Allow the bevy engine to serve authoritative scenes in preview

DCL_SERVER_ENGINE selects the implementation and defaults to hammurabi, so this is
inert until the bevy packages are published. Exit code 78 from the bevy launcher
means it can never run on this machine, so fall back rather than leave the preview
without a server.

Author: gonpombo8

packages/@dcl/sdk-commands/src/commands/start/hammurabi-server.ts

@@ -9,6 +9,20 @@ import { isElectronEnvironment, getSpawnEnv, findNpxCliJs, getNpxBin } from './u
 const HAMMURABI_PACKAGE = '@dcl/hammurabi-server'
 const HAMMURABI_VERSION = 'next'
 
+const BEVY_PACKAGE = '@dcl/bevy-headless-server'
+const BEVY_VERSION = 'next'
+
+// The bevy server exits with this when it can never run here (unsupported platform,
+// missing binary, bad arguments), which is our cue to fall back to hammurabi.
+const EXIT_UNAVAILABLE = 78
+
+type ServerEngine = 'bevy' | 'hammurabi'
+
+function selectedEngine(): ServerEngine {
+  const requested = process.env.DCL_SERVER_ENGINE
+  return requested === 'bevy' || requested === 'hammurabi' ? requested : 'hammurabi'
+}
+
 /**
  * Registers cleanup handlers on the global process object
  * Returns a function to remove the handlers
@@ -31,14 +45,17 @@ function registerProcessCleanup(cleanup: () => void): () => void {
 export function startHammurabiServer(
   components: Pick<CliComponents, 'logger'>,
   workingDir: string,
-  realm: string
+  realm: string,
+  engine: ServerEngine = 'hammurabi'
 ): ChildProcess {
+  const pkg = engine === 'bevy' ? `${BEVY_PACKAGE}@${BEVY_VERSION}` : `${HAMMURABI_PACKAGE}@${HAMMURABI_VERSION}`
+
   printProgressInfo(
     components.logger,
-    `Starting ${colors.bold('Multiplayer Server')} with realm: ${colors.bold(realm)}`
+    `Starting ${colors.bold('Multiplayer Server')} (${engine}) with realm: ${colors.bold(realm)}`
   )
 
-  const npxArgs = ['--yes', `${HAMMURABI_PACKAGE}@${HAMMURABI_VERSION}`, `--realm=${realm}`]
+  const npxArgs = ['--yes', pkg, `--realm=${realm}`]
   const npxCliJs = findNpxCliJs()
 
   // In Electron, override npm_config_prefix because npm derives its prefix from process.execPath,
@@ -80,18 +97,31 @@ export function startHammurabiServer(
  * In the auth-server SDK, all scenes are authoritative multiplayer.
  * Uses npx to handle installation and execution in a single step (works in Electron).
  *
+ * Which implementation runs is chosen by DCL_SERVER_ENGINE (bevy | hammurabi). When bevy
+ * reports itself unavailable on this machine, hammurabi is started instead.
+ *
  * @param components - Preview components including logger
  * @param project - The project to start the multiplayer server for
- * @param realm - The realm URL to pass to the hammurabi server
+ * @param realm - The realm URL to pass to the server
  * @returns The ChildProcess if started, undefined otherwise
  */
 export function spawnAuthServer(
   components: PreviewComponents,
   project: ProjectUnion,
   realm: string
 ): ChildProcess | undefined {
+  const engine = selectedEngine()
   try {
-    return startHammurabiServer(components, project.workingDirectory, realm)
+    const child = startHammurabiServer(components, project.workingDirectory, realm, engine)
+    if (engine === 'bevy') {
+      child.on('close', (code) => {
+        if (code === EXIT_UNAVAILABLE) {
+          printWarning(components.logger, 'Bevy multiplayer server unavailable here — falling back to hammurabi')
+          startHammurabiServer(components, project.workingDirectory, realm, 'hammurabi')
+        }
+      })
+    }
+    return child
   } catch (error: any) {
     printWarning(components.logger, `Failed to start Multiplayer Server: ${error.message}`)
     return undefined