Fixes a SharedDirectoryAnnounce incompatibility (#25931)

* Fixes a SharedDirectoryAnnounce incompatibility

The SharedDirectoryAnnounce message was changed to remove an unnecessary
completionID field, however this change caused an incompatibility between
the proxy and previous versions of the wds. This commit reverts that specific
change and adds notes to the code explaining the situation.

The original change is here: https://github.com/gravitational/teleport/pull/25260/files#diff-98a4bee57beb7f007614e4810d2cf8413bddf48484c8aad6dd0756a218797c36R677-R678

* changes completionId to discard

* adds discard bit to sda.Encode to make fuzz tests happy

lib/srv/desktop/tdp/proto.go

@@ -667,6 +667,8 @@ type SharedDirectoryAnnounce struct {
 func (s SharedDirectoryAnnounce) Encode() ([]byte, error) {
 	buf := new(bytes.Buffer)
 	buf.WriteByte(byte(TypeSharedDirectoryAnnounce))
+	// TODO(isaiah): The discard here allows fuzz tests to succeed, it should eventually be done away with.
+	writeUint32(buf, 0) // discard
 	writeUint32(buf, s.DirectoryID)
 	if err := encodeString(buf, s.Name); err != nil {
 		return nil, trace.Wrap(err)
@@ -675,8 +677,16 @@ func (s SharedDirectoryAnnounce) Encode() ([]byte, error) {
 }
 
 func decodeSharedDirectoryAnnounce(in io.Reader) (SharedDirectoryAnnounce, error) {
+	// TODO(isaiah): The discard here is a copy-paste error, but we need to keep it
+	// for now in order that the proxy stay compatible with previous versions of the wds.
+	var discard uint32
+	err := binary.Read(in, binary.BigEndian, &discard)
+	if err != nil {
+		return SharedDirectoryAnnounce{}, trace.Wrap(err)
+	}
+
 	var directoryID uint32
-	err := binary.Read(in, binary.BigEndian, &directoryID)
+	err = binary.Read(in, binary.BigEndian, &directoryID)
 	if err != nil {
 		return SharedDirectoryAnnounce{}, trace.Wrap(err)
 	}

web/packages/teleport/src/lib/tdp/client.ts

@@ -513,7 +513,7 @@ export default class Client extends EventEmitterWebAuthnSender {
       name = this.sdManager.getName();
       this.send(
         this.codec.encodeSharedDirectoryAnnounce({
-          completionId: 0, // This is always the first request.
+          discard: 0, // This is always the first request.
           // Hardcode directoryId for now since we only support sharing 1 directory.
           // We're using 2 because the smartcard device is hardcoded to 1 in the backend.
           directoryId: 2,

web/packages/teleport/src/lib/tdp/codec.ts

@@ -132,8 +132,10 @@ export type MfaJson = {
 };
 
 // | message type (11) | completion_id uint32 | directory_id uint32 | name_length uint32 | name []byte |
+// TODO(isaiah): The discard here is a copy-paste error, but we need to keep it
+// for now in order that the proxy stay compatible with previous versions of the wds.
 export type SharedDirectoryAnnounce = {
-  completionId: number;
+  discard: number;
   directoryId: number;
   name: string;
 };
@@ -601,7 +603,9 @@ export default class Codec {
     let offset = 0;
 
     view.setUint8(offset++, MessageType.SHARED_DIRECTORY_ANNOUNCE);
-    view.setUint32(offset, sharedDirAnnounce.completionId);
+    // TODO(isaiah): The discard here is a copy-paste error, but we need to keep it
+    // for now in order that the proxy stay compatible with previous versions of the wds.
+    view.setUint32(offset, sharedDirAnnounce.discard);
     offset += uint32Length;
     view.setUint32(offset, sharedDirAnnounce.directoryId);
     offset += uint32Length;