MSBuildSdkResolver: resolve symlink for 'dotnet' binary

On Linux the dotnet installed by the dotnet packages is a
symlink (/usr/bin/dotnet -> ../share/dotnet/dotnet).

We need to resolve the symlink so we're passing the path to
the actual .NET Core installation to libhostfxr.

Fixes https://github.com/dotnet/cli/issues/7125

src/Microsoft.DotNet.MSBuildSdkResolver/Interop.NETStandard.cs

@@ -7,6 +7,7 @@
 // on non-Windows machines.
 #if NETSTANDARD1_5
 
+using System;
 using System.Runtime.InteropServices;
 using System.Text;
 
@@ -16,6 +17,14 @@ namespace Microsoft.DotNet.MSBuildSdkResolver
     {
         internal static readonly bool RunningOnWindows = RuntimeInformation.IsOSPlatform(OSPlatform.Windows);
 
+        internal static string realpath(string path)
+        {
+            var ptr = unix_realpath(path, IntPtr.Zero);
+            var result = Marshal.PtrToStringAnsi(ptr); // uses UTF8 on Unix
+            unix_free(ptr);
+            return result;
+        }
+
         private static int hostfxr_resolve_sdk(string exe_dir, string working_dir, [Out] StringBuilder buffer, int buffer_size)
         {
             // hostfxr string encoding is platform -specific so dispatch to the 
@@ -31,6 +40,13 @@ namespace Microsoft.DotNet.MSBuildSdkResolver
         // CharSet.Ansi is UTF8 on Unix
         [DllImport("hostfxr", EntryPoint = nameof(hostfxr_resolve_sdk), CharSet = CharSet.Ansi, ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
         private static extern int unix_hostfxr_resolve_sdk(string exe_dir, string working_dir, [Out] StringBuilder buffer, int buffer_size);
+
+        // CharSet.Ansi is UTF8 on Unix
+        [DllImport("libc", EntryPoint = nameof(realpath), CharSet = CharSet.Ansi, ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
+        private static extern IntPtr unix_realpath(string path, IntPtr buffer);
+
+        [DllImport("libc", EntryPoint = "free", ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
+        private static extern void unix_free(IntPtr ptr);
     }
 }
 

src/Microsoft.DotNet.MSBuildSdkResolver/MSBuildSdkResolver.cs

@@ -174,8 +174,18 @@ namespace Microsoft.DotNet.MSBuildSdkResolver
             }
 
             var environmentProvider = new EnvironmentProvider(_getEnvironmentVariable);
+            var dotnetExe = environmentProvider.GetCommandPath("dotnet");
 
-            return Path.GetDirectoryName(environmentProvider.GetCommandPath("dotnet"));
+#if NETSTANDARD1_5
+            if (dotnetExe != null && !Interop.RunningOnWindows)
+            {
+                // e.g. on Linux the 'dotnet' command from PATH is a symlink so we need to
+                // resolve it to get the actual path to the binary
+                dotnetExe = Interop.realpath(dotnetExe) ?? dotnetExe;
+            }
+#endif
+
+            return Path.GetDirectoryName(dotnetExe);
         }
     }
 }