Refactor frontend message handling

* Frontend messages have been replaced with simple function calls on
  the connector. Rather than populating a message instance and then
  writing it, we now just call a method with parameters.
* Considerable reduction in code and complexity, plus no more
  need to cache frontend message instances on the connector.
* Made naming more consistent: write methods don't (necessarily)
  flush, etc.
* Refactored SASL authentication to better separate protocol-level
  logic from encryption logic.
* Enabled C# 8 for static local functions.

Closes #2353

Directory.Build.props

@@ -16,7 +16,7 @@
 
   <!-- Language configuration -->
   <PropertyGroup>
-    <LangVersion>latest</LangVersion>
+    <LangVersion>8.0</LangVersion>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
   </PropertyGroup>
 

Npgsql.sln.DotSettings

@@ -84,6 +84,7 @@
 	<s:Boolean x:Key="/Default/UserDictionary/Words/=Npgsql_0027s/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/UserDictionary/Words/=PGTZ/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/UserDictionary/Words/=Postgre/@EntryIndexedValue">True</s:Boolean>
+	<s:Boolean x:Key="/Default/UserDictionary/Words/=Pregenerated/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/UserDictionary/Words/=P_0020keepaliv/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/UserDictionary/Words/=resultset/@EntryIndexedValue">True</s:Boolean>
 	<s:Boolean x:Key="/Default/UserDictionary/Words/=UNLISTEN/@EntryIndexedValue">True</s:Boolean>

src/Npgsql/BackendMessages/CopyMessages.cs

@@ -89,21 +89,10 @@ internal CopyDataMessage Load(int len)
         }
     }
 
-    /// <remarks>
-    /// Note: This message is both a frontend and a backend message
-    /// </remarks>
-    class CopyDoneMessage : SimpleFrontendMessage, IBackendMessage
+    class CopyDoneMessage : IBackendMessage
     {
         public BackendMessageCode Code => BackendMessageCode.CopyDone;
         internal static readonly CopyDoneMessage Instance = new CopyDoneMessage();
         CopyDoneMessage() { }
-
-        internal override int Length => 5;
-
-        internal override void WriteFully(NpgsqlWriteBuffer buf)
-        {
-            buf.WriteByte((byte)BackendMessageCode.CopyDone);
-            buf.WriteInt32(4);
-        }
     }
 }

src/Npgsql/Common.cs

@@ -11,61 +11,6 @@ interface IBackendMessage
         BackendMessageCode Code { get; }
     }
 
-    /// <summary>
-    /// Base class for all classes which represent a message sent to the PostgreSQL backend.
-    /// Concrete classes which directly inherit this represent arbitrary-length messages which can chunked.
-    /// </summary>
-    abstract class FrontendMessage
-    {
-        /// <param name="buf">the buffer into which to write the message.</param>
-        /// <param name="async"></param>
-        /// <returns>
-        /// Whether there was enough space in the buffer to contain the entire message.
-        /// If false, the buffer should be flushed and write should be called again.
-        /// </returns>
-        internal abstract Task Write(NpgsqlWriteBuffer buf, bool async);
-
-        /// <summary>
-        /// Returns how many messages PostgreSQL is expected to send in response to this message.
-        /// Used for message prepending.
-        /// </summary>
-        internal virtual int ResponseMessageCount => 1;
-    }
-
-    /// <summary>
-    /// Represents a simple frontend message which is typically small and fits well within
-    /// the write buffer. The message is first queries for the number of bytes it requires,
-    /// and then writes itself out.
-    /// </summary>
-    abstract class SimpleFrontendMessage : FrontendMessage
-    {
-        /// <summary>
-        /// Returns the number of bytes needed to write this message.
-        /// </summary>
-        internal abstract int Length { get; }
-
-        /// <summary>
-        /// Writes the message contents into the buffer.
-        /// </summary>
-        internal abstract void WriteFully(NpgsqlWriteBuffer buf);
-
-        internal sealed override Task Write(NpgsqlWriteBuffer buf, bool async)
-        {
-            if (buf.WriteSpaceLeft < Length)
-                return FlushAndWrite(buf, async);
-            Debug.Assert(Length <= buf.WriteSpaceLeft, $"Message of type {GetType().Name} has length {Length} which is bigger than the buffer ({buf.WriteSpaceLeft})");
-            WriteFully(buf);
-            return Task.CompletedTask;
-        }
-
-        async Task FlushAndWrite(NpgsqlWriteBuffer buf, bool async)
-        {
-            await buf.Flush(async);
-            Debug.Assert(Length <= buf.WriteSpaceLeft, $"Message of type {GetType().Name} has length {Length} which is bigger than the buffer ({buf.WriteSpaceLeft})");
-            WriteFully(buf);
-        }
-    }
-
     enum BackendMessageCode : byte
     {
         AuthenticationRequest = (byte)'R',
@@ -95,6 +40,21 @@ enum BackendMessageCode : byte
         RowDescription        = (byte)'T',
     }
 
+    static class FrontendMessageCode
+    {
+        internal const byte Describe =  (byte)'D';
+        internal const byte Sync =      (byte)'S';
+        internal const byte Execute =   (byte)'E';
+        internal const byte Parse =     (byte)'P';
+        internal const byte Bind =      (byte)'B';
+        internal const byte Close =     (byte)'C';
+        internal const byte Query =     (byte)'Q';
+        internal const byte CopyDone =  (byte)'c';
+        internal const byte CopyFail =  (byte)'f';
+        internal const byte Terminate = (byte)'X';
+        internal const byte Password =  (byte)'p';
+    }
+
     enum StatementOrPortal : byte
     {
         Statement = (byte)'S',