[xabuild] Find Microsoft.Portable.CSharp.targets

Builds on Windows were failing because
`Microsoft.Portable.CSharp.targets` couldn't be found:

	> bin\Debug\bin\xabuild /bl Xamarin.Android-Tests.sln
	...
	...\xamarin-android\external\Java.Interop\lib\mono.linq.expressions\Mono.Linq.Expressions.csproj(108,3):
	error MSB4019: The imported project "...\xamarin-android\bin\Debug\lib\xamarin.android\xbuild\Microsoft\Portable\v4.0\Microsoft.Portable.CSharp.targets"
	was not found. Confirm that the path in the <Import> declaration is
	correct, and that the file exists on disk.

Turns Out that there are *multiple* `Microsoft` directories of
consequence, and we need *all* of them to build, including:

  * `%ProgramFiles(x86)%\Microsoft Visual Studio\2019\*\MSBuild\Microsoft`
  * `%ProgramFiles(x86)%\MSBuild\Microsoft\Portable`

`xabuild.exe` would symlink the first one into
`bin\$(Configuration)\lib\xamarin.android\xbuild`, but not the latter,
and lack of the `Microsoft\Portable` directory prevented
`Microsoft.Portable.CSharp.targets` from being found.

Update `xabuild.exe` to know about
`%ProgramFiles(x86)%\MSBuild\Microsoft`.

Furthermore, because we now have two `Microsoft` directories,
`Microsoft` must now become a "normal" directory, which contains
symlinks to *all* the known child directories.  To support this,
"invert" the `symbolicLinks` dictionary so that the keys are the
in-tree paths, making it easier to detect when we need to do such
merges.

Finally, add code to detect if an existing "conflicting" directory
(such as `Microsoft`) is a symlink, and remove it if necessary.
This should allow `git pull` + rebuild scenarios to work.

tools/xabuild/SymbolicLink.cs

@@ -10,6 +10,7 @@ static class SymbolicLink
 		public static bool Create (string source, string target)
 		{
 			if (!Directory.Exists (source)) {
+				Directory.CreateDirectory (Path.GetDirectoryName (source));
 				if (Environment.OSVersion.Platform == PlatformID.Win32NT) {
 					return CreateWindowsSymLink (source, target);
 				} else {
@@ -50,6 +51,74 @@ enum SymbolLinkFlag {
 			AllowUnprivilegedCreate = 2,
 		}
 
+		public static string GetRealPath (string path)
+		{
+			if (string.IsNullOrEmpty (path))
+				return null;
+
+			if (Environment.OSVersion.Platform == PlatformID.Win32NT) {
+				return GetWindowsRealPath (path);
+			} else {
+				return GetUnixRealPath (path);
+			}
+		}
+
+		static string GetWindowsRealPath (string path)
+		{
+			const FileAttributes    FILE_FLAG_BACKUP_SEMANTICS  = (FileAttributes) 0x02000000;
+			const FileAccess        GENERIC_READ                = unchecked((FileAccess) 0x80000000);
+			IntPtr handle = CreateFileW (lpFileName: path,
+					dwDesiredAccess:        GENERIC_READ,
+					dwShareMode:            FileShare.Read,
+					lpSecurityAttributes:   IntPtr.Zero,
+					dwCreationDisposition:  FileMode.Open,
+					dwFlagsAndAttributes:   FILE_FLAG_BACKUP_SEMANTICS,
+					hTemplateFile:          IntPtr.Zero);
+			if (handle == INVALID_FILE_HANDLE)
+				return null;
+			IntPtr finalPathBuf = IntPtr.Zero;
+			try  {
+				const FinalPathFlags flags = FinalPathFlags.FILE_NAME_OPENED;
+				uint len = GetFinalPathNameByHandleW (handle, IntPtr.Zero, 0, flags);
+				if (len == 0)
+					return null;
+				len = checked(len + 1);
+				finalPathBuf    = Marshal.AllocHGlobal (checked ((int) (sizeof (char)*(len))));
+				uint checkLen   = GetFinalPathNameByHandleW (handle, finalPathBuf, len, flags);
+				if (checkLen == 0 || checkLen > len) {
+					Console.Error.WriteLine ($"GetFinalPathNameByHandleW: expected {len}, got {checkLen}. Last Error: {Marshal.GetLastWin32Error()}");
+					return null;
+				}
+				const string LocalUncPathPrefix = @"\\?\";
+				string finalPath  = Marshal.PtrToStringUni (finalPathBuf);
+				if (finalPath?.StartsWith (LocalUncPathPrefix, StringComparison.Ordinal) ?? false)
+					finalPath = finalPath.Substring (LocalUncPathPrefix.Length);
+				return finalPath;
+			}
+			finally {
+				Marshal.FreeHGlobal (finalPathBuf);
+				CloseHandle (handle);
+			}
+		}
+
+		static string GetUnixRealPath (string path)
+		{
+			IntPtr buf = realpath (path, IntPtr.Zero);
+			try {
+				if (buf == IntPtr.Zero)
+					return null;
+				return Marshal.PtrToStringAnsi (buf);
+			}
+			finally {
+				free (buf);
+			}
+		}
+
+		public static bool IsPathSymlink (string path)
+		{
+			return Path.GetFullPath (path) != GetRealPath (path);
+		}
+
 		[DllImport ("kernel32.dll")]
 		[return: MarshalAs (UnmanagedType.I1)]
 		static extern bool CreateSymbolicLink (string lpSymlinkFileName, string lpTargetFileName, SymbolLinkFlag dwFlags);
@@ -59,5 +128,44 @@ enum SymbolLinkFlag {
 
