Check for Muxer path relative to base directory (#47664)

When determining the muxer (dotnet) path, search relative to the app base directory first. In most scenarios, this should correspond to the dotnet.dll directory of <root>/sdk/<version> where the dotnet executable is under <root>. If it doesn't exist, fall back to the existing logic.

With custom SDK search paths in global.json, the running dotnet process may not be the same as that corresponding to the running SDK. This updated logic finds the dotnet corresponding to the running SDK such that it can be properly propagated to anything launched via SDK commands (for example, build).

Author: elinor-fung

src/Cli/Microsoft.DotNet.Cli.Utils/Muxer.cs

@@ -36,31 +36,46 @@ public string MuxerPath
 
         public Muxer()
         {
-            // Best-effort search for muxer.
-            // SDK sets DOTNET_HOST_PATH as absolute path to current dotnet executable
+            // Most scenarios are running dotnet.dll as the app
+            // Root directory with muxer should be two above app base: <root>/sdk/<version>
+            string? rootPath = Path.GetDirectoryName(Path.GetDirectoryName(AppContext.BaseDirectory.TrimEnd(Path.DirectorySeparatorChar)));
+            if (rootPath is not null)
+            {
+                string muxerPathMaybe = Path.Combine(rootPath, $"{MuxerName}{FileNameSuffixes.CurrentPlatform.Exe}");
+                if (File.Exists(muxerPathMaybe))
+                {
+                    _muxerPath = muxerPathMaybe;
+                }
+            }
+
+            if (_muxerPath is null)
+            {
+                // Best-effort search for muxer.
+                // SDK sets DOTNET_HOST_PATH as absolute path to current dotnet executable
 #if NET6_0_OR_GREATER
-            string? processPath = Environment.ProcessPath;
+                string? processPath = Environment.ProcessPath;
 #else
-            string processPath = Process.GetCurrentProcess().MainModule.FileName;
+                string processPath = Process.GetCurrentProcess().MainModule.FileName;
 #endif
 
-            // The current process should be dotnet in most normal scenarios except when dotnet.dll is loaded in a custom host like the testhost
-            if (processPath is not null && !Path.GetFileNameWithoutExtension(processPath).Equals("dotnet", StringComparison.OrdinalIgnoreCase))
-            {
-	            // SDK sets DOTNET_HOST_PATH as absolute path to current dotnet executable
-	            processPath = Environment.GetEnvironmentVariable("DOTNET_HOST_PATH");
-	            if (processPath is null)
-	            {
-	                // fallback to DOTNET_ROOT which typically holds some dotnet executable
-	                var root = Environment.GetEnvironmentVariable("DOTNET_ROOT");
-	                if (root is not null)
-	                {
-	                    processPath = Path.Combine(root, $"dotnet{Constants.ExeSuffix}");
-	                }
-	            }
-            }
+                // The current process should be dotnet in most normal scenarios except when dotnet.dll is loaded in a custom host like the testhost
+                if (processPath is not null && !Path.GetFileNameWithoutExtension(processPath).Equals("dotnet", StringComparison.OrdinalIgnoreCase))
+                {
+                    // SDK sets DOTNET_HOST_PATH as absolute path to current dotnet executable
+                    processPath = Environment.GetEnvironmentVariable("DOTNET_HOST_PATH");
+                    if (processPath is null)
+                    {
+                        // fallback to DOTNET_ROOT which typically holds some dotnet executable
+                        var root = Environment.GetEnvironmentVariable("DOTNET_ROOT");
+                        if (root is not null)
+                        {
+                            processPath = Path.Combine(root, $"dotnet{Constants.ExeSuffix}");
+                        }
+                    }
+                }
 
-            _muxerPath = processPath;
+                _muxerPath = processPath;
+            }
         }
 
         public static string? GetDataFromAppDomain(string propertyName)