Repository: pion/ice (go)

Issue:
Allow users to pass custom 'CandidateSelectorControlling` and `CandidateSelectorControlled`
We have had two asks for custom ICE selection logic.

@lisay-yan in https://github.com/pion/webrtc/pull/2539 wants to effectively disable ICE. Just take the best candidate right away

@gbfarah wants to always take whatever the latest traffic coming in is in https://github.com/pion/webrtc/issues/2585

These are both greats ideas. However I am worried about the unintended side effects. I don't want to accidentally add a security/interoperability issue into Pion. I would like to support everyones use case though!

To do this I think we should make `CandidatePairSelector` a public interface (it is private now)

```
type CandidatePairSelector interface {
    Start(agent CandidatePairSelectorAgent)
    ContactCandidates(agent CandidatePairSelectorAgent)
    PingCandidate(agent CandidatePairSelectorAgent, local, remote Candidate)
    HandleSuccessResponse(agent CandidatePairSelectorAgent, m *stun.Message, local, remote Candidate, remoteAddr net.Addr)
    HandleBindingRequest(agent CandidatePairSelectorAgent, m *stun.Message, local, remote Candidate)
}
```

We will also create a new interface. This will allow us to selectively expose private APIs that users will need. We can add things as needed.

```
type CandidatePairSelectorAgent interface {
 GetSelectedPair()
 GetBestAvailableCandidatePair()
}
```

Users via `AgentConfig` can then pass in their `CandidatePairSelectorControlling` and `CandidatePairSelectorControlled`

Pull request description:
Add BindingRequestHandler
Allow the user to perform custom processing for inbound STUN Binding requests. This allows users to do some of the following

* Log incoming Binding Requests for debugging
* Implement draft-thatcher-ice-renomination
* Implement custom CandidatePair switching logic

Resolves pion/webrtc#2539
Resolves pion/webrtc#2585
Resolves #623

Patch:
diff --git a/agent.go b/agent.go
index 144f9c2..761c07c 100644
--- a/agent.go
+++ b/agent.go
@@ -119,6 +119,10 @@ type Agent struct {
 	// 1:1 D-NAT IP address mapping
 	extIPMapper *externalIPMapper
 
+	// Callback that allows user to implement custom behavior
+	// for STUN Binding Requests
+	userBindingRequestHandler func(m *stun.Message, local, remote Candidate, pair *CandidatePair) bool
+
 	gatherCandidateCancel func()
 	gatherCandidateDone   chan struct{}
 
@@ -213,6 +217,8 @@ func NewAgent(config *AgentConfig) (*Agent, error) { //nolint:gocognit
 		includeLoopback: config.IncludeLoopback,
 
 		disableActiveTCP: config.DisableActiveTCP,
+
+		userBindingRequestHandler: config.BindingRequestHandler,
 	}
 	a.connectionStateNotifier = &handlerNotifier{connectionStateFunc: a.onConnectionStateChange}
 	a.candidateNotifier = &handlerNotifier{candidateFunc: a.onCandidate}
diff --git a/agent_config.go b/agent_config.go
index 5edbea4..4c87fa1 100644
--- a/agent_config.go
+++ b/agent_config.go
@@ -193,6 +193,13 @@ type AgentConfig struct {
 	// DisableActiveTCP can be used to disable Active TCP candidates. Otherwise when TCP is enabled
 	// Active TCP candidates will be created when a new passive TCP remote candidate is added.
 	DisableActiveTCP bool
+
+	// BindingRequestHandler allows applications to perform logic on incoming STUN Binding Requests
+	// This was implemented to allow users to
+	// * Log incoming Binding Requests for debugging
+	// * Implement draft-thatcher-ice-renomination
+	// * Implement custom CandidatePair switching logic
+	BindingRequestHandler func(m *stun.Message, local, remote Candidate, pair *CandidatePair) bool
 }
 
