies the net.Error interface.
-	// If the connection was closed due to a timeout, Timeout() will be true.
+	// There is no signaling to the peer about new streams:
+	// The peer can only accept the stream after data has been sent on the stream,
+	// or the stream has been reset or closed.
 	OpenUniStreamSync(context.Context) (SendStream, error)
 	// LocalAddr returns the local address.
 	LocalAddr() net.Addr

streams_map.go

@@ -38,11 +38,21 @@ type streamOpenErr struct{ error }
 
 var _ net.Error = &streamOpenErr{}
 
-func (e streamOpenErr) Temporary() bool { return e.error == errTooManyOpenStreams }
 func (streamOpenErr) Timeout() bool   { return false }
+func (e streamOpenErr) Unwrap() error { return e.error }
 
-// errTooManyOpenStreams is used internally by the outgoing streams maps.
-var errTooManyOpenStreams = errors.New("too many open streams")
+func (e streamOpenErr) Temporary() bool {
+	// In older versions of quic-go, the stream limit error was documented to be a net.Error.Temporary.
+	// This function was since deprecated, but we keep the existing behavior.
+	return errors.Is(e, &StreamLimitReachedError{})
+}
+
+// StreamLimitReachedError is returned from Connection.OpenStream and Connection.OpenUniStream
+// when it is not possible to open a new stream because the number of opens streams reached
+// the peer's stream limit.
+type StreamLimitReachedError struct{}
+
+func (e StreamLimitReachedError) Error() string { return "too many open streams" }
 
 type streamsMap struct {
 	ctx         context.Context // not used for cancellations, but carries the values associated with the connection

streams_map_outgoing.go

@@ -60,7 +60,7 @@ func (m *outgoingStreamsMap[T]) OpenStream() (T, error) {
 	// if there are OpenStreamSync calls waiting, return an error here
 	if len(m.openQueue) > 0 || m.nextStream > m.maxStream {
 		m.maybeSendBlockedFrame()
-		return *new(T), streamOpenErr{errTooManyOpenStreams}
+		return *new(T), streamOpenErr{&StreamLimitReachedError{}}
 	}
 	return m.openStream(), nil
 }

streams_map_outgoing_test.go

@@ -361,8 +361,7 @@ var _ = Describe("Streams Map (outgoing)", func() {
 				Expect(bf.StreamLimit).To(BeEquivalentTo(6))
 			})
 			_, err := m.OpenStream()
-			Expect(err).To(HaveOccurred())
-			Expect(err.Error()).To(Equal(errTooManyOpenStreams.Error()))
+			Expect(err).To(MatchError(&StreamLimitReachedError{}))
 		})
 
 		It("only sends one STREAMS_BLOCKED frame for one stream ID", func() {
@@ -452,8 +451,7 @@ var _ = Describe("Streams Map (outgoing)", func() {
 				}
 				str, err := m.OpenStream()
 				if limit <= n {
-					Expect(err).To(HaveOccurred())
-					Expect(err.Error()).To(Equal(errTooManyOpenStreams.Error()))
+					Expect(err).To(MatchError(&StreamLimitReachedError{}))
 				} else {
 					Expect(str.num).To(Equal(protocol.StreamNum(n + 1)))
 				}

streams_map_test.go

@@ -33,11 +33,14 @@ type streamMapping struct {
 }
 
 func expectTooManyStreamsError(err error) {
-	ExpectWithOffset(1, err).To(HaveOccurred())
-	ExpectWithOffset(1, err.Error()).To(Equal(errTooManyOpenStreams.Error()))
+	ExpectWithOffset(1, err).To(MatchError(&StreamLimitReachedError{}))
 	nerr, ok := err.(net.Error)
 	ExpectWithOffset(1, ok).To(BeTrue())
 	ExpectWithOffset(1, nerr.Timeout()).To(BeFalse())
+	//nolint:staticcheck // SA1019
+	// In older versions of quic-go, the stream limit error was documented to be a net.Error.Temporary.
+	// This function was since deprecated, but we keep the existing behavior.
+	ExpectWithOffset(1, nerr.Temporary()).To(BeTrue())
 }
 
 var _ = Describe("Streams Map", func() {