Add mutex to getSnapState to prevent concurrent decryption (#3234)

This wraps `getSnapState` in a mutex, to ensure we only decrypt once
when calling it at the same time. The first call will decrypt the state
and cache it in the Snaps runtime, and the next call will just use the
cached result.

Author: Mrtenz

packages/snaps-controllers/coverage.json

@@ -1,6 +1,6 @@
 {
   "branches": 93.34,
-  "functions": 97.36,
+  "functions": 97.37,
   "lines": 98.33,
   "statements": 98.07
 }

packages/snaps-controllers/src/snaps/SnapController.ts

@@ -286,6 +286,11 @@ export type SnapRuntimeData = {
    * A mutex to prevent concurrent state updates.
    */
   stateMutex: Mutex;
+
+  /**
+   * A mutex to prevent concurrent state decryption.
+   */
+  getStateMutex: Mutex;
 };
 
 export type SnapError = {
@@ -2046,33 +2051,36 @@ export class SnapController extends BaseController<
    */
   async getSnapState(snapId: SnapId, encrypted: boolean): Promise<Json> {
     const runtime = this.#getRuntimeExpect(snapId);
-    const cachedState = encrypted ? runtime.state : runtime.unencryptedState;
+    return await runtime.getStateMutex.runExclusive(async () => {
+      const cachedState = encrypted ? runtime.state : runtime.unencryptedState;
 
-    if (cachedState !== undefined) {
-      return cachedState;
-    }
+      if (cachedState !== undefined) {
+        return cachedState;
+      }
 
-    const state = encrypted
-      ? this.state.snapStates[snapId]
-      : this.state.unencryptedSnapStates[snapId];
+      const state = encrypted
+        ? this.state.snapStates[snapId]
+        : this.state.unencryptedSnapStates[snapId];
 
-    if (state === null || state === undefined) {
-      return null;
-    }
+      if (state === null || state === undefined) {
+        return null;
+      }
 
-    if (!encrypted) {
-      // For performance reasons, we do not validate that the state is JSON,
-      // since we control serialization.
-      const json = JSON.parse(state);
-      runtime.unencryptedState = json;
+      if (!encrypted) {
+        // For performance reasons, we do not validate that the state is JSON,
+        // since we control serialization.
+        const json = JSON.parse(state);
+        runtime.unencryptedState = json;
 
-      return json;
-    }
+        return json;
+      }
 
-    const decrypted = await this.#decryptSnapState(snapId, state);
-    runtime.state = decrypted;
+      const decrypted = await this.#decryptSnapState(snapId, state);
+      // eslint-disable-next-line require-atomic-updates
+      runtime.state = decrypted;
 
-    return decrypted;
+      return decrypted;
+    });
   }
 
   /**
@@ -3968,6 +3976,7 @@ export class SnapController extends BaseController<
       interpreter,
       stopping: false,
       stateMutex: new Mutex(),
+      getStateMutex: new Mutex(),
     });
   }