fix(sdk): fs adapter flush() tracks in-flight writes; add to T13 contract suite (#1425)

* fix(sdk): fs adapter flush() tracks in-flight writes; add to T13 contract suite

* fix(sdk): document flush() first-error rejection semantics

Promise.all rejects on first write failure; errors also surface via
persist:error event channel per write.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Co-authored-by: Miguel Ángel <miguel07alm@protonmail.com>

---------

Co-authored-by: Miguel Ángel <miguel07alm@protonmail.com>

packages/sdk/src/adapters/fs.ts

@@ -18,6 +18,7 @@ class FsAdapter implements PersistAdapter {
   private readonly root: string;
   private readonly maxVersions: number;
   private errorHandlers: Array<(e: PersistErrorEvent) => void> = [];
+  private readonly inflightWrites = new Set<Promise<void>>();
   private _writeLocks = new Map<string, Promise<void>>();
 
   constructor(opts: FsAdapterOptions) {
@@ -35,6 +36,16 @@ class FsAdapter implements PersistAdapter {
   }
 
   async write(path: string, content: string): Promise<void> {
+    const p = this.doWrite(path, content);
+    this.inflightWrites.add(p);
+    try {
+      await p;
+    } finally {
+      this.inflightWrites.delete(p);
+    }
+  }
+
+  private async doWrite(path: string, content: string): Promise<void> {
     try {
       const abs = this.abs(path);
       await mkdir(dirname(abs), { recursive: true });
@@ -45,7 +56,11 @@ class FsAdapter implements PersistAdapter {
     }
   }
 
-  async flush(): Promise<void> {}
+  async flush(): Promise<void> {
+    // Promise.all rejects on the first write failure; per-write errors are also
+    // surfaced individually through the persist:error event channel.
+    await Promise.all([...this.inflightWrites]);
+  }
 
   async listVersions(path: string): Promise<PersistVersionEntry[]> {
     const dir = this.versionsDir(path);

packages/sdk/src/adapters/persistAdapter.contract.test.ts

@@ -9,8 +9,12 @@
  *   runPersistAdapterContract("s3", () => createS3Adapter({ bucket, prefix }))
  */
 
+import { mkdtempSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
 import { describe, it, expect, vi } from "vitest";
 import { createMemoryAdapter } from "./memory.js";
+import { createFsAdapter } from "./fs.js";
 import type { PersistAdapter } from "./types.js";
 
 export function runPersistAdapterContract(
@@ -126,3 +130,8 @@ export function runPersistAdapterContract(
 
 // Run the suite against the memory adapter immediately
 runPersistAdapterContract("memory", createMemoryAdapter);
+
+// Run against the fs adapter — each test gets an isolated tmpdir
+runPersistAdapterContract("fs", () =>
+  createFsAdapter({ root: mkdtempSync(join(tmpdir(), "hf-fs-test-")) }),
+);