[udp] make response message cache configurable

Modifies UDP conn implementation to allow for a custom response message
cache to be provided. This change is implemented in such a way that API
compatibility with previous v3 releases is preserved. Users may now
provide an option to supply their own cache implementation. Structured
messages, rather than their serialized representation, are now passed to
the cache implementation to allow for caching decisions to be made in
the cache implementation. For example, it may be desirable to skip
caching blockwise message responses if the entire underlying data being
transferred is also cached. The cache implementation is responsible for
cloning messages or otherwise ensuring that it is not storing data that
may subsequently be modified.

Signed-off-by: Daniel Mangum <georgedanielmangum@gmail.com>

udp/client/conn.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"math"
 	"net"
+	"strconv"
 	"sync"
 	"time"
 
@@ -127,6 +128,58 @@ func (m *midElement) GetMessage(cc *Conn) (*pool.Message, bool, error) {
 	return msg, true, nil
 }
 
+// MessageCache is a cache of CoAP messages.
+type MessageCache interface {
+	Load(key string, msg *pool.Message) (bool, error)
+	Store(key string, msg *pool.Message) error
+	CheckExpirations(time.Time)
+}
+
+// messageCache is a CoAP message cache backed by an in-memory cache.
+type messageCache struct {
+	c *cache.Cache[string, []byte]
+}
+
+// newMessageCache constructs a new CoAP message cache.
+func newMessageCache() *messageCache {
+	return &messageCache{
+		c: cache.NewCache[string, []byte](),
+	}
+}
+
+// Load loads a message from the cache if one exists with key.
+func (m *messageCache) Load(key string, msg *pool.Message) (bool, error) {
+	cachedResp := m.c.Load(key)
+	if cachedResp == nil {
+		return false, nil
+	}
+	if rawMsg := cachedResp.Data(); len(rawMsg) > 0 {
+		_, err := msg.UnmarshalWithDecoder(coder.DefaultCoder, rawMsg)
+		if err != nil {
+			return false, err
+		}
+		return true, nil
+	}
+	return false, nil
+}
+
+// Store stores a message in the cache.
+func (m *messageCache) Store(key string, msg *pool.Message) error {
+	marshaledResp, err := msg.MarshalWithEncoder(coder.DefaultCoder)
+	if err != nil {
+		return err
+	}
+	cacheMsg := make([]byte, len(marshaledResp))
+	copy(cacheMsg, marshaledResp)
+	m.c.LoadOrStore(key, cache.NewElement(cacheMsg, time.Now().Add(ExchangeLifetime), nil))
+	return nil
+}
+
+// CheckExpirations checks the cache for any expirations.
+func (m *messageCache) CheckExpirations(now time.Time) {
+	m.c.CheckExpirations(now)
+}
+
 // Conn represents a virtual connection to a conceptual endpoint, to perform COAPs commands.
 type Conn struct {
 	// This field needs to be the first in the struct to ensure proper word alignment on 32-bit platforms.
@@ -145,7 +198,7 @@ type Conn struct {
 
 	processReceivedMessage config.ProcessReceivedMessageFunc[*Conn]
 	errors                 ErrorFunc
-	responseMsgCache       *cache.Cache[string, []byte]
+	responseMsgCache       MessageCache
 	msgIDMutex             *MutexMap
 
 	tokenHandlerContainer *coapSync.Map[uint64, HandlerFunc]
@@ -192,6 +245,7 @@ type ConnOptions struct {
 	createBlockWise   func(cc *Conn) *blockwise.BlockWise[*Conn]
 	inactivityMonitor InactivityMonitor
 	requestMonitor    RequestMonitorFunc
+	responseMsgCache  MessageCache
 }
 
 type Option = func(opts *ConnOptions)
@@ -220,6 +274,23 @@ func WithRequestMonitor(requestMonitor RequestMonitorFunc) Option {
 	}
 }
 
