ensure MetadataReader fast path works with every FileStream on every OS

src/libraries/System.Reflection.Metadata/src/System.Reflection.Metadata.csproj

@@ -102,8 +102,9 @@
     <Compile Include="System\Reflection\Internal\Utilities\EmptyArray.cs" />
     <Compile Include="System\Reflection\Internal\Utilities\EncodingHelper.cs" Condition="$(TargetFramework.StartsWith('netstandard')) Or $(TargetFramework.StartsWith('net4'))" />
     <Compile Include="System\Reflection\Internal\Utilities\EncodingHelper.netcoreapp.cs" Condition="'$(TargetFramework)' == '$(NetCoreAppCurrent)'" />
-    <Compile Include="System\Reflection\Internal\Utilities\FileStreamReadLightUp.cs" Condition="'$(TargetFramework)' != 'netstandard1.1'" />
+    <Compile Include="System\Reflection\Internal\Utilities\FileStreamReadLightUp.netcoreapp.cs" Condition="'$(TargetFramework)' == '$(NetCoreAppCurrent)'" />
     <Compile Include="System\Reflection\Internal\Utilities\FileStreamReadLightUp.netstandard1.1.cs" Condition="'$(TargetFramework)' == 'netstandard1.1'" />
+    <Compile Include="System\Reflection\Internal\Utilities\FileStreamReadLightUp.netstandard2.0.cs" Condition="'$(TargetFramework)' == 'netstandard2.0' Or '$(TargetFramework)' == 'net461'" />
     <Compile Include="System\Reflection\Internal\Utilities\Hash.cs" />
     <Compile Include="System\Reflection\Internal\Utilities\ImmutableByteArrayInterop.cs" />
     <Compile Include="System\Reflection\Internal\Utilities\ImmutableMemoryStream.cs" />

src/libraries/System.Reflection.Metadata/src/System/Reflection/Internal/MemoryBlocks/StreamMemoryBlockProvider.cs

@@ -76,9 +76,11 @@ internal static unsafe NativeHeapMemoryBlock ReadMemoryBlockNoLock(Stream stream
             {
                 stream.Seek(start, SeekOrigin.Begin);
 
-                if (!isFileStream || !FileStreamReadLightUp.TryReadFile(stream, block.Pointer, start, size))
+                int bytesRead = 0;
+
+                if (!isFileStream || !FileStreamReadLightUp.TryReadFile(stream, block.Pointer, size, out bytesRead))
                 {
-                    stream.CopyTo(block.Pointer, size);
+                    stream.CopyTo(block.Pointer + bytesRead, size - bytesRead);
                 }
 
                 fault = false;

src/libraries/System.Reflection.Metadata/src/System/Reflection/Internal/Utilities/FileStreamReadLightUp.netcoreapp.cs

@@ -0,0 +1,19 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.IO;
+
+namespace System.Reflection.Internal
+{
+    internal static class FileStreamReadLightUp
+    {
+        internal static bool IsFileStream(Stream stream) => stream is FileStream;
+
+        internal static unsafe bool TryReadFile(Stream stream, byte* buffer, int size, out int bytesRead)
+        {
+            bytesRead = stream.Read(new Span<byte>(buffer, size));
+
+            return bytesRead == size;
+        }
+    }
+}

src/libraries/System.Reflection.Metadata/src/System/Reflection/Internal/Utilities/FileStreamReadLightUp.netstandard1.1.cs

@@ -86,21 +86,22 @@ internal static SafeHandle GetSafeFileHandle(Stream stream)
             return handle;
         }
 
-        internal static unsafe bool TryReadFile(Stream stream, byte* buffer, long start, int size)
+        internal static unsafe bool TryReadFile(Stream stream, byte* buffer, int size, out int bytesRead)
         {
             if (readFileNotAvailable)
             {
+                bytesRead = 0;
                 return false;
             }
 
             SafeHandle handle = GetSafeFileHandle(stream);
             if (handle == null)
             {
+                bytesRead = 0;
                 return false;
             }
 
             int result;
-            int bytesRead;
 
             try
             {
@@ -109,6 +110,7 @@ internal static unsafe bool TryReadFile(Stream stream, byte* buffer, long start,
             catch
             {
                 readFileNotAvailable = true;
+                bytesRead = 0;
                 return false;
             }
 

src/libraries/System.Reflection.Metadata/src/System/Reflection/Internal/Utilities/FileStreamReadLightUp.cs → src/libraries/System.Reflection.Metadata/src/System/Reflection/Internal/Utilities/FileStreamReadLightUp.netstandard2.0.cs

@@ -1,20 +1,14 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-using System.Diagnostics;
 using System.IO;
 using System.Runtime.InteropServices;
 
 namespace System.Reflection.Internal
 {
     internal static class FileStreamReadLightUp
     {
-        private static bool IsReadFileAvailable =>
-#if NETCOREAPP
-            OperatingSystem.IsWindows();
-#else
-            Path.DirectorySeparatorChar == '\\';
-#endif
+        private static bool IsReadFileAvailable => Path.DirectorySeparatorChar == '\\';
 
         internal static bool IsFileStream(Stream stream) => stream is FileStream;
 
@@ -42,20 +36,22 @@ internal static class FileStreamReadLightUp
             return handle;
         }
 
-        internal static unsafe bool TryReadFile(Stream stream, byte* buffer, long start, int size)
+        internal static unsafe bool TryReadFile(Stream stream, byte* buffer, int size, out int bytesRead)
         {
             if (!IsReadFileAvailable)
             {
+                bytesRead = 0;
                 return false;
             }
 
             SafeHandle? handle = GetSafeFileHandle(stream);
             if (handle == null)
             {
+                bytesRead = 0;
                 return false;
             }
 
-            int result = Interop.Kernel32.ReadFile(handle, buffer, size, out int bytesRead, IntPtr.Zero);
+            int result = Interop.Kernel32.ReadFile(handle, buffer, size, out bytesRead, IntPtr.Zero);
 
             if (result == 0 || bytesRead != size)
             {