fix(daemon): pet-store uses mutex to ensure serial file operations

packages/daemon/src/mutex.js

@@ -0,0 +1,49 @@
+import { makePromiseKit } from '@endo/promise-kit';
+
+/**
+ * @returns {{ put: (value: any) => void, get: () => Promise<any> }}
+ */
+export const makeQueue = () => {
+  let { promise: tailPromise, resolve: tailResolve } = makePromiseKit();
+  return {
+    put(value) {
+      const next = makePromiseKit();
+      const promise = next.promise;
+      tailResolve({ value, promise });
+      tailResolve = next.resolve;
+    },
+    get() {
+      const promise = tailPromise.then(next => next.value);
+      tailPromise = tailPromise.then(next => next.promise);
+      return promise;
+    },
+  };
+};
+
+/**
+ * @returns {{ lock: () => Promise<void>, unlock: () => void, enqueue: (asyncFn: () => Promise<any>) => Promise<any> }}
+ */
+export const makeMutex = () => {
+  const queue = makeQueue();
+  const lock = () => {
+    return queue.get()
+  }
+  const unlock = () => {
+    queue.put()
+  }
+  unlock()
+
+  return {
+    lock,
+    unlock,
+    // helper for correct usage
+    enqueue: async (asyncFn) => {
+      await lock()
+      try {
+        return await asyncFn()
+      } finally {
+        unlock()
+      }
+    },
+  };
+}

packages/daemon/src/pet-store.js

@@ -3,6 +3,7 @@
 import { Far } from '@endo/far';
 import { makeChangeTopic } from './pubsub.js';
 import { makeIteratorRef } from './reader-ref.js';
+import { makeMutex } from './mutex.js';
 
 const { quote: q } = assert;
 
@@ -27,6 +28,7 @@ export const makePetStoreMaker = (filePowers, locator) => {
     const formulaIdentifiers = new Map();
     /** @type {import('./types.js').Topic<unknown>} */
     const changesTopic = makeChangeTopic();
+    const writeLock = makeMutex();
 
     /** @param {string} petName */
     const read = async petName => {
@@ -103,8 +105,11 @@ export const makePetStoreMaker = (filePowers, locator) => {
 
       const petNamePath = filePowers.joinPath(petNameDirectoryPath, petName);
       const petNameText = `${formulaIdentifier}\n`;
-      await filePowers.writeFileText(petNamePath, petNameText);
       changesTopic.publisher.next({ add: petName });
+
+      return writeLock.enqueue(async () => {
+        await filePowers.writeFileText(petNamePath, petNameText);
+      });
     };
 
     const list = () => harden([...petNames.keys()].sort());
@@ -136,7 +141,6 @@ export const makePetStoreMaker = (filePowers, locator) => {
       }
 
       const petNamePath = filePowers.joinPath(petNameDirectoryPath, petName);
-      await filePowers.removePath(petNamePath);
       petNames.delete(petName);
       const formulaPetNames = formulaIdentifiers.get(petName);
       if (formulaPetNames !== undefined) {
@@ -145,6 +149,10 @@ export const makePetStoreMaker = (filePowers, locator) => {
       changesTopic.publisher.next({ remove: petName });
       // TODO consider retaining a backlog of deleted names for recovery
       // TODO consider tracking historical pet names for formulas
+
+      return writeLock.enqueue(async () => {
+        await filePowers.removePath(petNamePath);
+      });
     };
 
     /**
@@ -178,7 +186,6 @@ export const makePetStoreMaker = (filePowers, locator) => {
 
       const fromPath = filePowers.joinPath(petNameDirectoryPath, fromName);
       const toPath = filePowers.joinPath(petNameDirectoryPath, toName);
-      await filePowers.renamePath(fromPath, toPath);
       petNames.set(toName, formulaIdentifier);
       petNames.delete(fromName);
 
@@ -202,6 +209,10 @@ export const makePetStoreMaker = (filePowers, locator) => {
       changesTopic.publisher.next({ add: toName });
       changesTopic.publisher.next({ remove: fromName });
       // TODO consider retaining a backlog of overwritten names for recovery
+
+      return writeLock.enqueue(async () => {
+        await filePowers.renamePath(fromPath, toPath);
+      });
     };
 
     /**