improvement(client-container-loader): tighten Signal expectations

Signals specify `type` within message `content` next to inner content that is stringified JSON.
Assert that internally (to avoid legacy API change) and leverage. Additionally, clean up system signal typing and type guard to reduce casts.

Author: jason-ha

packages/loader/container-loader/src/connectionManager.ts

@@ -14,6 +14,8 @@ import {
 	type ITelemetryBaseProperties,
 	LogLevel,
 } from "@fluidframework/core-interfaces";
+import type { JsonString } from "@fluidframework/core-interfaces/internal";
+import { JsonStringify } from "@fluidframework/core-interfaces/internal";
 import { assert } from "@fluidframework/core-utils/internal";
 import type {
 	ConnectionMode,
@@ -197,6 +199,19 @@ interface IPendingConnection {
 	connectionMode: ConnectionMode;
 }
 
+function assertExpectedSignals(
+	signals: ISignalMessage[],
+): asserts signals is ISignalMessage<{ type: never; content: JsonString<unknown> }>[] {
+	for (const signal of signals) {
+		if ("type" in signal) {
+			throw new Error("Unexpected type in ISignalMessage");
+		}
+		if (typeof signal.content !== "string") {
+			throw new TypeError("Non-string content in ISignalMessage");
+		}
+	}
+}
+
 /**
  * Implementation of IConnectionManager, used by Container class
  * Implements constant connectivity to relay service, by reconnecting in case of lost connection or error.
@@ -1003,29 +1018,29 @@ export class ConnectionManager implements IConnectionManager {
 		// Synthesize clear & join signals out of initialClients state.
 		// This allows us to have single way to process signals, and makes it simpler to initialize
 		// protocol in Container.
-		const clearSignal: ISignalMessage = {
+		const clearSignal = {
 			// API uses null
 			// eslint-disable-next-line unicorn/no-null
 			clientId: null, // system message
-			content: JSON.stringify({
+			content: JsonStringify({
 				type: SignalType.Clear,
 			}),
 		};
 
 		// list of signals to process due to this new connection
-		let signalsToProcess: ISignalMessage[] = [clearSignal];
-
-		const clientJoinSignals: ISignalMessage[] = (connection.initialClients ?? []).map(
-			(priorClient) => ({
-				// API uses null
-				// eslint-disable-next-line unicorn/no-null
-				clientId: null, // system signal
-				content: JSON.stringify({
-					type: SignalType.ClientJoin,
-					content: priorClient, // ISignalClient
-				}),
+		let signalsToProcess: ISignalMessage<{ type: never; content: JsonString<unknown> }>[] = [
+			clearSignal,
+		];
+
+		const clientJoinSignals = (connection.initialClients ?? []).map((priorClient) => ({
+			// API uses null
+			// eslint-disable-next-line unicorn/no-null
+			clientId: null, // system signal
+			content: JsonStringify({
+				type: SignalType.ClientJoin,
+				content: priorClient, // ISignalClient
 			}),
-		);
+		}));
 		if (clientJoinSignals.length > 0) {
 			signalsToProcess = [...signalsToProcess, ...clientJoinSignals];
 		}
@@ -1035,6 +1050,7 @@ export class ConnectionManager implements IConnectionManager {
 		// for "self" and connection.initialClients does not contain "self", so we have to process them after
 		// "clear" signal above.
 		if (connection.initialSignals !== undefined && connection.initialSignals.length > 0) {
+			assertExpectedSignals(connection.initialSignals);
 			signalsToProcess = [...signalsToProcess, ...connection.initialSignals];
 		}
 
@@ -1261,6 +1277,7 @@ export class ConnectionManager implements IConnectionManager {
 
 	private readonly signalHandler = (signalsArg: ISignalMessage | ISignalMessage[]): void => {
 		const signals = Array.isArray(signalsArg) ? signalsArg : [signalsArg];
+		assertExpectedSignals(signals);
 		this.props.signalHandler(signals);
 	};
 

packages/loader/container-loader/src/container.ts

@@ -140,9 +140,10 @@ import { NoopHeuristic } from "./noopHeuristic.js";
 import { pkgVersion } from "./packageVersion.js";
 import type { IQuorumSnapshot } from "./protocol/index.js";
 import {
-	type IProtocolHandler,
+	type InternalProtocolHandlerBuilder,
 	ProtocolHandler,
 	type ProtocolHandlerBuilder,
+	type ProtocolHandlerInternal,
 	protocolHandlerShouldProcessSignal,
 } from "./protocol.js";
 import { initQuorumValuesFromCodeDetails } from "./quorum.js";
@@ -495,7 +496,7 @@ export class Container
 	private readonly scope: FluidObject;
 	private readonly subLogger: ITelemetryLoggerExt;
 	private readonly detachedBlobStorage: MemoryDetachedBlobStorage | undefined;
-	private readonly protocolHandlerBuilder: ProtocolHandlerBuilder;
+	private readonly protocolHandlerBuilder: InternalProtocolHandlerBuilder;
 	private readonly client: IClient;
 
