From f45452ae6ca966869e95c6f259bb317ee147a50b Mon Sep 17 00:00:00 2001
From: YeonGyu-Kim <code.yeon.gyu@gmail.com>
Date: Sun, 10 May 2026 14:53:28 +0900
Subject: [PATCH] fix(config): use r+ mode for fsync on Windows to prevent
 migration retry loop

openSync with read-only mode fails fsync on Windows because FlushFileBuffers requires write-permission FD. This caused atomic writes to fail silently, leaving migrated config unwritten and triggering repeated migration + .bak.<timestamp> generation on every startup.

Same root cause as PR #3644 (#3643). Hyperplan disappear is a secondary symptom of plugin load instability.

Fixes #3877
---
 src/shared/write-file-atomically.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/shared/write-file-atomically.ts b/src/shared/write-file-atomically.ts
index 09ce5b7d5..7f6544761 100644
--- a/src/shared/write-file-atomically.ts
+++ b/src/shared/write-file-atomically.ts
@@ -16,7 +16,7 @@ export function writeFileAtomically(
 ): void {
   const tempPath = `${filePath}.tmp`
   writeFileSync(tempPath, content, "utf-8")
-  const tempFileDescriptor = openSync(tempPath, "r")
+  const tempFileDescriptor = openSync(tempPath, "r+")
   try {
     tolerantFsyncSync(tempFileDescriptor, `writeFileAtomically:${filePath}`, deps.fsyncSync)
   } finally {