add a AllowConnectionWindowIncrease config option

config.go

@@ -110,6 +110,7 @@ func populateConfig(config *Config) *Config {
 		MaxStreamReceiveWindow:           maxStreamReceiveWindow,
 		InitialConnectionReceiveWindow:   initialConnectionReceiveWindow,
 		MaxConnectionReceiveWindow:       maxConnectionReceiveWindow,
+		AllowConnectionWindowIncrease:    config.AllowConnectionWindowIncrease,
 		MaxIncomingStreams:               maxIncomingStreams,
 		MaxIncomingUniStreams:            maxIncomingUniStreams,
 		ConnectionIDLength:               config.ConnectionIDLength,

config_test.go

@@ -45,7 +45,7 @@ var _ = Describe("Config", func() {
 			}
 
 			switch fn := typ.Field(i).Name; fn {
-			case "AcceptToken", "GetLogWriter":
+			case "AcceptToken", "GetLogWriter", "AllowConnectionWindowIncrease":
 				// Can't compare functions.
 			case "Versions":
 				f.Set(reflect.ValueOf([]VersionNumber{1, 2, 3}))
@@ -100,13 +100,16 @@ var _ = Describe("Config", func() {
 
 	Context("cloning", func() {
 		It("clones function fields", func() {
-			var calledAcceptToken bool
+			var calledAcceptToken, calledAllowConnectionWindowIncrease bool
 			c1 := &Config{
-				AcceptToken: func(_ net.Addr, _ *Token) bool { calledAcceptToken = true; return true },
+				AcceptToken:                   func(_ net.Addr, _ *Token) bool { calledAcceptToken = true; return true },
+				AllowConnectionWindowIncrease: func(Session, int) bool { calledAllowConnectionWindowIncrease = true; return true },
 			}
 			c2 := c1.Clone()
 			c2.AcceptToken(&net.UDPAddr{}, &Token{})
 			Expect(calledAcceptToken).To(BeTrue())
+			c2.AllowConnectionWindowIncrease(nil, 1234)
+			Expect(calledAllowConnectionWindowIncrease).To(BeTrue())
 		})
 
 		It("clones non-function fields", func() {

interface.go

@@ -266,6 +266,11 @@ type Config struct {
 	// MaxConnectionReceiveWindow is the connection-level flow control window for receiving data.
 	// If this value is zero, it will default to 15 MB.
 	MaxConnectionReceiveWindow uint64
+	// AllowConnectionWindowIncrease is called every time the connection flow controller attempts
+	// to increase the connection flow control window.
+	// If set, the caller can prevent an increase of the window. Typically, it would do so to
+	// limit the memory usage.
+	AllowConnectionWindowIncrease func(sess Session, delta int) bool
 	// MaxIncomingStreams is the maximum number of concurrent bidirectional streams that a peer is allowed to open.
 	// Values above 2^60 are invalid.
 	// If not set, it will default to 100.

session.go

@@ -506,7 +506,12 @@ func (s *session) preSetup() {
 		protocol.ByteCount(s.config.InitialConnectionReceiveWindow),
 		protocol.ByteCount(s.config.MaxConnectionReceiveWindow),
 		s.onHasConnectionWindowUpdate,
-		func(protocol.ByteCount) bool { return true },
+		func(size protocol.ByteCount) bool {
+			if s.config.AllowConnectionWindowIncrease == nil {
+				return true
+			}
+			return s.config.AllowConnectionWindowIncrease(s, int(size))
+		},
 		s.rttStats,
 		s.logger,
 	)