 // initWithDefaults populates an agent and falls back to defaults if fields are unset
diff --git a/selection.go b/selection.go
index 55ad20b..09a0988 100644
--- a/selection.go
+++ b/selection.go
@@ -111,6 +111,12 @@ func (s *controllingSelector) HandleBindingRequest(m *stun.Message, local, remot
 			s.nominatePair(p)
 		}
 	}
+
+	if s.agent.userBindingRequestHandler != nil {
+		if shouldSwitch := s.agent.userBindingRequestHandler(m, local, remote, p); shouldSwitch {
+			s.agent.setSelectedPair(p)
+		}
+	}
 }
 
 func (s *controllingSelector) HandleSuccessResponse(m *stun.Message, local, remote Candidate, remoteAddr net.Addr) {
@@ -242,14 +248,12 @@ func (s *controlledSelector) HandleSuccessResponse(m *stun.Message, local, remot
 }
 
 func (s *controlledSelector) HandleBindingRequest(m *stun.Message, local, remote Candidate) {
-	useCandidate := m.Contains(stun.AttrUseCandidate)
-
 	p := s.agent.findPair(local, remote)
 	if p == nil {
 		p = s.agent.addPair(local, remote)
 	}
 
-	if useCandidate {
+	if m.Contains(stun.AttrUseCandidate) {
 		// https://tools.ietf.org/html/rfc8445#section-7.3.1.5
 
 		if p.state == CandidatePairStateSucceeded {
@@ -257,8 +261,8 @@ func (s *controlledSelector) HandleBindingRequest(m *stun.Message, local, remote
 			// previously sent by this pair produced a successful response and
 			// generated a valid pair (Section 7.2.5.3.2).  The agent sets the
 			// nominated flag value of the valid pair to true.
-			if selectedPair := s.agent.getSelectedPair(); selectedPair == nil ||
-				(selectedPair != p && selectedPair.priority() <= p.priority()) {
+			selectedPair := s.agent.getSelectedPair()
+			if selectedPair == nil || (selectedPair != p && selectedPair.priority() <= p.priority()) {
 				s.agent.setSelectedPair(p)
 			} else if selectedPair != p {
 				s.log.Tracef("Ignore nominate new pair %s, already nominated pair %s", p, selectedPair)
@@ -278,6 +282,12 @@ func (s *controlledSelector) HandleBindingRequest(m *stun.Message, local, remote
 
 	s.agent.sendBindingSuccess(m, local, remote)
 	s.PingCandidate(local, remote)
+
+	if s.agent.userBindingRequestHandler != nil {
+		if shouldSwitch := s.agent.userBindingRequestHandler(m, local, remote, p); shouldSwitch {
+			s.agent.setSelectedPair(p)
+		}
+	}
 }
 
