[xaprepare] Support Gentoo Linux for OS detection (#7442)

Fixes: #7441

`make prepare` fails during OS detection on Gentoo Linux:

	Failed to detect your Linux distribution. Additional info: name: $Gentoo; release: $2.8; id: $gentoo
	System.InvalidOperationException: Failed to detect your Linux distribution. Additional info: name: $Gentoo; release: $2.8; id: $gentoo
	   at Xamarin.Android.Prepare.Linux.DetectAndCreate(Context context)
	   at Xamarin.Android.Prepare.Context.InitOS()
	   at Xamarin.Android.Prepare.Context.Init(String scenarioName)
	   at Xamarin.Android.Prepare.App.Run(String[] args)

Update build-tools/xaprepare to handle Gentoo Linux OS detection and
package handling.

Implementation is based on ArchLinux implementation.

Author: cyborgyn

build-tools/xaprepare/xaprepare/Application/Program.GentooLinux.cs

@@ -0,0 +1,63 @@
+using System;
+using System.Threading.Tasks;
+
+namespace Xamarin.Android.Prepare
+{
+	class GentooLinuxProgram : LinuxProgram
+	{
+		public GentooLinuxProgram (string packageName, string? executableName = null)
+			: base (packageName, executableName)
+		{}
+
+		protected override bool CheckWhetherInstalled ()
+		{
+			var output = Utilities.GetStringFromStdout ("equery", "--quiet", "list", PackageName).Replace ($"{PackageName.Split (':') [0]}-", "").Split ('-', '_');
+			if (output.Length >= 1 && !String.IsNullOrEmpty (output [0])) {
+				CurrentVersion = output [0];
+				return true;
+			}
+
+			return false;
+		}
+
+#pragma warning disable CS1998
+		public override async Task<bool> Install ()
+		{
+			ProcessRunner runner;
+			if (NeedsSudoToInstall) {
+				runner = new ProcessRunner ("sudo", "emerge", "--oneshot", PackageName) {
+					EchoStandardOutput = true,
+					EchoStandardError = true,
+					ProcessTimeout = TimeSpan.FromMinutes (60),     // gcc most probably will not compile in 60 minutes...
+				};
+			}
+			else
+			{
+				runner = new ProcessRunner ("emerge", "--oneshot", PackageName) {
+					EchoStandardOutput = true,
+					EchoStandardError = true,
+					ProcessTimeout = TimeSpan.FromMinutes (60),     // gcc most probably will not compile in 60 minutes...
+				};
+			}
+
+			bool failed = await Task.Run (() => !runner.Run ());
+			if (failed) {
+				Log.Error ($"Installation of {PackageName} timed out");
+				failed = true;
+			}
+
+			if (runner.ExitCode != 0) {
+				Log.Error ($"Installation failed with error code {runner.ExitCode}");
+				failed = true;
+			}
+
+			return !failed;
+		}
+#pragma warning restore CS1998
+
+		protected override bool DeterminePackageVersion()
+		{
+			return true;
+		}
+	}
+}

build-tools/xaprepare/xaprepare/Application/Utilities.cs

@@ -820,7 +820,7 @@ public static string GetStringFromStdout (ProcessRunner runner, bool throwOnErro
 					LogError ($"failed with exit code {runner.ExitCode}");
 					return String.Empty;
 				}
-
+				
 				string ret = sw.ToString ();
 				if (trimTrailingWhitespace)
 					return ret.TrimEnd ();

build-tools/xaprepare/xaprepare/ConfigAndData/Dependencies/Linux.Gentoo.cs

@@ -0,0 +1,59 @@
+using System.Collections.Generic;
+
+namespace Xamarin.Android.Prepare
+{
+	class LinuxGentoo : Linux
+	{
+		static readonly List<GentooLinuxProgram> packages = new List<GentooLinuxProgram> {
+			new GentooLinuxProgram ("sys-devel/autoconf"),
+			new GentooLinuxProgram ("sys-devel/automake"),
+			new GentooLinuxProgram ("sys-devel/binutils"),
+			new GentooLinuxProgram ("sys-devel/bison"),
+			new GentooLinuxProgram ("net-misc/curl"),
+			new GentooLinuxProgram ("sys-apps/fakeroot"),
+			new GentooLinuxProgram ("sys-apps/file"),
+			new GentooLinuxProgram ("sys-apps/findutils"),
+			new GentooLinuxProgram ("sys-devel/flex"),
+			new GentooLinuxProgram ("sys-apps/gawk"),
+			new GentooLinuxProgram ("sys-devel/gcc"),
+			new GentooLinuxProgram ("sys-devel/gettext"),
+			new GentooLinuxProgram ("dev-vcs/git"),
+			new GentooLinuxProgram ("sys-apps/grep"),
+			new GentooLinuxProgram ("sys-apps/groff"),
+			//new GentooLinuxProgram ("gtk-sharp-2"),
+			new GentooLinuxProgram ("app-arch/gzip"),
+			new GentooLinuxProgram ("dev-java/openjdk-bin:8"),
+			new GentooLinuxProgram ("sys-devel/libtool"),
+			new GentooLinuxProgram ("dev-libs/libzip"),
+			new GentooLinuxProgram ("sys-devel/m4"),
+			new GentooLinuxProgram ("sys-devel/make"),
+			//new GentooLinuxProgram ("nuget"),
+			new GentooLinuxProgram ("sys-devel/patch"),
+			new GentooLinuxProgram ("dev-util/pkgconf"),
+			//new GentooLinuxProgram ("referenceassemblies-pcl"),
+			new GentooLinuxProgram ("sys-apps/sed"),
+			new GentooLinuxProgram ("sys-apps/texinfo"),
+			new GentooLinuxProgram ("app-arch/unzip"),
+			new GentooLinuxProgram ("sys-apps/which"),
+			new GentooLinuxProgram ("app-arch/zip"),
+			new GentooLinuxProgram ("app-arch/p7zip"),
+		};
+
+		public LinuxGentoo (Context context)
+			: base (context)
+		{
+			Dependencies.AddRange (packages);
+		}
+
+		protected override void InitializeDependencies ()
+		{}
+
+		protected override bool InitOS ()
+		{
+			if (!base.InitOS ())
+				return false;
+
+			return true;
+		}
+	};
+}

build-tools/xaprepare/xaprepare/OperatingSystems/Linux.cs

@@ -36,14 +36,16 @@ echo 1 > /proc/sys/fs/binfmt_misc/status
 			{"LinuxMint", (ctx) => new LinuxMint   (ctx)},
 			{"Arch",      (ctx) => new LinuxArch   (ctx)},
 			{"Fedora",    (ctx) => new LinuxFedora (ctx)},
+			{"Gentoo",    (ctx) => new LinuxGentoo (ctx)},
 		};
 
 		static readonly Dictionary<string, string> distroIdMap = new Dictionary<string, string> (StringComparer.OrdinalIgnoreCase) {
 			{"debian",    "Debian"},
 			{"ubuntu",    "Ubuntu"},
 			{"arch",      "Arch"},
 			{"linuxmint", "LinuxMint"},
-			{"fedora",    "Fedora"}
+			{"fedora",    "Fedora"},
+			{"gentoo",    "Gentoo"},
 		};
 
 		bool warnBinFmt;