 	private readonly mc: MonitoringContext;
@@ -597,8 +598,8 @@ export class Container
 		}
 		return this._runtime;
 	}
-	private _protocolHandler: IProtocolHandler | undefined;
-	private get protocolHandler(): IProtocolHandler {
+	private _protocolHandler: ProtocolHandlerInternal | undefined;
+	private get protocolHandler(): ProtocolHandlerInternal {
 		if (this._protocolHandler === undefined) {
 			throw new Error("Attempted to access protocolHandler before it was defined");
 		}
@@ -830,7 +831,7 @@ export class Container
 				attributes: IDocumentAttributes,
 				quorumSnapshot: IQuorumSnapshot,
 				sendProposal: (key: string, value: unknown) => number,
-			): ProtocolHandler =>
+			): ProtocolHandlerInternal =>
 				new ProtocolHandler(
 					attributes,
 					quorumSnapshot,

packages/loader/container-loader/src/contracts.ts

@@ -12,6 +12,7 @@ import {
 	type IConnectionDetails,
 } from "@fluidframework/container-definitions/internal";
 import type { IErrorBase, ITelemetryBaseProperties } from "@fluidframework/core-interfaces";
+import type { JsonString } from "@fluidframework/core-interfaces/internal";
 import type { ConnectionMode, IClientDetails } from "@fluidframework/driver-definitions";
 import type {
 	IContainerPackageInfo,
@@ -145,7 +146,9 @@ export interface IConnectionManagerFactoryArgs {
 	 * Called by connection manager for each incoming signal.
 	 * May be called before connectHandler is called (due to initial signals on socket connection)
 	 */
-	readonly signalHandler: (signals: ISignalMessage[]) => void;
+	readonly signalHandler: (
+		signals: ISignalMessage<{ type: never; content: JsonString<unknown> }>[],
+	) => void;
 
 	/**
 	 * Called when connection manager experiences delay in connecting to relay service.

packages/loader/container-loader/src/deltaManager.ts

@@ -16,7 +16,8 @@ import type {
 	ITelemetryBaseEvent,
 	ITelemetryBaseProperties,
 } from "@fluidframework/core-interfaces";
-import type { IThrottlingWarning } from "@fluidframework/core-interfaces/internal";
+import { JsonParse } from "@fluidframework/core-interfaces/internal";
+import type { IThrottlingWarning, JsonString } from "@fluidframework/core-interfaces/internal";
 import { assert } from "@fluidframework/core-utils/internal";
 import type { ConnectionMode } from "@fluidframework/driver-definitions";
 import {
@@ -210,7 +211,9 @@ export class DeltaManager<TConnectionManager extends IConnectionManager>
 	private initSequenceNumber: number = 0;
 
 	private readonly _inbound: DeltaQueue<ISequencedDocumentMessage>;
-	private readonly _inboundSignal: DeltaQueue<ISignalMessage>;
+	private readonly _inboundSignal: DeltaQueue<
+		ISignalMessage<{ type: never; content: JsonString<unknown> }>
+	>;
 
 	private _closed = false;
 	private _disposed = false;
@@ -433,7 +436,9 @@ export class DeltaManager<TConnectionManager extends IConnectionManager>
 					this.close(normalizeError(error));
 				}
 			},
-			signalHandler: (signals: ISignalMessage[]) => {
+			signalHandler: (
+				signals: ISignalMessage<{ type: never; content: JsonString<unknown> }>[],
+			) => {
 				for (const signal of signals) {
 					this._inboundSignal.push(signal);
 				}
@@ -474,14 +479,16 @@ export class DeltaManager<TConnectionManager extends IConnectionManager>
 		});
 
 		// Inbound signal queue
-		this._inboundSignal = new DeltaQueue<ISignalMessage>((message) => {
+		this._inboundSignal = new DeltaQueue<
+			ISignalMessage<{ type: never; content: JsonString<unknown> }>
+		>((message) => {
 			if (this.handler === undefined) {
 				throw new Error("Attempted to process an inbound signal without a handler attached");
 			}
 
 			this.handler.processSignal({
 				...message,
-				content: JSON.parse(message.content as string),
+				content: JsonParse(message.content),
 			});
 		});
 

packages/loader/container-loader/src/protocol.ts

@@ -21,12 +21,39 @@ import {
 } from "./protocol/index.js";
 
