Coverage for "await foreach" loops (issue coverlet-coverage#1104)

This commit attempts to fix code coverage for the "await foreach"
loop introduced in C# 8.  The presence of an "await foreach" loop
causes four new kinds of branch patterns to be generated in the
async state machine.  Three of these are to be eliminated, while
the fourth is actually the "should I stay in the loop or not?"
branch and is best left in a code coverage report.

CecilSymbolHelper now eliminates the three branch patterns that
need to be eliminated.  There are tests both in CecilSymbolHelperTests
as well as a new set of coverage tests in CoverageTests.AsyncForeach.cs.

src/coverlet.core/Symbols/CecilSymbolHelper.cs

@@ -396,6 +396,166 @@ private bool SkipGeneratedBranchesForExceptionHandlers(MethodDefinition methodDe
             return _compilerGeneratedBranchesToExclude[methodDefinition.FullName].Contains(instruction.Offset);
         }
 
+        private bool SkipGeneratedBranchesForAwaitForeach(List<Instruction> instructions, Instruction instruction)
+        {
+            // An "await foreach" causes four additional branches to be generated
+            // by the compiler.  We want to skip the last three, but we want to
+            // keep the first one.
+            //
+            // (1) At each iteration of the loop, a check that there is another
+            //     item in the sequence.  This is a branch that we want to keep,
+            //     because it's basically "should we stay in the loop or not?",
+            //     which is germane to code coverage testing.
+            // (2) A check near the end for whether the IAsyncEnumerator was ever
+            //     obtained, so it can be disposed.
+            // (3) A check for whether an exception was thrown in a most recent
+            //     loop iteration.
+            // (4) A check for whether the exception thrown in the most recent
+            //     loop iteration has (at least) the type System.Exception.
+            //
+            // If we're looking at any of three the last three of those four
+            // branches, we should be skipping it.
+
+            int currentIndex = instructions.BinarySearch(instruction, new InstructionByOffsetComparer());
+
+            return SkipGeneratedBranchForAwaitForeach_CheckForAsyncEnumerator(instructions, instruction, currentIndex) ||
+                   SkipGeneratedBranchForAwaitForeach_CheckIfExceptionThrown(instructions, instruction, currentIndex) ||
+                   SkipGeneratedBranchForAwaitForeach_CheckThrownExceptionType(instructions, instruction, currentIndex);
+        }
+
+        // The pattern for the "should we stay in the loop or not?", which we don't
+        // want to skip (so we have no method to try to find it), looks like this:
+        //
+        // IL_0111: ldloca.s 4
+        // IL_0113: call instance !0 valuetype [System.Private.CoreLib]System.Runtime.CompilerServices.ValueTaskAwaiter`1<bool>::GetResult()
+        // IL_0118: brtrue IL_0047
+        //
+        // In Debug mode, there are additional things that happen in between
+        // the "call" and branch, but it's the same idea either way: branch
+        // if GetResult() returned true.
+
+        private bool SkipGeneratedBranchForAwaitForeach_CheckForAsyncEnumerator(List<Instruction> instructions, Instruction instruction, int currentIndex)
+        {
+            // We're looking for the following pattern, which checks whether a
+            // compiler-generated field of type IAsyncEnumerator<> is null.
+            //
+            // IL_012c: ldfld class [System.Private.CoreLib]System.Collections.Generic.IAsyncEnumerator`1<int32> AwaitForeachStateMachine/'<AsyncAwait>d__0'::'<>7__wrap1'
+            // IL_0131: brfalse.s IL_0196
+
+            if (instruction.OpCode != OpCodes.Brfalse &&
+                instruction.OpCode != OpCodes.Brfalse_S)
+            {
+                return false;
+            }
+
+            if (currentIndex >= 1 &&
+                instructions[currentIndex - 1].OpCode == OpCodes.Ldfld &&
+                instructions[currentIndex - 1].Operand is FieldDefinition field &&
+                IsCompilerGenerated(field) && field.FieldType.FullName.StartsWith("System.Collections.Generic.IAsyncEnumerator"))
+            {
+                return true;
+            }
+
+            return false;
+        }
+
+        private bool SkipGeneratedBranchForAwaitForeach_CheckIfExceptionThrown(List<Instruction> instructions, Instruction instruction, int currentIndex)
+        {
+            // Here, we want to find a pattern where we're checking whether a
+            // compiler-generated field of type Object is null.  To narrow our
+            // search down and reduce the odds of false positives, we'll also
+            // expect a call to GetResult() to precede the loading of the field's
+            // value.  The basic pattern looks like this:
+            //
+            // IL_018f: ldloca.s 2