+// WithResponseMessageCache sets the cache used for response messages. All
+// response messages are submitted to the cache, but it is up to the cache
+// implementation to determine which messages are stored and for how long.
+// Caching responses enables sending the same Acknowledgment for retransmitted
+// confirmable messages within an EXCHANGE_LIFETIME. It may be desirable to
+// relax this behavior in some scenarios.
+// https://datatracker.ietf.org/doc/html/rfc7252#section-4.5
+// The default response message cache stores all responses with an expiration of
+// 247 seconds, which is EXCHANGE_LIFETIME when using default CoAP transmission
+// parameters.
+// https://datatracker.ietf.org/doc/html/rfc7252#section-4.8.2
+func WithResponseMessageCache(cache MessageCache) Option {
+	return func(opts *ConnOptions) {
+		opts.responseMsgCache = cache
+	}
+}
+
 func NewConnWithOpts(session Session, cfg *Config, opts ...Option) *Conn {
 	if cfg.Errors == nil {
 		cfg.Errors = func(error) {
@@ -248,6 +319,10 @@ func NewConnWithOpts(session Session, cfg *Config, opts ...Option) *Conn {
 	for _, o := range opts {
 		o(&cfgOpts)
 	}
+	// Only construct cache if one was not set via options.
+	if cfgOpts.responseMsgCache == nil {
+		cfgOpts.responseMsgCache = newMessageCache()
+	}
 	cc := Conn{
 		session: session,
 		transmission: &Transmission{
@@ -262,7 +337,7 @@ func NewConnWithOpts(session Session, cfg *Config, opts ...Option) *Conn {
 		processReceivedMessage:    cfg.ProcessReceivedMessage,
 		errors:                    cfg.Errors,
 		msgIDMutex:                NewMutexMap(),
-		responseMsgCache:          cache.NewCache[string, []byte](),
+		responseMsgCache:          cfgOpts.responseMsgCache,
 		inactivityMonitor:         cfgOpts.inactivityMonitor,
 		requestMonitor:            cfgOpts.requestMonitor,
 		messagePool:               cfg.MessagePool,
@@ -609,34 +684,14 @@ func (cc *Conn) Sequence() uint64 {
 	return cc.sequence.Add(1)
 }
 
-func (cc *Conn) responseMsgCacheID(msgID int32) string {
-	return fmt.Sprintf("resp-%v-%d", cc.RemoteAddr(), msgID)
+// getResponseFromCache gets a message from the response message cache.
+func (cc *Conn) getResponseFromCache(mid int32, resp *pool.Message) (bool, error) {
+	return cc.responseMsgCache.Load(strconv.Itoa(int(mid)), resp)
 }
 
+// addResponseToCache adds a message to the response message cache.
 func (cc *Conn) addResponseToCache(resp *pool.Message) error {
-	marshaledResp, err := resp.MarshalWithEncoder(coder.DefaultCoder)
-	if err != nil {
-		return err
-	}
-	cacheMsg := make([]byte, len(marshaledResp))
-	copy(cacheMsg, marshaledResp)
-	cc.responseMsgCache.LoadOrStore(cc.responseMsgCacheID(resp.MessageID()), cache.NewElement(cacheMsg, time.Now().Add(ExchangeLifetime), nil))
-	return nil
-}
-
-func (cc *Conn) getResponseFromCache(mid int32, resp *pool.Message) (bool, error) {
-	cachedResp := cc.responseMsgCache.Load(cc.responseMsgCacheID(mid))
-	if cachedResp == nil {
-		return false, nil
-	}
-	if rawMsg := cachedResp.Data(); len(rawMsg) > 0 {
-		_, err := resp.UnmarshalWithDecoder(coder.DefaultCoder, rawMsg)
-		if err != nil {
-			return false, err
-		}
-		return true, nil
-	}
-	return false, nil
+	return cc.responseMsgCache.Store(strconv.Itoa(int(resp.MessageID())), resp)
 }
 
 // checkMyMessageID compare client msgID against peer messageID and if it is near < 0xffff/4 then increase msgID.