More tests for FileStream edge cases (#49754)

* add StreamConformanceTests to the solution file so we can read source code from VS

* add FileOffsetIsPreservedWhenFileStreamIsCreatedFromSafeFileHandle tests

* add WriteAsyncStartsWherePreviousReadAsyncHasFinished test

src/libraries/System.IO.FileSystem/System.IO.FileSystem.sln

@@ -27,6 +27,8 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "src", "src", "{D9FB1730-B75
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "System.IO.FileSystem.Legacy.Tests", "tests\LegacyTests\System.IO.FileSystem.Legacy.Tests.csproj", "{48E07F12-8597-40DE-8A37-CCBEB9D54012}"
 EndProject
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "StreamConformanceTests", "..\Common\tests\StreamConformanceTests\StreamConformanceTests.csproj", "{FEF03BCC-509F-4646-9132-9DE27FA3DA6F}"
+EndProject
 Global
 	GlobalSection(NestedProjects) = preSolution
 		{D350D6E7-52F1-40A4-B646-C178F6BBB689} = {1A727AF9-4F39-4109-BB8F-813286031DC9}
@@ -40,6 +42,7 @@ Global
 		{877E39A8-51CB-463A-AF4C-6FAE4F438075} = {D9FB1730-B750-4C0D-8D24-8C992DEB6034}
 		{D7DF8034-3AE5-4DEF-BCC4-6353239391BF} = {D9FB1730-B750-4C0D-8D24-8C992DEB6034}
 		{48E07F12-8597-40DE-8A37-CCBEB9D54012} = {1A727AF9-4F39-4109-BB8F-813286031DC9}
+		{FEF03BCC-509F-4646-9132-9DE27FA3DA6F} = {1A727AF9-4F39-4109-BB8F-813286031DC9}
 	EndGlobalSection
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -90,6 +93,10 @@ Global
 		{48E07F12-8597-40DE-8A37-CCBEB9D54012}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{48E07F12-8597-40DE-8A37-CCBEB9D54012}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{48E07F12-8597-40DE-8A37-CCBEB9D54012}.Release|Any CPU.Build.0 = Release|Any CPU
+		{FEF03BCC-509F-4646-9132-9DE27FA3DA6F}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{FEF03BCC-509F-4646-9132-9DE27FA3DA6F}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{FEF03BCC-509F-4646-9132-9DE27FA3DA6F}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{FEF03BCC-509F-4646-9132-9DE27FA3DA6F}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE

src/libraries/System.IO.FileSystem/tests/FileStream/FileStreamConformanceTests.cs

@@ -3,6 +3,7 @@
 
 using Microsoft.Win32.SafeHandles;
 using System.IO.Pipes;
+using System.Linq;
 using System.Threading.Tasks;
 using Xunit;
 
@@ -29,6 +30,74 @@ private Task<Stream> CreateStream(byte[] initialData, FileAccess access)
         protected override Task<Stream> CreateWriteOnlyStreamCore(byte[] initialData) => CreateStream(initialData, FileAccess.Write);
 
         protected override bool NopFlushCompletesSynchronously => OperatingSystem.IsWindows();
+
+        [Theory]
+        [MemberData(nameof(AllReadWriteModes))]
+        public async Task FileOffsetIsPreservedWhenFileStreamIsCreatedFromSafeFileHandle_Reads(ReadWriteMode mode)
+        {
+            byte[] initialData = new byte[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
+            using FileStream stream = (FileStream)await CreateReadOnlyStreamCore(initialData);
+            byte[] buffer = new byte[5];
+            int bytesRead = await ReadAsync(mode, stream, buffer, 0, buffer.Length);
+
+            Assert.Equal(bytesRead, stream.Position);
+
+            using FileStream createdFromHandle = new FileStream(stream.SafeFileHandle, FileAccess.Read);
+
+            Assert.Equal(bytesRead, stream.Position); // accessing SafeFileHandle must not change the position
+            Assert.Equal(stream.Position, createdFromHandle.Position); // but it should sync the offset with OS
+        }
+
+        [Theory]
+        [MemberData(nameof(AllReadWriteModes))]
+        public async Task FileOffsetIsPreservedWhenFileStreamIsCreatedFromSafeFileHandle_Writes(ReadWriteMode mode)
+        {
+            using FileStream stream = (FileStream)await CreateWriteOnlyStreamCore(Array.Empty<byte>());
+            byte[] buffer = new byte[] { 0, 1, 2, 3, 4 };
+            await WriteAsync(mode, stream, buffer, 0, buffer.Length);
+
+            Assert.Equal(buffer.Length, stream.Position);
+
+            using FileStream createdFromHandle = new FileStream(stream.SafeFileHandle, FileAccess.Write);
+
+            Assert.Equal(buffer.Length, stream.Position); 
+            Assert.Equal(stream.Position, createdFromHandle.Position);
+        }
+
+        [Theory]
+        [MemberData(nameof(AllReadWriteModes))]
+        public async Task WriteAsyncStartsWherePreviousReadAsyncHasFinished(ReadWriteMode mode)
+        {
+            if (mode == ReadWriteMode.SyncByte)
+            {
+                // it reads a single byte even if buffer.Length > 1
+                return;
+            }
+
+            byte[] initialData = new byte[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
+            byte[] readBuffer = new byte[initialData.Length * 2]; // the edge case: reading more than available
+            byte[] writeBuffer = new byte[] { 10, 11, 12, 13, 14, 15 };
+            string filePath;
+
+            using (FileStream stream = (FileStream)await CreateReadWriteStreamCore(initialData))
+            {
+                filePath = stream.Name;
+
+                int bytesRead = await ReadAsync(mode, stream, readBuffer, 0, readBuffer.Length);
+
+                Assert.Equal(bytesRead, initialData.Length);
+                Assert.Equal(initialData.Length, stream.Position);
+                Assert.Equal(stream.Position, stream.Length);
+
+                await WriteAsync(mode, stream, writeBuffer, 0, writeBuffer.Length);
+
+                Assert.Equal(initialData.Length + writeBuffer.Length, stream.Position);
+                Assert.Equal(stream.Position, stream.Length);
+            }
+
+            byte[] allBytes = File.ReadAllBytes(filePath);
+            Assert.Equal(initialData.Concat(writeBuffer), allBytes);
+        }
     }
 
     public class UnbufferedSyncFileStreamStandaloneConformanceTests : FileStreamStandaloneConformanceTests