Add locking to ensure safe concurrent access to config file (#881)

Reviewed-on: https://gitea.com/gitea/tea/pulls/881
Co-authored-by: techknowlogick <techknowlogick@gitea.com>
Co-committed-by: techknowlogick <techknowlogick@gitea.com>

modules/config/lock_windows.go

@@ -0,0 +1,48 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+//go:build windows
+
+package config
+
+import (
+	"fmt"
+	"os"
+	"time"
+
+	"golang.org/x/sys/windows"
+)
+
+// lockFile acquires an exclusive lock on the file using LockFileEx.
+// It polls with non-blocking LockFileEx until timeout.
+func lockFile(file *os.File, timeout time.Duration) error {
+	deadline := time.Now().Add(timeout)
+	handle := windows.Handle(file.Fd())
+
+	// LOCKFILE_EXCLUSIVE_LOCK | LOCKFILE_FAIL_IMMEDIATELY
+	const flags = windows.LOCKFILE_EXCLUSIVE_LOCK | windows.LOCKFILE_FAIL_IMMEDIATELY
+
+	for {
+		// Lock the first byte (advisory lock)
+		var overlapped windows.Overlapped
+		err := windows.LockFileEx(handle, flags, 0, 1, 0, &overlapped)
+		if err == nil {
+			return nil
+		}
+		if err != windows.ERROR_LOCK_VIOLATION {
+			return fmt.Errorf("LockFileEx failed: %w", err)
+		}
+
+		if time.Now().After(deadline) {
+			return fmt.Errorf("timeout waiting for file lock")
+		}
+		time.Sleep(fileLockPollInterval)
+	}
+}
+
+// unlockFile releases the lock on the file.
+func unlockFile(file *os.File) error {
+	handle := windows.Handle(file.Fd())
+	var overlapped windows.Overlapped
+	return windows.UnlockFileEx(handle, 0, 1, 0, &overlapped)
+}