Fix encoding polyfill issue around null pointers (npgsql#5453)

Fixes npgsql#5446

Signed-off-by: monjowe <jonas.westman@monitor.se>

src/Npgsql/Shims/EncodingExtensions.cs

@@ -1,6 +1,7 @@
 // ReSharper disable RedundantUsingDirective
 using System.Buffers;
 using System.Collections.Generic;
+using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 // ReSharper restore RedundantUsingDirective
 
@@ -10,70 +11,87 @@ namespace System.Text;
 static class EncodingExtensions
 {
 #if NETSTANDARD2_0
+
+    /// <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)]
+    internal static unsafe ref readonly T GetNonNullPinnableReference<T>(ReadOnlySpan<T> span)
+        => ref span.Length != 0 ? ref span.GetPinnableReference() : 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)]
+    internal static unsafe ref T GetNonNullPinnableReference<T>(Span<T> span)
+        => ref span.Length != 0 ? ref span.GetPinnableReference() : ref Unsafe.AsRef<T>((void*)1);
+
     public static unsafe int GetByteCount(this Encoding encoding, ReadOnlySpan<char> chars)
     {
-        fixed (char* charsPtr = chars)
+        fixed (char* charsPtr = &GetNonNullPinnableReference(chars))
         {
             return encoding.GetByteCount(charsPtr, chars.Length);
         }
     }
 
     public static unsafe int GetBytes(this Encoding encoding, ReadOnlySpan<char> chars, Span<byte> bytes)
     {
-        fixed (char* charsPtr = chars)
-        fixed (byte* bytesPtr = bytes)
+        fixed (char* charsPtr = &GetNonNullPinnableReference(chars))
+        fixed (byte* bytesPtr = &GetNonNullPinnableReference(bytes))
         {
             return encoding.GetBytes(charsPtr, chars.Length, bytesPtr, bytes.Length);
         }
     }
 
     public static unsafe int GetCharCount(this Encoding encoding, ReadOnlySpan<byte> bytes)
     {
-        fixed (byte* bytesPtr = bytes)
+        fixed (byte* bytesPtr = &GetNonNullPinnableReference(bytes))
         {
             return encoding.GetCharCount(bytesPtr, bytes.Length);
         }
     }
 
     public static unsafe int GetCharCount(this Decoder encoding, ReadOnlySpan<byte> bytes, bool flush)
     {
-        fixed (byte* bytesPtr = bytes)
+        fixed (byte* bytesPtr = &GetNonNullPinnableReference(bytes))
         {
             return encoding.GetCharCount(bytesPtr, bytes.Length, flush);
         }
     }
 
     public static unsafe int GetChars(this Decoder encoding, ReadOnlySpan<byte> bytes, Span<char> chars, bool flush)
     {
-        fixed (byte* bytesPtr = bytes)
-        fixed (char* charsPtr = chars)
+        fixed (byte* bytesPtr = &GetNonNullPinnableReference(bytes))
+        fixed (char* charsPtr = &GetNonNullPinnableReference(chars))
         {
             return encoding.GetChars(bytesPtr, bytes.Length, charsPtr, chars.Length, flush);
         }
     }
 
     public static unsafe int GetChars(this Encoding encoding, ReadOnlySpan<byte> bytes, Span<char> chars)
     {
-        fixed (byte* bytesPtr = bytes)
-        fixed (char* charsPtr = chars)
+        fixed (byte* bytesPtr = &GetNonNullPinnableReference(bytes))
+        fixed (char* charsPtr = &GetNonNullPinnableReference(chars))
         {
             return encoding.GetChars(bytesPtr, bytes.Length, charsPtr, chars.Length);
         }
     }
 
     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* charsPtr = chars)
-        fixed (byte* bytesPtr = bytes)
+        fixed (char* charsPtr = &GetNonNullPinnableReference(chars))
+        fixed (byte* bytesPtr = &GetNonNullPinnableReference(bytes))
         {
             encoder.Convert(charsPtr, chars.Length, bytesPtr, bytes.Length, flush, out charsUsed, out bytesUsed, out completed);
         }
     }
 
     public static unsafe void Convert(this Decoder encoder, ReadOnlySpan<byte> bytes, Span<char> chars, bool flush, out int bytesUsed, out int charsUsed, out bool completed)
     {
-        fixed (byte* bytesPtr = bytes)
-        fixed (char* charsPtr = chars)
+        fixed (byte* bytesPtr = &GetNonNullPinnableReference(bytes))
+        fixed (char* charsPtr = &GetNonNullPinnableReference(chars))
         {
             encoder.Convert(bytesPtr, bytes.Length, charsPtr, chars.Length, flush, out bytesUsed, out charsUsed, out completed);
         }