 type liteSelector struct {


Test patch:
diff --git a/selection_test.go b/selection_test.go
new file mode 100644
index 0000000..a5e17fc
--- /dev/null
+++ b/selection_test.go
@@ -0,0 +1,156 @@
+// SPDX-FileCopyrightText: 2023 The Pion community <https://pion.ly>
+// SPDX-License-Identifier: MIT
+
+//go:build !js
+// +build !js
+
+package ice
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"io"
+	"net"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/pion/stun/v2"
+	"github.com/pion/transport/v3/test"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func sendUntilDone(t *testing.T, writingConn, readingConn net.Conn, maxAttempts int) bool {
+	testMessage := []byte("Hello World")
+	testBuffer := make([]byte, len(testMessage))
+
+	readDone, readDoneCancel := context.WithCancel(context.Background())
+	go func() {
+		_, err := readingConn.Read(testBuffer)
+		if errors.Is(err, io.EOF) {
+			return
+		}
+
+		require.NoError(t, err)
+		require.True(t, bytes.Equal(testMessage, testBuffer))
+
+		readDoneCancel()
+	}()
+
+	attempts := 0
+	for {
+		select {
+		case <-time.After(5 * time.Millisecond):
+			if attempts > maxAttempts {
+				return false
+			}
+
+			_, err := writingConn.Write(testMessage)
+			require.NoError(t, err)
+			attempts++
+		case <-readDone.Done():
+			return true
+		}
+	}
+}
+
+func TestBindingRequestHandler(t *testing.T) {
+	defer test.CheckRoutines(t)()
+	defer test.TimeOut(time.Second * 30).Stop()
+
+	var switchToNewCandidatePair, controlledLoggingFired atomic.Value
+	oneHour := time.Hour
+	keepaliveInterval := time.Millisecond * 20
+
+	aNotifier, aConnected := onConnected()
+	bNotifier, bConnected := onConnected()
+	controllingAgent, err := NewAgent(&AgentConfig{
+		NetworkTypes:      []NetworkType{NetworkTypeUDP4, NetworkTypeUDP6},
+		MulticastDNSMode:  MulticastDNSModeDisabled,
+		KeepaliveInterval: &keepaliveInterval,
+		CheckInterval:     &oneHour,
+		BindingRequestHandler: func(_ *stun.Message, _, _ Candidate, _ *CandidatePair) bool {
+			controlledLoggingFired.Store(true)
+			return false
+		},
+	})
+	require.NoError(t, err)
+	require.NoError(t, controllingAgent.OnConnectionStateChange(aNotifier))
+
+	controlledAgent, err := NewAgent(&AgentConfig{
+		NetworkTypes:      []NetworkType{NetworkTypeUDP4},
+		MulticastDNSMode:  MulticastDNSModeDisabled,
+		KeepaliveInterval: &keepaliveInterval,
+		CheckInterval:     &oneHour,
+		BindingRequestHandler: func(_ *stun.Message, _, _ Candidate, _ *CandidatePair) bool {
+			// Don't switch candidate pair until we are ready
+			val, ok := switchToNewCandidatePair.Load().(bool)
+			return ok && val
+		},
+	})
+	require.NoError(t, err)
+	require.NoError(t, controlledAgent.OnConnectionStateChange(bNotifier))
+
+	controlledConn, controllingConn := connect(controlledAgent, controllingAgent)
+	<-aConnected
+	<-bConnected
+
+	// Assert we have connected and can send data
+	require.True(t, sendUntilDone(t, controlledConn, controllingConn, 100))
+
+	// Take the lock on the controlling Agent and unset state
+	assert.NoError(t, controlledAgent.loop.Run(controlledAgent.loop, func(_ context.Context) {
+		for net, cs := range controlledAgent.remoteCandidates {
+			for _, c := range cs {
+				require.NoError(t, c.close())
+			}
+			delete(controlledAgent.remoteCandidates, net)
+		}
+
+		for _, c := range controlledAgent.localCandidates[NetworkTypeUDP4] {
+			cast, ok := c.(*CandidateHost)
+			require.True(t, ok)
+			cast.remoteCandidateCaches = map[AddrPort]Candidate{}
+		}
+
+		controlledAgent.setSelectedPair(nil)
+		controlledAgent.checklist = make([]*CandidatePair, 0)
+	}))
+
+	// Assert that Selected Candidate pair has only been unset on Controlled side
+	candidatePair, err := controlledAgent.GetSelectedCandidatePair()
+	assert.Nil(t, candidatePair)
+	assert.NoError(t, err)
+
+	candidatePair, err = controllingAgent.GetSelectedCandidatePair()
+	assert.NotNil(t, candidatePair)
+	assert.NoError(t, err)
+
+	// Sending will fail, we no longer have a selected candidate pair
+	require.False(t, sendUntilDone(t, controlledConn, controllingConn, 20))
+
+	// Send STUN Binding requests until a new Selected Candidate Pair has been set by BindingRequestHandler
+	switchToNewCandidatePair.Store(true)
+	for {
+		controllingAgent.requestConnectivityCheck()
+
+		candidatePair, err = controlledAgent.GetSelectedCandidatePair()
+		require.NoError(t, err)
+		if candidatePair != nil {
+			break
+		}
+
+		time.Sleep(time.Millisecond * 5)
+	}
+
+	// We have a new selected candidate pair because of BindingRequestHandler, test that it works
+	require.True(t, sendUntilDone(t, controllingConn, controlledConn, 100))
+
+	fired, ok := controlledLoggingFired.Load().(bool)
+	require.True(t, ok)
+	require.True(t, fired)
+
+	closePipe(t, controllingConn, controlledConn)
+}