[gh-216] Fixed edge-case bug in exception rewriter.

source/Server/Execution/Internal/CecilExtensions.cs

@@ -4,6 +4,10 @@ using Mono.Cecil.Cil;
 
 namespace SharpLab.Server.Execution.Internal {
     internal static class CecilExtensions {
+        public static bool IsLeave(this Code code) {
+            return code == Code.Leave || code == Code.Leave_S;
+        }
+
         public static Instruction CreateLdargBest(this ILProcessor il, ParameterReference parameter) {
             var index = parameter.Index;
             if (il.Body.Method.HasThis)

source/Server/Execution/Internal/FlowReportingRewriter.cs

@@ -169,20 +169,29 @@ namespace SharpLab.Server.Execution.Internal {
         }
 
         private void RewriteFinally(ExceptionHandler handler, ref int handlerIndex, ILProcessor il, ReportMethods flow) {
-            var oldTryLeave = handler.TryEnd.Previous;
+            // for try/finally, the only thing we can do is to
+            // wrap internals of try into a new try+filter+catch
+            var outerTryLeave = handler.TryEnd.Previous;
+            if (!outerTryLeave.OpCode.Code.IsLeave()) {
+                // in some cases (e.g. exception throw) outer handler does
+                // not end with `leave` -- but we do need it once we wrap
+                // that throw
+                outerTryLeave = il.Create(OpCodes.Leave, handler.TryEnd);
+                il.InsertBefore(handler.TryEnd, outerTryLeave);
+            }
 
-            var newTryLeave = il.Create(OpCodes.Leave_S, (Instruction)oldTryLeave.Operand);
+            var innerTryLeave = il.Create(OpCodes.Leave_S, (Instruction)outerTryLeave.Operand);
             var reportCall = il.CreateCall(flow.ReportException);
             var catchHandler = il.Create(OpCodes.Pop);
 
-            InsertBeforeAndRetargetAll(il, oldTryLeave, newTryLeave);
-            il.InsertBefore(oldTryLeave, reportCall);
-            il.InsertBefore(oldTryLeave, il.Create(OpCodes.Ldc_I4_0));
-            il.InsertBefore(oldTryLeave, il.Create(OpCodes.Endfilter));
-            il.InsertBefore(oldTryLeave, catchHandler);
-            
+            InsertBeforeAndRetargetAll(il, outerTryLeave, innerTryLeave);
+            il.InsertBefore(outerTryLeave, reportCall);
+            il.InsertBefore(outerTryLeave, il.Create(OpCodes.Ldc_I4_0));
+            il.InsertBefore(outerTryLeave, il.Create(OpCodes.Endfilter));
+            il.InsertBefore(outerTryLeave, catchHandler);
+
             for (var i = 0; i < handlerIndex; i++) {
-                il.Body.ExceptionHandlers[i].RetargetAll(oldTryLeave.Next, newTryLeave.Next);
+                il.Body.ExceptionHandlers[i].RetargetAll(outerTryLeave.Next, innerTryLeave.Next);
             }
 
             il.Body.ExceptionHandlers.Insert(handlerIndex, new ExceptionHandler(ExceptionHandlerType.Filter) {
@@ -190,7 +199,7 @@ namespace SharpLab.Server.Execution.Internal {
                 TryEnd = reportCall,
                 FilterStart = reportCall,
                 HandlerStart = catchHandler,
-                HandlerEnd = oldTryLeave.Next
+                HandlerEnd = outerTryLeave.Next
             });
             handlerIndex += 1;
         }

source/Tests/ExecutionTests.cs

@@ -29,6 +29,7 @@ namespace SharpLab.Tests {
         [InlineData("Exception.DivideByZero.Catch.Finally.cs", 5, "DivideByZeroException")]
         [InlineData("Exception.DivideByZero.Catch.Finally.WriteLine.cs", 5, "DivideByZeroException", Optimize.Debug)]
         [InlineData("Exception.DivideByZero.Catch.Finally.WriteLine.cs", 5, "DivideByZeroException", Optimize.Release)]
+        [InlineData("Exception.Throw.New.Finally.cs", 8, "Exception", Optimize.Debug)]
         public async Task SlowUpdate_ReportsExceptionInFlow(string resourceName, int expectedLineNumber, string expectedExceptionTypeName, string optimize = Optimize.Debug) {
             var driver = await NewTestDriverAsync(LoadCodeFromResource(resourceName), optimize: optimize);
 

source/Tests/TestCode/Execution/Exception.Throw.New.Finally.cs

@@ -0,0 +1,14 @@
+using System;
+public class C
+{
+    public static void Main()
+    {
+        try
+        {
+            throw new Exception();
+        }
+        finally
+        {
+        }
+    }
+}