Bug 60791 - Path.GetTempFileName is not thread-safe
Summary: Path.GetTempFileName is not thread-safe
Alias: None
Product: Class Libraries
Classification: Mono
Component: mscorlib ()
Version: master
Hardware: PC Mac OS
: Normal normal
Target Milestone: Untriaged
Assignee: Rodrigo Kumpera
Depends on:
Reported: 2017-11-21 21:00 UTC by Jérémie Laval
Modified: 2018-03-14 00:40 UTC (History)
4 users (show)

Is this bug a regression?: ---
Last known good build:

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Description Jérémie Laval 2017-11-21 21:00:55 UTC
The naive implementation of Path.GetTempFileName is not thread-safe and can return the same filename multiple time.


using System;
using System.IO;
using System.Threading;
using System.Threading.Tasks;
using System.Collections.Generic;
using System.Collections.Concurrent;

class T
    public static async Task Main ()
        var createdPaths = new HashSet<string> ();
        for (int i = 0; i < 10000; i++) {
            var queue = new ConcurrentQueue<string> ();
            await Task.WhenAll (
                Task.Run (() => queue.Enqueue (Path.GetTempFileName ())),
                Task.Run (() => queue.Enqueue (Path.GetTempFileName ())),
                Task.Run (() => queue.Enqueue (Path.GetTempFileName ())),
                Task.Run (() => queue.Enqueue (Path.GetTempFileName ())),
                Task.Run (() => queue.Enqueue (Path.GetTempFileName ())),
                Task.Run (() => queue.Enqueue (Path.GetTempFileName ())),
                Task.Run (() => queue.Enqueue (Path.GetTempFileName ()))
            while (queue.TryDequeue (out var path)) {
                if (!createdPaths.Add (path))
                    Console.WriteLine ($"/!\\ Path {path} has been created twice");
                File.Delete (path);


% mono repro-gettempfilename.exe
/!\ Path /var/folders/sj/m6n49_qx5w3_9n7s0pdmc3gc0000gn/T/tmp5408018.tmp has been created twice
/!\ Path /var/folders/sj/m6n49_qx5w3_9n7s0pdmc3gc0000gn/T/tmp52146c7b.tmp has been created twice

To compare, .NET core uses the proper mktemp family of syscalls which are correct.
Comment 1 Rodrigo Kumpera 2018-03-14 00:22:49 UTC
working on it.
Comment 2 Rodrigo Kumpera 2018-03-14 00:40:53 UTC
I tried your test case multiple times and it did not find a single case of duplicated file paths.

I checked how it's implemented in mono and could not find what you refer as a naive implementation. It's doing exactly the same thing as mktemp does.

The API is unfortunately racy, as it should return a FileStream instead of a string, but that's a separate story.