harden state durability and disable destructive snapshot sync

src/server.js

@@ -64,13 +64,19 @@ function createHttpServer({ app }) {
 
 function createMutationPersister({ stateStore, logger = console }) {
   let queue = Promise.resolve();
+  let lastError = null;
 
   return {
     enqueue(state) {
       queue = queue
-        .then(() => stateStore.save(state))
+        .then(
+          () => stateStore.save(state),
+          () => stateStore.save(state),
+        )
         .catch((error) => {
+          lastError = error;
           logger.error({ err: error }, "failed to persist state");
+          throw error;
         });
 
       return queue;
@@ -78,6 +84,9 @@ function createMutationPersister({ stateStore, logger = console }) {
     flush() {
       return queue;
     },
+    getLastError() {
+      return lastError;
+    },
   };
 }
 
@@ -92,10 +101,15 @@ async function createRuntime({ runtimeConfig = config, logger = console, stateSt
   try {
     initialState = await effectiveStateStore.load();
   } catch (error) {
-    logger.warn(
-      { err: error },
-      "failed to initialize configured state store; falling back to in-memory state",
-    );
+    const allowFallback = runtimeConfig.allowInMemoryStateFallback !== undefined
+      ? Boolean(runtimeConfig.allowInMemoryStateFallback)
+      : true;
+
+    if (!allowFallback) {
+      throw new Error("state_store_unavailable_without_fallback", { cause: error });
+    }
+
+    logger.warn({ err: error }, "failed to initialize configured state store; falling back to in-memory state");
     effectiveStateStore = new InMemoryStateStore();
     initialState = await effectiveStateStore.load();
   }