flowcontrol: fix start of the flow-control auto-tuning period (#4730)

The period should start when the first frame is received, not when the data is
first read. This makes a difference when the first STREAM frame is received
out of order.

internal/flowcontrol/base_flow_controller.go

@@ -66,11 +66,6 @@ func (c *baseFlowController) sendWindowSize() protocol.ByteCount {
 
 // needs to be called with locked mutex
 func (c *baseFlowController) addBytesRead(n protocol.ByteCount) {
-	// pretend we sent a WindowUpdate when reading the first byte
-	// this way auto-tuning of the window size already works for the first WindowUpdate
-	if c.bytesRead == 0 {
-		c.startNewAutoTuningEpoch(time.Now())
-	}
 	c.bytesRead += n
 }
 

internal/flowcontrol/connection_flow_controller.go

@@ -46,7 +46,12 @@ func (c *connectionFlowController) IncrementHighestReceived(increment protocol.B
 	c.mutex.Lock()
 	defer c.mutex.Unlock()
 
+	// If this is the first frame received on this connection, start flow-control auto-tuning.
+	if c.highestReceived == 0 {
+		c.startNewAutoTuningEpoch(time.Now())
+	}
 	c.highestReceived += increment
+
 	if c.checkFlowControlViolation() {
 		return &qerr.TransportError{
 			ErrorCode:    qerr.FlowControlError,

internal/flowcontrol/stream_flow_controller.go

@@ -2,6 +2,7 @@ package flowcontrol
 
 import (
 	"fmt"
+	"time"
 
 	"github.com/quic-go/quic-go/internal/protocol"
 	"github.com/quic-go/quic-go/internal/qerr"
@@ -70,8 +71,7 @@ func (c *streamFlowController) UpdateHighestReceived(offset protocol.ByteCount,
 	if offset == c.highestReceived {
 		return nil
 	}
-	// A higher offset was received before.
+	// A higher offset was received before. This can happen due to reordering.
-	// This can happen due to reordering.
 	if offset <= c.highestReceived {
 		if final {
 			return &qerr.TransportError{
@@ -82,8 +82,13 @@ func (c *streamFlowController) UpdateHighestReceived(offset protocol.ByteCount,
 		return nil
 	}
 
+	// If this is the first frame received for this stream, start flow-control auto-tuning.
+	if c.highestReceived == 0 {
+		c.startNewAutoTuningEpoch(time.Now())
+	}
 	increment := offset - c.highestReceived
 	c.highestReceived = offset
+
 	if c.checkFlowControlViolation() {
 		return &qerr.TransportError{
 			ErrorCode:    qerr.FlowControlError,