Migrate FormatUrl to multithreaded execution model

src/Framework.UnitTests/AbsolutePath_Tests.cs

@@ -308,5 +308,23 @@ public void AbsolutePath_NotRooted_ShouldThrowWithLocalizedMessage()
             var exception = Should.Throw<ArgumentException>(() => new AbsolutePath("relative/path"));
             exception.Message.ShouldContain("Path must be rooted");
         }
+
+        [WindowsOnlyFact]
+        public void AbsolutePath_WhitespacePath_ShouldThrowOnWindows()
+        {
+            var basePath = GetTestBasePath();
+
+            Should.Throw<ArgumentException>(() => new AbsolutePath(" ", basePath));
+        }
+
+        [UnixOnlyFact]
+        public void AbsolutePath_WhitespacePath_ShouldNotThrowOnUnix()
+        {
+            var basePath = GetTestBasePath();
+
+            // Whitespace is a valid filename character on Unix — should not throw.
+            var absolutePath = new AbsolutePath(" ", basePath);
+            absolutePath.Value.ShouldNotBeNullOrEmpty();
+        }
     }
 }

src/Framework/PathHelpers/AbsolutePath.cs

@@ -106,6 +106,16 @@ public AbsolutePath(string path, AbsolutePath basePath)
         {
             ArgumentException.ThrowIfNullOrEmpty(path);
 
+            // Before MSBuild's migration from multi-process to multi-threaded, Path.GetFullPath(" ") threw
+            // ArgumentException on Windows because whitespace-only is not a legal path. After combining with
+            // a rooted base directory, Path.GetFullPath silently trims the trailing whitespace, masking the
+            // error. Preserve the historical Windows contract by explicitly rejecting whitespace-only input.
+            // Unix retains the historical accepting behavior because whitespace is a valid filename character there.
+            if (NativeMethods.IsWindows && string.IsNullOrWhiteSpace(path))
+            {
+                throw new ArgumentException(SR.WhitespacePathNotAllowedOnWindows, nameof(path));
+            }
+
             // This function should not throw when path has illegal characters.
             // For .NET Framework, Microsoft.IO.Path.Combine should be used instead of System.IO.Path.Combine to achieve it.
             // For .NET Core, System.IO.Path.Combine already does not throw in this case.

src/Framework/Resources/SR.resx

@@ -153,6 +153,9 @@
   <data name="PathMustBeRooted" xml:space="preserve">
     <value>Path must be rooted.</value>
   </data>
+  <data name="WhitespacePathNotAllowedOnWindows" xml:space="preserve">
+    <value>Path cannot be null or whitespace on Windows.</value>
+  </data>
   <data name="PathTooLong" xml:space="preserve">
     <value>Path: {0} exceeds the OS max path limit. The fully qualified file name must be less than {1} characters.</value>
   </data>