Fix/apphosting port validation

src/command.ts

@@ -165,8 +165,7 @@ export class Command {
     if (this.aliases) {
       cmd.aliases(this.aliases);
     }
-    this.options.forEach((args) => {
-      const flags = args.shift();
+    this.options.forEach(([flags, ...args]) => {
       cmd.option(flags, ...args);
     });
 

src/emulator/apphosting/serve.spec.ts

@@ -94,17 +94,27 @@ describe("serve", () => {
       expect(spawnWithCommandStringStub.getCall(0).args[0]).to.eq(startCommand + " --port 5002");
     });
 
-    it("should reject the custom command if a port is specified", async () => {
-      const startCommand = "ng serve --port 5004";
+    it("should allow custom command with port specified and show warning", async () => {
+      const startCommand = "npm run dev -- --port 5004";
       checkListenableStub.onFirstCall().returns(true);
       configsStub.getLocalAppHostingConfiguration.resolves(AppHostingYamlConfig.empty());
 
-      await expect(serve.start({ startCommand })).to.be.rejectedWith(
-        FirebaseError,
-        /Specifying a port in the start command is not supported by the apphosting emulator/,
-      );
+      await serve.start({ startCommand });
+
+      expect(spawnWithCommandStringStub).to.be.called;
+      expect(spawnWithCommandStringStub.getCall(0).args[0]).to.eq(startCommand);
+      // Should log a warning about port mismatch
+    });
+
+    it("should not add port to ng serve if already specified", async () => {
+      const startCommand = "ng serve --port 5002";
+      checkListenableStub.onFirstCall().returns(true);
+      configsStub.getLocalAppHostingConfiguration.resolves(AppHostingYamlConfig.empty());
+
+      await serve.start({ startCommand });
 
-      expect(spawnWithCommandStringStub).to.not.be.called;
+      expect(spawnWithCommandStringStub).to.be.called;
+      expect(spawnWithCommandStringStub.getCall(0).args[0]).to.eq(startCommand);
     });
 
     it("Should pass plaintext environment variables", async () => {

src/emulator/apphosting/serve.ts

@@ -106,26 +106,32 @@ export async function start(options?: StartOptions): Promise<{ hostname: string;
   let startCommand;
   if (options?.startCommand) {
     startCommand = options?.startCommand;
-    // Angular and nextjs CLIs allow for specifying port options but the emulator is setting and
-    // specifying specific ports rather than use framework defaults or w/e the user has set, so we
-    // need to reject such custom commands.
-    // NOTE: this is not robust, a command could be a wrapper around another command and we cannot
-    // detect --port there.
-    if (startCommand.includes("--port") || startCommand.includes(" -p ")) {
-      throw new FirebaseError(
-        "Specifying a port in the start command is not supported by the apphosting emulator",
-      );
-    }
     // Angular does not respect the NodeJS.ProcessEnv.PORT set below. Port needs to be
-    // set directly in the CLI.
-    if (startCommand.includes("ng serve")) {
+    // set directly in the CLI. Only add --port if not already specified by the user.
+    if (
+      startCommand.includes("ng serve") &&
+      !startCommand.includes("--port") &&
+      !startCommand.includes(" -p ")
+    ) {
       startCommand += ` --port ${port}`;
     }
     logger.logLabeled(
       "BULLET",
       Emulators.APPHOSTING,
       `running custom start command: '${startCommand}'`,
     );
+    // Warn if user specified a port that differs from the emulator port
+    const portMatch = startCommand.match(/--port[= ](\d+)|-p[= ](\d+)/);
+    if (portMatch) {
+      const userPort = parseInt(portMatch[1] || portMatch[2]);
+      if (userPort !== port) {
+        logLabeledWarning(
+          Emulators.APPHOSTING,
+          `Custom start command specifies port ${userPort}, but emulator is using port ${port}. ` +
+            `Make sure your command uses the PORT environment variable or matches the emulator port.`,
+        );
+      }
+    }
   } else {
     // TODO: port may be specified in an underlying command. But we will need to parse the package.json
     // file to be sure.