 // ADO: #1986: Start using enum from protocol-base.
-export enum SignalType {
-	ClientJoin = "join", // same value as MessageType.ClientJoin,
-	ClientLeave = "leave", // same value as MessageType.ClientLeave,
-	Clear = "clear", // used only by client for synthetic signals
+export const SignalType = {
+	ClientJoin: "join", // same value as MessageType.ClientJoin,
+	ClientLeave: "leave", // same value as MessageType.ClientLeave,
+	Clear: "clear", // used only by client for synthetic signals
+} as const;
+
+interface SystemSignalContent {
+	type: (typeof SignalType)[keyof typeof SignalType];
+	content?: unknown;
+}
+
+interface InboundSystemSignal<TSignalContent extends SystemSignalContent>
+	extends ISignalMessage<{ type: never; content: TSignalContent }> {
+	// eslint-disable-next-line @rushstack/no-new-null -- `null` is used in JSON protocol to indicate system message
+	readonly clientId: null;
 }
 
+type ClientJoinSignal = InboundSystemSignal<{
+	type: typeof SignalType.ClientJoin;
+	content: ISignalClient;
+}>;
+
+type ClientLeaveSignal = InboundSystemSignal<{
+	type: typeof SignalType.ClientLeave;
+	content: string; // clientId of leaving client
+}>;
+
+type ClearClientsSignal = InboundSystemSignal<{
+	type: typeof SignalType.Clear;
+}>;
+
+type AudienceSignal = ClientJoinSignal | ClientLeaveSignal | ClearClientsSignal;
+
 /**
  * Function to be used for creating a protocol handler.
  * @legacy @beta
@@ -47,7 +74,34 @@ export interface IProtocolHandler extends IBaseProtocolHandler {
 	processSignal(message: ISignalMessage);
 }
 
-export class ProtocolHandler extends ProtocolOpHandler implements IProtocolHandler {
+/**
+ * More specific version of {@link IProtocolHandler} with narrower call
+ * constraints for processSignal.
+ */
+export interface ProtocolHandlerInternal extends IProtocolHandler {
+	/**
+	 * Process the audience related signal.
+	 * @privateRemarks Internally only AudienceSignal messages need handled
+	 */
+	processSignal(message: AudienceSignal): void;
+}
+
+/**
+ * Function to be used for creating a protocol handler.
+ *
+ * @remarks This is the same are {@link ProtocolHandlerBuilder} but
+ * returns the {@link ProtocolHandlerInternal} which has narrower
+ * expectations for `processSignal`.
+ */
+export type InternalProtocolHandlerBuilder = (
+	attributes: IDocumentAttributes,
+	snapshot: IQuorumSnapshot,
+	// TODO: use a real type (breaking change)
+	// eslint-disable-next-line @typescript-eslint/no-explicit-any
+	sendProposal: (key: string, value: any) => number,
+) => ProtocolHandlerInternal;
+
+export class ProtocolHandler extends ProtocolOpHandler implements ProtocolHandlerInternal {
 	constructor(
 		attributes: IDocumentAttributes,
 		quorumSnapshot: IQuorumSnapshot,
@@ -104,8 +158,8 @@ export class ProtocolHandler extends ProtocolOpHandler implements IProtocolHandl
 		return super.processMessage(message, local);
 	}
 
-	public processSignal(message: ISignalMessage): void {
-		const innerContent = message.content as { content: unknown; type: string };
+	public processSignal(message: AudienceSignal): void {
+		const innerContent = message.content;
 		switch (innerContent.type) {
 			case SignalType.Clear: {
 				const members = this.audience.getMembers();
@@ -117,15 +171,15 @@ export class ProtocolHandler extends ProtocolOpHandler implements IProtocolHandl
 				break;
 			}
 			case SignalType.ClientJoin: {
-				const newClient = innerContent.content as ISignalClient;
+				const newClient = innerContent.content;
 				// Ignore write clients - quorum will control such clients.
 				if (newClient.client.mode === "read") {
 					this.audience.addMember(newClient.clientId, newClient.client);
 				}
 				break;
 			}
 			case SignalType.ClientLeave: {
-				const leftClientId = innerContent.content as string;
+				const leftClientId = innerContent.content;
 				// Ignore write clients - quorum will control such clients.
 				if (this.audience.getMember(leftClientId)?.mode === "read") {
 					this.audience.removeMember(leftClientId);
@@ -144,7 +198,9 @@ export class ProtocolHandler extends ProtocolOpHandler implements IProtocolHandl
  * The protocol handler should strictly handle only ClientJoin, ClientLeave
  * and Clear signal types.
  */
-export function protocolHandlerShouldProcessSignal(message: ISignalMessage): boolean {
+export function protocolHandlerShouldProcessSignal(
+	message: ISignalMessage,
+): message is AudienceSignal {
 	// Signal originates from server
 	if (message.clientId === null) {
 		const innerContent = message.content as { content: unknown; type: string };