From 24e3e8d1d2ea70ed3fe65590530d39b406abde3f Mon Sep 17 00:00:00 2001
From: Tom Deseyn <tom.deseyn@gmail.com>
Date: Sun, 16 Apr 2023 08:44:06 +0200
Subject: [PATCH] Fix NREs on netstandard2.0 while reading empty strings.
 (#195)

---
 .../Netstandard2_0Extensions.cs               | 35 +++++++++++++------
 1 file changed, 25 insertions(+), 10 deletions(-)

diff --git a/src/Tmds.DBus.Protocol/Netstandard2_0Extensions.cs b/src/Tmds.DBus.Protocol/Netstandard2_0Extensions.cs
index a4dc2860..2946863f 100644
--- a/src/Tmds.DBus.Protocol/Netstandard2_0Extensions.cs
+++ b/src/Tmds.DBus.Protocol/Netstandard2_0Extensions.cs
@@ -18,8 +18,8 @@ public static bool Remove<TKey, TValue>(this Dictionary<TKey, TValue> dictionary
 
     public static unsafe int GetBytes(this Encoding encoding, ReadOnlySpan<char> chars, Span<byte> bytes)
     {
-        fixed (char* pChars = chars)
-        fixed (byte* pBytes = bytes)
+        fixed (char* pChars = &GetNonNullPinnableReference(chars))
+        fixed (byte* pBytes = &GetNonNullPinnableReference(bytes))
         {
             return encoding.GetBytes(pChars, chars.Length, pBytes, bytes.Length);
         }
@@ -27,8 +27,8 @@ public static unsafe int GetBytes(this Encoding encoding, ReadOnlySpan<char> cha
 
     public static unsafe int GetChars(this Encoding encoding, ReadOnlySpan<byte> bytes, Span<char> chars)
     {
-        fixed (char* pChars = chars)
-        fixed (byte* pBytes = bytes)
+        fixed (char* pChars = &GetNonNullPinnableReference(chars))
+        fixed (byte* pBytes = &GetNonNullPinnableReference(bytes))
         {
             return encoding.GetChars(pBytes, bytes.Length, pChars, chars.Length);
         }
@@ -36,7 +36,7 @@ public static unsafe int GetChars(this Encoding encoding, ReadOnlySpan<byte> byt
 
     public static unsafe string GetString(this Encoding encoding, ReadOnlySpan<byte> bytes)
     {
-        fixed (byte* pBytes = bytes)
+        fixed (byte* pBytes = &GetNonNullPinnableReference(bytes))
         {
             return encoding.GetString(pBytes, bytes.Length);
         }
@@ -44,7 +44,7 @@ public static unsafe string GetString(this Encoding encoding, ReadOnlySpan<byte>
 
     public static unsafe int GetCharCount(this Encoding encoding, ReadOnlySpan<byte> bytes)
     {
-        fixed (byte* pBytes = bytes)
+        fixed (byte* pBytes = &GetNonNullPinnableReference(bytes))
         {
             return encoding.GetCharCount(pBytes, bytes.Length);
         }
@@ -52,7 +52,7 @@ public static unsafe int GetCharCount(this Encoding encoding, ReadOnlySpan<byte>
 
     public static unsafe int GetByteCount(this Encoding encoding, ReadOnlySpan<char> chars)
     {
-        fixed (char* pChars = chars)
+        fixed (char* pChars = &GetNonNullPinnableReference(chars))
         {
             return encoding.GetByteCount(pChars, chars.Length);
         }
@@ -60,7 +60,7 @@ public static unsafe int GetByteCount(this Encoding encoding, ReadOnlySpan<char>
 
     public static unsafe int GetByteCount(this Encoder encoder, ReadOnlySpan<char> chars, bool flush)
     {
-        fixed (char* pChars = chars)
+        fixed (char* pChars = &GetNonNullPinnableReference(chars))
         {
             return encoder.GetByteCount(pChars, chars.Length, flush);
         }
@@ -68,8 +68,8 @@ public static unsafe int GetByteCount(this Encoder encoder, ReadOnlySpan<char> c
 
     public static unsafe void Convert(this Encoder encoder, ReadOnlySpan<char> chars, Span<byte> bytes, bool flush, out int charsUsed, out int bytesUsed, out bool completed)
     {
-        fixed (char* pChars = chars)
-        fixed (byte* pBytes = bytes)
+        fixed (char* pChars = &GetNonNullPinnableReference(chars))
+        fixed (byte* pBytes = &GetNonNullPinnableReference(bytes))
         {
             encoder.Convert(pChars, chars.Length, pBytes, bytes.Length, flush, out charsUsed, out bytesUsed, out completed);
         }
@@ -109,7 +109,22 @@ public static async ValueTask<int> SendAsync(this Socket socket, ReadOnlyMemory<
 
         throw new NotSupportedException();
     }
+
+    /// <summary>
+    /// Returns a reference to the 0th element of the Span. If the Span is empty, returns a reference to fake non-null pointer. Such a reference can be used
+    /// for pinning but must never be dereferenced. This is useful for interop with methods that do not accept null pointers for zero-sized buffers.
+    /// </summary>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    private static unsafe ref T GetNonNullPinnableReference<T>(Span<T> span) => ref (span.Length != 0) ? ref MemoryMarshal.GetReference(span) : ref Unsafe.AsRef<T>((void*)1);
+
+    /// <summary>
+    /// Returns a reference to the 0th element of the ReadOnlySpan. If the ReadOnlySpan is empty, returns a reference to fake non-null pointer. Such a reference
+    /// can be used for pinning but must never be dereferenced. This is useful for interop with methods that do not accept null pointers for zero-sized buffers.
+    /// </summary>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    private static unsafe ref T GetNonNullPinnableReference<T>(ReadOnlySpan<T> span) => ref (span.Length != 0) ? ref MemoryMarshal.GetReference(span) : ref Unsafe.AsRef<T>((void*)1);
 }
+
 internal sealed class UnixDomainSocketEndPoint : EndPoint
 {
     private const AddressFamily EndPointAddressFamily = AddressFamily.Unix;