fix: race condition in RPC filesystem cache. (#7531)

Author: dts

packages/vitest/src/node/pools/rpc.ts

@@ -3,8 +3,8 @@ import type { RuntimeRPC } from '../../types/rpc'
 import type { TestProject } from '../project'
 import type { ResolveSnapshotPathHandlerContext } from '../types/config'
 import { mkdirSync } from 'node:fs'
-import { writeFile } from 'node:fs/promises'
-import { join } from 'pathe'
+import { rename, stat, unlink, writeFile } from 'node:fs/promises'
+import { dirname, join } from 'pathe'
 import { hash } from '../hash'
 
 const created = new Set()
@@ -65,7 +65,11 @@ export function createMethodsRPC(project: TestProject, options: MethodsOptions =
       }
       promises.set(
         tmp,
-        writeFile(tmp, code, 'utf-8').finally(() => promises.delete(tmp)),
+
+        atomicWriteFile(tmp, code)
+        // Fallback to non-atomic write for windows case where file already exists:
+          .catch(() => writeFile(tmp, code, 'utf-8'))
+          .finally(() => promises.delete(tmp)),
       )
       await promises.get(tmp)
       Object.assign(result, { id: tmp })
@@ -146,3 +150,35 @@ function handleRollupError(e: unknown): never {
   }
   throw e
 }
+
+/**
+ * Performs an atomic write operation using the write-then-rename pattern.
+ *
+ * Why we need this:
+ * - Ensures file integrity by never leaving partially written files on disk
+ * - Prevents other processes from reading incomplete data during writes
+ * - Particularly important for test files where incomplete writes could cause test failures
+ *
+ * The implementation writes to a temporary file first, then renames it to the target path.
+ * This rename operation is atomic on most filesystems (including POSIX-compliant ones),
+ * guaranteeing that other processes will only ever see the complete file.
+ *
+ * Added in https://github.com/vitest-dev/vitest/pull/7531
+ */
+async function atomicWriteFile(realFilePath: string, data: string): Promise<void> {
+  const dir = dirname(realFilePath)
+  const tmpFilePath = join(dir, `.tmp-${Date.now()}-${Math.random().toString(36).slice(2)}`)
+
+  try {
+    await writeFile(tmpFilePath, data, 'utf-8')
+    await rename(tmpFilePath, realFilePath)
+  }
+  finally {
+    try {
+      if (await stat(tmpFilePath)) {
+        await unlink(tmpFilePath)
+      }
+    }
+    catch {}
+  }
+}