Use LockFile for directory locking on windows

diggy · June 11, 2020, 1:22pm

Moved from GitHub badger/1364

Badger acquires an exclusive lock on the directory to prevent multiple processes from using it. The existing code in dir_windows.go file relies on an in-memory handle to prevent multiple files from accessing it.

github.com

dgraph-io/badger/blob/14386ac9b764cb6655cb0da7c848e244369a2a27/dir_windows.go#L82-L98


	// This call creates a file handler in memory that only one process can use at a time. When
	// that process ends, the file is deleted by the system.
	// FILE_ATTRIBUTE_TEMPORARY is used to tell Windows to try to create the handle in memory.
	// FILE_FLAG_DELETE_ON_CLOSE is not specified in syscall_windows.go but tells Windows to delete
	// the file when all processes holding the handler are closed.
	// XXX: this works but it's a bit klunky. i'd prefer to use LockFileEx but it needs unsafe pkg.
	h, err := syscall.CreateFile(
		syscall.StringToUTF16Ptr(absLockFilePath), 0, 0, nil,
		syscall.OPEN_ALWAYS,
		uint32(FILE_ATTRIBUTE_TEMPORARY|FILE_FLAG_DELETE_ON_CLOSE),
		0)
	if err != nil {
		return nil, errors.Wrapf(err,
			"Cannot create lock file %q.  Another process is using this Badger database",
			absLockFilePath)
	}

While opening we set the sharing mode to zero which means only one process can access it
From Create File Documentation

If this parameter is zero and CreateFile succeeds, the file or device cannot be shared and cannot be opened again until the handle to the file or device is closed. For more information, see the Remarks section.

We can use LockFile for instead.

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Use LockFile for directory locking on windows

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