rpc: remove NotifierContextKey

Context keys must have a unique type in order to prevent
any unintented clashes. The code used int(1) as key.

Fix it by implementing the pattern recommended by package context.

eth/api.go

@@ -603,7 +603,7 @@ type NewBlocksArgs struct {
 // NewBlocks triggers a new block event each time a block is appended to the chain. It accepts an argument which allows
 // the caller to specify whether the output should contain transactions and in what format.
 func (s *PublicBlockChainAPI) NewBlocks(ctx context.Context, args NewBlocksArgs) (rpc.Subscription, error) {
-	notifier, supported := ctx.Value(rpc.NotifierContextKey).(rpc.Notifier)
+	notifier, supported := rpc.NotifierFromContext(ctx)
 	if !supported {
 		return nil, rpc.ErrNotificationsUnsupported
 	}
@@ -1345,7 +1345,7 @@ func (s *PublicTransactionPoolAPI) PendingTransactions() []*RPCTransaction {
 // NewPendingTransaction creates a subscription that is triggered each time a transaction enters the transaction pool
 // and is send from one of the transactions this nodes manages.
 func (s *PublicTransactionPoolAPI) NewPendingTransactions(ctx context.Context) (rpc.Subscription, error) {
-	notifier, supported := ctx.Value(rpc.NotifierContextKey).(rpc.Notifier)
+	notifier, supported := rpc.NotifierFromContext(ctx)
 	if !supported {
 		return nil, rpc.ErrNotificationsUnsupported
 	}

eth/downloader/api.go

@@ -85,7 +85,7 @@ type SyncingResult struct {
 
 // Syncing provides information when this nodes starts synchronising with the Ethereum network and when it's finished.
 func (api *PublicDownloaderAPI) Syncing(ctx context.Context) (rpc.Subscription, error) {
-	notifier, supported := ctx.Value(rpc.NotifierContextKey).(rpc.Notifier)
+	notifier, supported := rpc.NotifierFromContext(ctx)
 	if !supported {
 		return nil, rpc.ErrNotificationsUnsupported
 	}

eth/filters/api.go

@@ -234,7 +234,7 @@ func (s *PublicFilterAPI) newLogFilter(earliest, latest int64, addresses []commo
 }
 
 func (s *PublicFilterAPI) Logs(ctx context.Context, args NewFilterArgs) (rpc.Subscription, error) {
-	notifier, supported := ctx.Value(rpc.NotifierContextKey).(rpc.Notifier)
+	notifier, supported := rpc.NotifierFromContext(ctx)
 	if !supported {
 		return nil, rpc.ErrNotificationsUnsupported
 	}

rpc/notification.go

@@ -23,6 +23,7 @@ import (
 
 	"github.com/ethereum/go-ethereum/logger"
 	"github.com/ethereum/go-ethereum/logger/glog"
+	"golang.org/x/net/context"
 )
 
 var (
@@ -62,6 +63,14 @@ type Notifier interface {
 	Unsubscribe(id string) error
 }
 
+type notifierKey struct{}
+
+// NotifierFromContext returns the Notifier value stored in ctx, if any.
+func NotifierFromContext(ctx context.Context) (Notifier, bool) {
+	n, ok := ctx.Value(notifierKey{}).(Notifier)
+	return n, ok
+}
+
 // Subscription defines the interface for objects that can notify subscribers
 type Subscription interface {
 	// Inform client of an event

rpc/notification_test.go

@@ -36,7 +36,7 @@ func (s *NotificationTestService) Unsubscribe(subid string) {
 }
 
 func (s *NotificationTestService) SomeSubscription(ctx context.Context, n, val int) (Subscription, error) {
-	notifier, supported := ctx.Value(NotifierContextKey).(Notifier)
+	notifier, supported := NotifierFromContext(ctx)
 	if !supported {
 		return nil, ErrNotificationsUnsupported
 	}

rpc/server.go

@@ -32,9 +32,6 @@ import (
 const (
 	stopPendingRequestTimeout = 3 * time.Second // give pending requests stopPendingRequestTimeout the time to finish when the server is stopped
 
-	// NotifierContextKey is the key where the notifier associated with the codec is stored in the context
-	NotifierContextKey = 1
-
 	notificationBufferSize = 10000 // max buffered notifications before codec is closed
 
 	DefaultIPCApis  = "admin,eth,debug,miner,net,shh,txpool,personal,web3"
@@ -171,7 +168,7 @@ func (s *Server) serveRequest(codec ServerCodec, singleShot bool, options CodecO
 	// to send notification to clients. It is thight to the codec/connection. If the
 	// connection is closed the notifier will stop and cancels all active subscriptions.
 	if options&OptionSubscriptions == OptionSubscriptions {
-		ctx = context.WithValue(ctx, NotifierContextKey, newBufferedNotifier(codec, notificationBufferSize))
+		ctx = context.WithValue(ctx, notifierKey{}, newBufferedNotifier(codec, notificationBufferSize))
 	}
 	s.codecsMu.Lock()
 	if atomic.LoadInt32(&s.run) != 1 { // server stopped
@@ -275,7 +272,7 @@ func (s *Server) handle(ctx context.Context, codec ServerCodec, req *serverReque
 
 	if req.isUnsubscribe { // cancel subscription, first param must be the subscription id
 		if len(req.args) >= 1 && req.args[0].Kind() == reflect.String {
-			notifier, supported := ctx.Value(NotifierContextKey).(*bufferedNotifier)
+			notifier, supported := NotifierFromContext(ctx)
 			if !supported { // interface doesn't support subscriptions (e.g. http)
 				return codec.CreateErrorResponse(&req.id, &callbackError{ErrNotificationsUnsupported.Error()}), nil
 			}
@@ -298,8 +295,8 @@ func (s *Server) handle(ctx context.Context, codec ServerCodec, req *serverReque
 
 		// active the subscription after the sub id was successful sent to the client
 		activateSub := func() {
-			notifier, _ := ctx.Value(NotifierContextKey).(*bufferedNotifier)
+			notifier, _ := NotifierFromContext(ctx)
-			notifier.activate(subid)
+			notifier.(*bufferedNotifier).activate(subid)
 		}
 
 		return codec.CreateResponse(req.id, subid), activateSub