Wrong branch coverage with EnumeratorCancellation attribute

Documentation/Changelog.md

@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## Unreleased
 
 ### Fixed
+-Fix wrong branch coverage with EnumeratorCancellation attribute [#1275](https://github.com/coverlet-coverage/coverlet/issues/1275)
 -Fix negative coverage exceeding int.MaxValue [#1266](https://github.com/coverlet-coverage/coverlet/issues/1266)  
 -Fix summary output format for culture de-DE [#1263](https://github.com/coverlet-coverage/coverlet/issues/1263)  
 -Fix branch coverage issue for finally block with await [#1233](https://github.com/coverlet-coverage/coverlet/issues/1233)  

src/coverlet.core/Symbols/CecilSymbolHelper.cs

@@ -855,7 +855,6 @@ instructions[i].Operand is FieldDefinition field &&
                 return false;
             }
 
-
             static bool DisposeCheck(List<Instruction> instructions, Instruction instruction, int currentIndex)
             {
                 // Within the compiler-generated async iterator, there are at least a
@@ -891,6 +890,40 @@ static bool DisposeCheck(List<Instruction> instructions, Instruction instruction
             }
         }
 
+        private bool SkipGeneratedBranchesForEnumeratorCancellationAttribute(List<Instruction> instructions, Instruction instruction)
+        {
+            // For async-enumerable methods an additional cancellation token despite the default one can be passed.
+            // The EnumeratorCancellation attribute marks the parameter whose value is received by GetAsyncEnumerator(CancellationToken).
+            // Therefore the compiler generates the field x__combinedTokens and generates some additional branch points.
+            //
+            // IL_0118: ldarg.0
+            // IL_0119: ldfld     class [System.Runtime]System.Threading.CancellationTokenSource Issue1275.AwaitForeachReproduction/'<AsyncEnumerable>d__1'::'<>x__combinedTokens'
+            // IL_011E: brfalse.s IL_0133
+            //
+            // We'll eliminate these wherever they appear.  It's reasonable to just look for a "brfalse" or "brfalse.s" instruction, preceded
+            // immediately by "ldfld" of the compiler-generated "<>x__combinedTokens" field.
+
+            int branchIndex = instructions.BinarySearch(instruction, new InstructionByOffsetComparer());
+
+            if (instruction.OpCode != OpCodes.Brfalse &&
+                instruction.OpCode != OpCodes.Brfalse_S)
+            {
+                return false;
+            }
+
+            if (branchIndex >= 2 &&
+                instructions[branchIndex - 1].OpCode == OpCodes.Ldfld &&
+                instructions[branchIndex - 1].Operand is FieldDefinition field &&
+                field.FieldType.FullName.Equals("System.Threading.CancellationTokenSource") &&
+                field.FullName.EndsWith("x__combinedTokens") &&
+                (instructions[branchIndex - 2].OpCode == OpCodes.Ldarg ||
+                 instructions[branchIndex - 2].OpCode == OpCodes.Ldarg_0))
+            {
+                return true;
+            }
+            return false;
+        }
+
         // https://github.com/dotnet/roslyn/blob/master/docs/compilers/CSharp/Expression%20Breakpoints.md
         private static bool SkipExpressionBreakpointsBranches(Instruction instruction) => instruction.Previous is not null && instruction.Previous.OpCode == OpCodes.Ldc_I4 &&
                                                                                           instruction.Previous.Operand is int operandValue && operandValue == 1 &&
@@ -973,6 +1006,11 @@ public IReadOnlyList<BranchPoint> GetBranchPoints(MethodDefinition methodDefinit
                         }
                     }
 
+                    if (SkipGeneratedBranchesForEnumeratorCancellationAttribute(instructions, instruction))
+                    {
+                        continue;
+                    }
+
                     if (SkipBranchGeneratedExceptionFilter(instruction, methodDefinition))
                     {
                         continue;

test/coverlet.core.tests/Coverage/CoverageTests.AsyncAwait.cs

@@ -1,5 +1,6 @@
 using System.IO;
 using System.Linq;
+using System.Threading;
 using System.Threading.Tasks;
 
 using Coverlet.Core.Samples.Tests;
@@ -182,5 +183,35 @@ public void AsyncAwait_Issue_1233()
                 File.Delete(path);
             }
         }
+
+        [Fact]
+        public void AsyncAwait_Issue_1275()
+        {
+            string path = Path.GetTempFileName();
+            try
+            {
+                FunctionExecutor.Run(async (string[] pathSerialize) =>
+                {
+                    CoveragePrepareResult coveragePrepareResult = await TestInstrumentationHelper.Run<Issue_1275>(instance =>
+                        {
+                            var cts = new CancellationTokenSource();
+                            ((Task)instance.Execute(cts.Token)).ConfigureAwait(false).GetAwaiter().GetResult();
+                            return Task.CompletedTask;
+                        },
+                        persistPrepareResultToFile: pathSerialize[0]);
+
+                    return 0;
+                }, new string[] { path });
+
+                var document = TestInstrumentationHelper.GetCoverageResult(path).Document("Instrumentation.AsyncAwait.cs");
+                document.AssertLinesCoveredFromTo(BuildConfiguration.Debug, 170, 176);
+                document.AssertBranchesCovered(BuildConfiguration.Debug, (171, 0, 1), (171, 1, 1));
+                Assert.DoesNotContain(document.Branches, x => x.Key.Line == 176);
+            }
+            finally
+            {
+                File.Delete(path);
+            }
+        }
     }
 }

test/coverlet.core.tests/Samples/Instrumentation.AsyncAwait.cs

@@ -151,4 +151,28 @@ async public Task Test()
             }
         }
     }
+
+    public class Issue_1275
+    {
+        public async Task<int> Execute(System.Threading.CancellationToken token)
+        {
+            int sum = 0;
+
+            await foreach (int result in AsyncEnumerable(token))
+            {
+                sum += result;
+            }
+
+            return sum;
+        }
+
+        async System.Collections.Generic.IAsyncEnumerable<int> AsyncEnumerable([System.Runtime.CompilerServices.EnumeratorCancellation] System.Threading.CancellationToken cancellationToken)
+        {
+            for (int i = 0; i < 1; i++)
+            {
+                await Task.Delay(1, cancellationToken);
+                yield return i;
+            }
+        }
+    }
 }