[xamarin] fix for DebugConfig and read-only file system

[jonathanpeppers]

Using DebugInProcessConfig results in an error on Android:

System.IO.IOException: Read-only file system
    at System.IO.FileSystem.CreateDirectory (System.String fullPath) [0x00187] in /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/external/corefx/src/System.IO.FileSystem/src/System/IO/FileSystem.Unix.cs:317
    at System.IO.Directory.CreateDirectory (System.String path) [0x0002c] in /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/external/corefx/src/System.IO.FileSystem/src/System/IO/Directory.cs:40
    at BenchmarkDotNet.Extensions.CommonExtensions.CreateIfNotExists (System.String directoryPath) [0x0000e] in C:\src\BenchmarkDotNet\src\BenchmarkDotNet\Extensions\CommonExtensions.cs:71
    at BenchmarkDotNet.Running.BenchmarkRunnerClean.GetRootArtifactsFolderPath (BenchmarkDotNet.Running.BenchmarkRunInfo[] benchmarkRunInfos) [0x00058] in C:\src\BenchmarkDotNet\src\BenchmarkDotNet\Running\BenchmarkRunnerClean.cs:534
    at BenchmarkDotNet.Running.BenchmarkRunnerClean.Run (BenchmarkDotNet.Running.BenchmarkRunInfo[] benchmarkRunInfos) [0x00014] in C:\src\BenchmarkDotNet\src\BenchmarkDotNet\Running\BenchmarkRunnerClean.cs:40
    at BenchmarkDotNet.Running.BenchmarkRunner.RunWithDirtyAssemblyResolveHelper (System.Type type, BenchmarkDotNet.Configs.IConfig config) [0x00000] in C:\src\BenchmarkDotNet\src\BenchmarkDotNet\Running\BenchmarkRunnerDirty.cs:77
    at BenchmarkDotNet.Running.BenchmarkRunner+<>c__DisplayClass0_0`1[T].<Run>b__0 () [0x00000] in C:\src\BenchmarkDotNet\src\BenchmarkDotNet\Running\BenchmarkRunnerDirty.cs:23
    at BenchmarkDotNet.Running.BenchmarkRunner.RunWithExceptionHandling (System.Func`1[TResult] run) [0x00002] in C:\src\BenchmarkDotNet\src\BenchmarkDotNet\Running\BenchmarkRunnerDirty.cs:107
    at BenchmarkDotNet.Running.BenchmarkRunner.Run[T] (BenchmarkDotNet.Configs.IConfig config) [0x00014] in C:\src\BenchmarkDotNet\src\BenchmarkDotNet\Running\BenchmarkRunnerDirty.cs:23
    at BenchmarkDotNet.Samples.Forms.MainPage+<>c__DisplayClass1_0.<Button_Clicked>b__0 () [0x0000f] in C:\src\BenchmarkDotNet\samples\BenchmarkDotNet.Samples.Forms\MainPage.xaml.cs:32
    at System.Threading.Tasks.Task.InnerInvoke () [0x0000f] in /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/external/corert/src/System.Private.CoreLib/src/System/Threading/Tasks/Task.cs:2476
    at System.Threading.Tasks.Task.Execute () [0x00000] in /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/external/corert/src/System.Private.CoreLib/src/System/Threading/Tasks/Task.cs:2319
    --- End of stack trace from previous location where exception was thrown ---
    at BenchmarkDotNet.Samples.Forms.MainPage.Button_Clicked (System.Object sender, System.EventArgs e) [0x00077] in C:\src\BenchmarkDotNet\samples\BenchmarkDotNet.Samples.Forms\MainPage.xaml.cs:26

In 9c0f523, I fixed DefaultConfig.ArtifactPath's usage of
Directory.GetCurrentDirectory() so that ArtifactPath defaults to a
writeable directory on Android.

We need the same fix in DebugConfig.ArtifactPath, but I also added
RuntimeInformation.IsAndroid() and RuntimeInformation.IsiOS() to
make things a bit better.

I updated the Xamarin sample to use a DebugInProcessConfig for
Debug configurations.

[adamsitnik]

Is this the best way to check if we are running on Android? How about:

Suggested change

	if (root == "/")
	if (RuntimeInformation.IsOSPlatform(OSPlatform.Create("ANDROID")))

@jonathanpeppers could you please add a comment here that explains why we need this check (it's not obvious)

[jonathanpeppers]

This is the same code as here:

BenchmarkDotNet/src/BenchmarkDotNet/Configs/DefaultConfig.cs

Lines 80 to 91 in 92474e8

	public string ArtifactsPath
	{
	get
	{
	var root = Directory.GetCurrentDirectory();
	if (root == "/")
	{
	root = Environment.GetFolderPath(Environment.SpecialFolder.UserProfile);
	}
	return Path.Combine(root, "BenchmarkDotNet.Artifacts");
	}
	}

My thought was that if the current directory is /, it's not going to be writeable on any platform.

Let me see if the IsOSPlatform check works for me, though.

[jonathanpeppers]

@adamsitnik neither of these checks actually work on the current Xamarin:

RuntimeInformation.IsOSPlatform(OSPlatform.Create("ANDROID"))
RuntimeInformation.IsOSPlatform(OSPlatform.Create("Android"))

I found some discussion here.

Hopefully, OSPlatform.Android will exist in .NET 6, but we probably can't use until then.

I updated BDN's RuntimeInformation instead.

[adamsitnik]

@jonathanpeppers Looks very good to me. Big thanks for your work on Xamarin support!