Review extensionless Windows executable resolution

src/Fallout.Tooling/ToolPathResolver.cs

@@ -15,6 +15,12 @@ public static string TryGetEnvironmentExecutable(string environmentExecutable)
             return null;
         }
 
+        if (!File.Exists(environmentExecutablePath) && EnvironmentInfo.IsWin &&
+            !Path.HasExtension(environmentExecutablePath) && File.Exists(environmentExecutablePath + ".exe"))
+        {
+            environmentExecutablePath += ".exe";
+        }
+
         Assert.FileExists(environmentExecutablePath,
             $"Path '{environmentExecutablePath}' from environment variable '{environmentExecutable}' does not exist");
 

tests/Fallout.Tooling.Specs/EnvironmentExecutableSpecs.cs

@@ -0,0 +1,62 @@
+using System;
+using Fallout.Common.IO;
+using Fallout.Common.Tooling;
+using FluentAssertions;
+using Xunit;
+
+namespace Fallout.Common.Specs;
+
+[Collection(ToolPathResolverStateCollection.Name)]
+public sealed class EnvironmentExecutableSpecs : IDisposable
+{
+    private readonly AbsolutePath root = AbsolutePath.Temp("environment-executables").CreateDirectory();
+    private readonly string variable = "FALLOUT_SPEC_TOOL_" + Guid.NewGuid().ToString("N");
+
+    [Fact]
+    public void Existing_paths_with_spaces_keep_their_exact_value()
+    {
+        var path = root / "directory with spaces" / "tool";
+        path.Parent.CreateDirectory();
+        path.WriteAllText(string.Empty);
+        ((AbsolutePath)(path + ".exe")).WriteAllText(string.Empty);
+        Environment.SetEnvironmentVariable(variable, path);
+
+        ToolPathResolver.TryGetEnvironmentExecutable(variable).Should().Be(path);
+    }
+
+    [Fact]
+    public void Extensionless_paths_resolve_exe_files_only_on_windows()
+    {
+        var path = root / "directory with spaces" / "tool";
+        path.Parent.CreateDirectory();
+        ((AbsolutePath)(path + ".exe")).WriteAllText(string.Empty);
+        Environment.SetEnvironmentVariable(variable, path);
+
+        if (EnvironmentInfo.IsWin)
+        {
+            ToolPathResolver.TryGetEnvironmentExecutable(variable).Should().Be(path + ".exe");
+        }
+        else
+        {
+            var resolve = () => ToolPathResolver.TryGetEnvironmentExecutable(variable);
+            resolve.Should().Throw<Exception>().WithMessage("*does not exist*");
+        }
+    }
+
+    [Fact]
+    public void Missing_explicit_paths_still_fail()
+    {
+        (root / "missing.exe.exe").WriteAllText(string.Empty);
+        Environment.SetEnvironmentVariable(variable, root / "missing.exe");
+
+        var resolve = () => ToolPathResolver.TryGetEnvironmentExecutable(variable);
+
+        resolve.Should().Throw<Exception>().WithMessage("*does not exist*");
+    }
+
+    public void Dispose()
+    {
+        Environment.SetEnvironmentVariable(variable, null);
+        root.DeleteDirectory();
+    }
+}