Ensure that at least one character can be converted. Fixes #974

Author: bgrainger

src/MySqlConnector/MySqlBulkCopy.cs

@@ -619,12 +619,16 @@ static bool WriteSubstring(string value, ref int inputIndex, ref Encoder? utf8En
 						if (nextIndex == -1)
 							nextIndex = value.Length;
 
-						utf8Encoder ??= Encoding.UTF8.GetEncoder();
+						utf8Encoder ??= s_utf8Encoding.GetEncoder();
 #if NETSTANDARD1_3
+						if (output.Length <= 4 && utf8Encoder.GetByteCount(value.ToCharArray(), inputIndex, 1, flush: false) > output.Length)
+							return false;
 						var buffer = new byte[output.Length];
 						utf8Encoder.Convert(value.ToCharArray(), inputIndex, nextIndex - inputIndex, buffer, 0, buffer.Length, nextIndex == value.Length, out var charsUsed, out var bytesUsed, out var completed);
 						buffer.AsSpan().CopyTo(output);
 #else
+						if (output.Length <= 4 && utf8Encoder.GetByteCount(value.AsSpan(inputIndex, 1), flush: false) > output.Length)
+							return false;
 						utf8Encoder.Convert(value.AsSpan(inputIndex, nextIndex - inputIndex), output, nextIndex == value.Length, out var charsUsed, out var bytesUsed, out var completed);
 #endif
 
@@ -659,6 +663,7 @@ static bool WriteBytes(ReadOnlySpan<byte> value, ref int inputIndex, Span<byte>
 
 		private static readonly char[] s_specialCharacters = new char[] { '\t', '\\', '\n' };
 		private static readonly IMySqlConnectorLogger Log = MySqlConnectorLogManager.CreateLogger(nameof(MySqlBulkCopy));
+		private static readonly Encoding s_utf8Encoding = new UTF8Encoding(encoderShouldEmitUTF8Identifier: false, throwOnInvalidBytes: true);
 
 		readonly MySqlConnection m_connection;
 		readonly MySqlTransaction? m_transaction;

src/MySqlConnector/Utilities/Utility.cs

@@ -77,6 +77,16 @@ public static unsafe void Convert(this Encoder encoder, ReadOnlySpan<char> chars
 				encoder.Convert(charsPtr, chars.Length, bytesPtr is null ? (byte*) 1 : bytesPtr, bytes.Length, flush, out charsUsed, out bytesUsed, out completed);
 			}
 		}
+
+		public static unsafe int GetByteCount(this Encoder encoder, ReadOnlySpan<char> chars, bool flush)
+		{
+			fixed (char* charsPtr = &MemoryMarshal.GetReference(chars))
+			{
+				// MemoryMarshal.GetNonNullPinnableReference is internal, so fake it by using an invalid but non-null pointer; this
+				// prevents Convert from throwing an exception when the output buffer is empty
+				return encoder.GetByteCount(charsPtr, chars.Length, flush);
+			}
+		}
 #endif
 
 		/// <summary>