+            // IL_0191: call instance void [System.Private.CoreLib]System.Runtime.CompilerServices.ValueTaskAwaiter::GetResult()
+            // IL_0196: ldarg.0
+            // IL_0197: ldfld object AwaitForeachStateMachine/'<AsyncAwait>d__0'::'<>7__wrap2'
+            // IL_019c: stloc.s 6
+            // IL_019e: ldloc.s 6
+            // IL_01a0: brfalse.s IL_01b9
+            //
+            // Variants are possible (e.g., a "dup" instruction instead of a
+            // "stloc.s" and "ldloc.s" pair), so we'll just look for the
+            // highlights.
+
+            if (instruction.OpCode != OpCodes.Brfalse &&
+                instruction.OpCode != OpCodes.Brfalse_S)
+            {
+                return false;
+            }
+
+            // We expect the field to be loaded no more than thre instructions before
+            // the branch, so that's how far we're willing to search for it.
+            int minFieldIndex = Math.Max(0, currentIndex - 3);
+
+            for (int i = currentIndex - 1; i >= minFieldIndex; --i)
+            {
+                if (instructions[i].OpCode == OpCodes.Ldfld &&
+                    instructions[i].Operand is FieldDefinition field &&
+                    IsCompilerGenerated(field) && field.FieldType.FullName == "System.Object")
+                {
+                    // We expect the call to GetResult() to be no more than three
+                    // instructions before the loading of the field's value.
+                    int minCallIndex = Math.Max(0, i - 3);
+
+                    for (int j = i - 1; j >= minCallIndex; --j)
+                    {
+                        if (instructions[j].OpCode == OpCodes.Call &&
+                            instructions[j].Operand is MethodReference callRef &&
+                            callRef.DeclaringType.FullName.StartsWith("System.Runtime.CompilerServices") &&
+                            callRef.DeclaringType.FullName.Contains("TaskAwait") &&
+                            callRef.Name == "GetResult")
+                        {
+                            return true;
+                        }
+                    }
+                }
+            }
+
+            return false;
+        }
+
+        private bool SkipGeneratedBranchForAwaitForeach_CheckThrownExceptionType(List<Instruction> instructions, Instruction instruction, int currentIndex)
+        {
+            // In this case, we're looking for a branch generated by the compiler to
+            // check whether a previously-thrown exception has (at least) the type
+            // System.Exception, the pattern for which looks like this:
+            //
+            // IL_01db: ldloc.s 7
+            // IL_01dd: isinst [System.Private.CoreLib]System.Exception
+            // IL_01e2: stloc.s 9
+            // IL_01e4: ldloc.s 9
+            // IL_01e6: brtrue.s IL_01eb
+            //
+            // Once again, variants are possible here, such as a "dup" instruction in
+            // place of the "stloc.s" and "ldloc.s" pair, and we'll reduce the odds of
+            // a false positive by requiring a "ldloc.s" instruction to precede the
+            // "isinst" instruction.
+
+            if (instruction.OpCode != OpCodes.Brtrue &&
+                instruction.OpCode != OpCodes.Brtrue_S)
+            {
+                return false;
+            }
+
+            int minTypeCheckIndex = Math.Max(1, currentIndex - 3);
+
+            for (int i = currentIndex - 1; i >= minTypeCheckIndex; --i)
+            {
+                if (instructions[i].OpCode == OpCodes.Isinst &&
+                    instructions[i].Operand is TypeReference typeRef &&
+                    typeRef.FullName == "System.Exception" &&
+                    (instructions[i - 1].OpCode == OpCodes.Ldloc ||
+                     instructions[i - 1].OpCode == OpCodes.Ldloc_S))
+                {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
         // https://github.com/dotnet/roslyn/blob/master/docs/compilers/CSharp/Expression%20Breakpoints.md
         private bool SkipExpressionBreakpointsBranches(Instruction instruction) => instruction.Previous is not null && instruction.Previous.OpCode == OpCodes.Ldc_I4 &&
                                                                                     instruction.Previous.Operand is int operandValue && operandValue == 1 &&
@@ -461,7 +621,8 @@ public IReadOnlyList<BranchPoint> GetBranchPoints(MethodDefinition methodDefinit
                     if (isAsyncStateMachineMoveNext)
                     {
                         if (SkipGeneratedBranchesForExceptionHandlers(methodDefinition, instruction, instructions) ||
-                            SkipGeneratedBranchForExceptionRethrown(instructions, instruction))
+                            SkipGeneratedBranchForExceptionRethrown(instructions, instruction) ||
+                            SkipGeneratedBranchesForAwaitForeach(instructions, instruction))
                         {
                             continue;
                         }