 		[DllImport ("libc")]
 		static extern void perror (string s);
+
+		[DllImport ("libc")]
+		static extern IntPtr realpath (string file_name, IntPtr resolved_name);
+
+		[DllImport ("libc")]
+		static extern void free (IntPtr p);
+
+		static readonly IntPtr INVALID_FILE_HANDLE = new IntPtr (-1);
+
+		[DllImport("kernel32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
+		public static extern IntPtr CreateFileW(
+			[MarshalAs(UnmanagedType.LPWStr)] string lpFileName,
+			[MarshalAs(UnmanagedType.U4)] FileAccess dwDesiredAccess,
+			[MarshalAs(UnmanagedType.U4)] FileShare dwShareMode,
+			IntPtr lpSecurityAttributes,
+			[MarshalAs(UnmanagedType.U4)] FileMode dwCreationDisposition,
+			[MarshalAs(UnmanagedType.U4)] FileAttributes dwFlagsAndAttributes,
+			IntPtr hTemplateFile);
+
+		[DllImport("kernel32.dll", SetLastError=true)]
+		[return: MarshalAs(UnmanagedType.Bool)]
+		static extern bool CloseHandle(IntPtr hObject);
+
+		[DllImport("Kernel32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
+		static extern uint GetFinalPathNameByHandleW(
+			IntPtr hFile,
+			IntPtr lpszFilePath,
+			uint cchFilePath,
+			FinalPathFlags dwFlags);
+	}
+
+	[Flags]
+	enum FinalPathFlags : uint {
+		VOLUME_NAME_DOS      = 0x0,
+		FILE_NAME_NORMALIZED = 0x0,
+		VOLUME_NAME_GUID     = 0x1,
+		VOLUME_NAME_NT       = 0x2,
+		VOLUME_NAME_NONE     = 0x4,
+		FILE_NAME_OPENED     = 0x8,
 	}
 }

tools/xabuild/XABuild.cs

@@ -38,22 +38,39 @@ static int Main ()
 				// Create a Microsoft.Build.NuGetSdkResolver.xml
 				CreateSdkResolverConfig (paths);
 
-				//Symbolic links to be created: key=system, value=in-tree
+				//Symbolic links to be created: key=in-tree-dir, value=system-dir
 				var symbolicLinks = new Dictionary<string, string> ();
 				foreach (var dir in Directory.EnumerateDirectories (paths.SystemFrameworks)) {
 					if (Path.GetFileName (dir) != "MonoAndroid") {
-						symbolicLinks [dir] = Path.Combine (paths.FrameworksDirectory, Path.GetFileName (dir));
+						var inTreeFramework = Path.Combine (paths.FrameworksDirectory, Path.GetFileName (dir));
+						symbolicLinks [inTreeFramework]	= dir;
 					}
 				}
 				foreach (var dir in paths.SystemTargetsDirectories) {
-					symbolicLinks [dir] = Path.Combine (paths.MSBuildExtensionsPath, Path.GetFileName (dir));
+					var inTreeTargetsDir  = Path.Combine (paths.MSBuildExtensionsPath, Path.GetFileName (dir));
+					if (!symbolicLinks.ContainsKey (inTreeTargetsDir)) {
+						symbolicLinks [inTreeTargetsDir] = dir;
+						continue;
+					}
+					var prevTargetDir = symbolicLinks [inTreeTargetsDir];
+					symbolicLinks.Remove (inTreeTargetsDir);
+					if (Directory.Exists (inTreeTargetsDir) && SymbolicLink.IsPathSymlink (inTreeTargetsDir)) {
+						Console.WriteLine ($"# jonp: Removing path? {inTreeTargetsDir}");
+						Directory.Delete (inTreeTargetsDir);
+					}
+					var subTargetDirs = Directory.EnumerateDirectories (prevTargetDir)
+						.Concat (Directory.EnumerateDirectories (dir));
+					foreach (var subDir in subTargetDirs) {
+						var inTreeTargetSubdir = Path.Combine (inTreeTargetsDir, Path.GetFileName (subDir));
+						symbolicLinks [inTreeTargetSubdir] = subDir;
+					}
 				}
-				if (symbolicLinks.Values.Any (d => !Directory.Exists (d))) {
+				if (symbolicLinks.Keys.Any (d => !Directory.Exists (d))) {
 					//Hold open the file while creating the symbolic links
 					using (var writer = OpenSysLinksFile (paths)) {
 						foreach (var pair in symbolicLinks) {
-							var systemDirectory = pair.Key;
-							var symbolicLink = pair.Value;
+							var systemDirectory = pair.Value;
+							var symbolicLink = pair.Key;
 							Console.WriteLine ($"[xabuild] creating symbolic link '{symbolicLink}' -> '{systemDirectory}'");
 							if (!SymbolicLink.Create (symbolicLink, systemDirectory)) {
 								return 1;

tools/xabuild/XABuildPaths.cs

@@ -147,7 +147,11 @@ public XABuildPaths ()
 				MSBuildSdksPath          = DotNetSdkPath ?? Path.Combine (MSBuildPath, "Sdks");
 				SystemFrameworks         = Path.Combine (programFiles, "Reference Assemblies", "Microsoft", "Framework");
 				string msbuildDir        = Path.GetDirectoryName (MSBuildBin);
-				SystemTargetsDirectories = new [] { msbuildDir, Path.Combine (MSBuildPath, "Microsoft") };
+				SystemTargetsDirectories = new [] {
+					msbuildDir,
+					Path.Combine (MSBuildPath, "Microsoft"),
+					Path.Combine (programFiles, "MSBuild", "Microsoft"),
+				};
 				SearchPathsOS            = "windows";
 				string nuget             = Path.Combine (MSBuildPath, "Microsoft", "NuGet", "16.0");
 				if (!Directory.Exists (nuget)) {