Tool produces a 0-byte binary when compilation fails

Fixes #35

src/Microsoft.AspNetCore.Mvc.Razor.ViewCompilation.Design/Internal/PrecompileRunCommand.cs

@@ -9,7 +9,6 @@ using System.Linq;
 using System.Reflection;
 using System.Text;
 using System.Threading.Tasks;
-using Microsoft.AspNetCore.Mvc.ApplicationParts;
 using Microsoft.AspNetCore.Mvc.Razor.Compilation;
 using Microsoft.AspNetCore.Mvc.Razor.Internal;
 using Microsoft.CodeAnalysis;
@@ -104,7 +103,11 @@ namespace Microsoft.AspNetCore.Mvc.Razor.ViewCompilation.Design.Internal
             var resources = GetResources(results);
 
             var assemblyPath = Path.Combine(OutputPathOption.Value(), precompileAssemblyName + ".dll");
-            var emitResult = EmitAssembly(compilation, assemblyPath, resources);
+            var emitResult = EmitAssembly(
+                compilation,
+                MvcServiceProvider.Compiler.EmitOptions,
+                assemblyPath,
+                resources);
 
             if (!emitResult.Success)
             {
@@ -143,23 +146,41 @@ namespace Microsoft.AspNetCore.Mvc.Razor.ViewCompilation.Design.Internal
             return resources;
         }
 
-        private EmitResult EmitAssembly(
+        public EmitResult EmitAssembly(
             CSharpCompilation compilation,
+            EmitOptions emitOptions,
             string assemblyPath,
             ResourceDescription[] resources)
         {
-            Directory.CreateDirectory(Path.GetDirectoryName(assemblyPath));
-
             EmitResult emitResult;
-            using (var assemblyStream = File.OpenWrite(assemblyPath))
+            using (var assemblyStream = new MemoryStream())
             {
-                using (var pdbStream = File.OpenWrite(Path.ChangeExtension(assemblyPath, ".pdb")))
+                using (var pdbStream = new MemoryStream())
                 {
                     emitResult = compilation.Emit(
                         assemblyStream,
                         pdbStream,
                         manifestResources: resources,
-                        options: MvcServiceProvider.Compiler.EmitOptions);
+                        options: emitOptions);
+
+                    if (emitResult.Success)
+                    {
+                        Directory.CreateDirectory(Path.GetDirectoryName(assemblyPath));
+                        var pdbPath = Path.ChangeExtension(assemblyPath, ".pdb");
+                        assemblyStream.Position = 0;
+                        pdbStream.Position = 0;
+
+                        // Avoid writing to disk unless the compilation is successful.
+                        using (var assemblyFileStream = File.OpenWrite(assemblyPath))
+                        {
+                            assemblyStream.CopyTo(assemblyFileStream);
+                        }
+
+                        using (var pdbFileStream = File.OpenWrite(pdbPath))
+                        {
+                            pdbStream.CopyTo(pdbFileStream);
+                        }
+                    }
                 }
             }
 

test/Microsoft.AspNetCore.Mvc.Razor.ViewCompilation.Design.Test/PrecompileRunCommandTest.cs

@@ -6,6 +6,9 @@ using System.IO;
 using System.Linq;
 using System.Reflection;
 using Microsoft.AspNetCore.Mvc.Razor.ViewCompilation.Design.Internal;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.Emit;
 using Xunit;
 
 namespace Microsoft.AspNetCore.Mvc.Razor.ViewCompilation.Tools
@@ -150,6 +153,29 @@ Options:
             Assert.Equal(expectedError, result.Error.Trim());
         }
 
+        [Fact]
+        public void EmitAssembly_DoesNotWriteAssembliesToDisk_IfCompilationFails()
+        {
+            // Arrange
+            var assemblyDirectory = Path.Combine(Path.GetTempPath(), Path.GetRandomFileName());
+            var assemblyPath = Path.Combine(assemblyDirectory, "out.dll");
+            var precompileRunCommand = new PrecompileRunCommand();
+            var syntaxTree = CSharpSyntaxTree.ParseText("using Microsoft.DoestNotExist");
+            var compilation = CSharpCompilation.Create("Test.dll", new[] { syntaxTree });
+
+            // Act
+            var emitResult = precompileRunCommand.EmitAssembly(
+                compilation,
+                new EmitOptions(),
+                assemblyPath,
+                new ResourceDescription[0]);
+
+            // Assert
+            Assert.False(emitResult.Success);
+            Assert.False(Directory.Exists(assemblyDirectory));
+            Assert.False(File.Exists(assemblyPath));
+        }
+
         private static string GetToolVersion()
         {
             return typeof(Program)