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

@@ -0,0 +1,73 @@
+using System.Collections.Generic;
+using System.IO;
+using System.Threading.Tasks;
+
+using Coverlet.Core.Samples.Tests;
+using Coverlet.Tests.Xunit.Extensions;
+using Xunit;
+
+namespace Coverlet.Core.Tests
+{
+    public partial class CoverageTests
+    {
+        async private static IAsyncEnumerable<T> ToAsync<T>(IEnumerable<T> values)
+        {
+            foreach (T value in values)
+            {
+                yield return await Task.FromResult(value);
+            }
+        }
+
+        [Fact]
+        public void AsyncForeach()
+        {
+            string path = Path.GetTempFileName();
+            try
+            {
+                FunctionExecutor.Run(async (string[] pathSerialize) =>
+                {
+                    CoveragePrepareResult coveragePrepareResult = await TestInstrumentationHelper.Run<AsyncForeach>(instance =>
+                    {
+                        int res = ((ValueTask<int>)instance.SumWithATwist(ToAsync(new int[] { 1, 2, 3, 4, 5 }))).GetAwaiter().GetResult();
+                        res += ((ValueTask<int>)instance.Sum(ToAsync(new int[] { 1, 2, 3 }))).GetAwaiter().GetResult();
+                        res += ((ValueTask<int>)instance.SumEmpty()).GetAwaiter().GetResult();
+
+                        return Task.CompletedTask;
+                    }, persistPrepareResultToFile: pathSerialize[0]);
+                    return 0;
+                }, new string[] { path });
+
+                TestInstrumentationHelper.GetCoverageResult(path)
+                .Document("Instrumentation.AsyncForeach.cs")
+                .AssertLinesCovered(BuildConfiguration.Debug,
+                                    // SumWithATwist(IAsyncEnumerable<int>)
+                                    // Apparently due to entering and exiting the async state machine, line 17
+                                    // (the top of an "await foreach" loop) is reached three times *plus* twice
+                                    // per loop iteration.  So, in this case, with five loop iterations, we end
+                                    // up with 3 + 5 * 2 = 13 hits.
+                                    (14, 1), (15, 1), (17, 13), (18, 5), (19, 5), (20, 5), (21, 5), (22, 5),
+                                    (24, 0), (25, 0), (26, 0), (27, 5), (29, 1), (30, 1),
+                                    // Sum(IAsyncEnumerable<int>)
+                                    (34, 1), (35, 1), (37, 9), (38, 3), (39, 3), (40, 3), (42, 1), (43, 1),
+                                    // SumEmpty()
+                                    (47, 1), (48, 1), (50, 3), (51, 0), (52, 0), (53, 0), (55, 1), (56, 1)
+                                    )
+                .AssertBranchesCovered(BuildConfiguration.Debug,
+                                       // SumWithATwist(IAsyncEnumerable<int>)
+                                       (17, 2, 1), (17, 3, 5), (19, 0, 5), (19, 1, 0),
+                                       // Sum(IAsyncEnumerable<int>)
+                                       (37, 0, 1), (37, 1, 3),
+                                       // SumEmpty()
+                                       // If we never entered the loop, that's a branch not taken, which is
+                                       // what we want to see.
+                                       (50, 0, 1), (50, 1, 0)
+                                       )
+                .ExpectedTotalNumberOfBranches(BuildConfiguration.Debug, 4);
+            }
+            finally
+            {
+                File.Delete(path);
+            }
+        }
+    }
+}

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

@@ -0,0 +1,58 @@
+// Remember to use full name because adding new using directives change line numbers
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Coverlet.Core.Samples.Tests
+{
+    public class AsyncForeach
+    {
+        async public ValueTask<int> SumWithATwist(IAsyncEnumerable<int> ints)
+        {
+            int sum = 0;
+
+            await foreach (int i in ints)
+            {
+                if (i > 0)
+                {
+                    sum += i;
+                }
+                else
+                {
+                    sum = 0;
+                }
+            }
+
+            return sum;
+        }
+
+
+        async public ValueTask<int> Sum(IAsyncEnumerable<int> ints)
+        {
+            int sum = 0;
+
+            await foreach (int i in ints)
+            {
+                sum += i;
+            }
+
+            return sum;
+        }
+
+
+        async public ValueTask<int> SumEmpty()
+        {
+            int sum = 0;
+
+            await foreach (int i in AsyncEnumerable.Empty<int>())
+            {
+                sum += i;
+            }
+
+            return sum;
+        }
+    }
+}

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

@@ -197,6 +197,39 @@ async public ValueTask AsyncAwait()
         }
     }
 
+    public class AwaitForeachStateMachine
+    {
+        async public ValueTask AsyncAwait(IAsyncEnumerable<int> ints)
+        {
+            await foreach (int i in ints)
+            {
+                await default(ValueTask);
+            }
+        }
+    }
+
+    public class AwaitForeachStateMachine_WithBranches
+    {
+        async public ValueTask<int> SumWithATwist(IAsyncEnumerable<int> ints)
+        {
+            int sum = 0;
+
+            await foreach (int i in ints)
+            {
+                if (i > 0)
+                {
+                    sum += i;
+                }
+                else
+                {
+                    sum = 0;
+                }
+            }
+
+            return sum;
+        }
+    }
+
     [ExcludeFromCoverage]
     public class ClassExcludedByCoverletCodeCoverageAttr
     {