Question

On a Windows 7 (or server) box, we have a folder on a UNC share (cross machine UNC, not localhost). We rename that folder, and then check for the existence of a file at the new folder location. Even though it exists, it takes almost 5 seconds for File.Exists to return true on it.

Full repro can be found on https://github.com/davidebbo/NpmFolderRenameIssue. Here is the core code:

// This file doesn't exist yet
// Note that the presence of this existence check is what triggers the bug below!!
Console.WriteLine("Exists (should be false): " + File.Exists("test/test2/myfile"));

// Create a directory, with a file in it
Directory.CreateDirectory("test/subdir/test");
File.WriteAllText("test/subdir/test/myfile", "Hello");

// Rename the directory
Directory.Move("test/subdir/test", "test/test2");

var start = DateTime.UtcNow;

// List the files at the new location. Here, our file shows up fine
foreach (var path in Directory.GetFiles("test/test2"))
{
    Console.WriteLine(path);
}

for (; ; )
{
    // Now do a simple existence test. It should also be true, but when
    // running on a (cross machine) UNC share, it takes almost 5 seconds to become true!
    if (File.Exists("test/test2/myfile")) break;

    Console.WriteLine("After {0} milliseconds, test/test2/myfile doesn't show as existing",
        (DateTime.UtcNow - start).TotalMilliseconds);
    Thread.Sleep(100);
}

Console.WriteLine("After {0} milliseconds, test/test2/myfile correctly shows as existing!",
    (DateTime.UtcNow - start).TotalMilliseconds);

So it seems like the initial existence check causes the existence value to be cached, causing this bogus behavior.

Questions: what is the explanation for this? What’s the best way to avoid it?

NOTE: this issue initially arose when using npm (Node Package Manager) on Windows. The code I have here is a C# port of the repro. See https://github.com/isaacs/npm/issues/2230 for the original Node/npm issue. The goal is to find a way to address it.

Answer 1

David,
The redirector implements a negative “File Not Found” cache which prevents a client from flooding a server with file not found requests. The default cache time is 5 seconds but you can modify the FileNotFoundCacheLifetime registry value to control the cache or disable it by setting this value to 0.

Details: http://technet.microsoft.com/en-us/library/ff686200(v=WS.10).aspx