Targets may be run more than once when running in parallel

[adamralph]

Version(s)

From 2.0.0 to 3.7.1 inclusive.

To reproduce

Steps to reproduce the behaviour:

1. Create a console app targetting netcoreapp3.0 or later
2. Create some targets with diamond dependencies, e.g.

   var names = new List<string>();
   
   foreach (var name in Enumerable.Range(1, 10).Select(i => i.ToString(CultureInfo.InvariantCulture)))
   {
       Target(name, names.TakeLast(2), () => { });
       names.Add(name);
   }
   
   Target("default", DependsOn(names.Last()));
   
   RunTargetsAndExit(args);

3. Run with the --parallel option.

Expected behaviour

Actual behaviour

Workarounds

Target netcoreapp2.x or earlier, or do not use the --parallel option.

Additional context

Thanks to @alibresco for raising #619 which led to discovery of the bug!

It is due to the behaviour of ConcurrentDictionary.GetOrAdd:

If you call GetOrAdd simultaneously on different threads, valueFactory may be called multiple times, but only one key/value pair will be added to the dictionary.

Although this remark is also present in versions of the documentation for frameworks for netcoreapp2.x and earlier, I was unable to reproduce the bug in those versions, suggesting that the chance of valueFactory being called multiple times increased significantly in netcoreapp3.0.

[adamralph]

released in 3.8.0-alpha.2