feat(proto): add backend message writer

proto/src/writer/backend.rs

@@ -0,0 +1,59 @@
+use crate::message::backend::BackendMessage;
+use crate::writer::oneway::OneWayProtoWriter;
+use crate::writer::protowriter::ProtoWriter;
+use async_trait::async_trait;
+use tokio::io::{AsyncWrite, AsyncWriteExt};
+
+#[async_trait]
+pub trait BackendProtoWriter: OneWayProtoWriter<BackendMessage> {
+    async fn write_ssl_reject(&mut self) -> anyhow::Result<()>;
+}
+
+#[async_trait]
+impl<W> BackendProtoWriter for ProtoWriter<W>
+where
+    W: AsyncWrite + Unpin + Send,
+{
+    async fn write_ssl_reject(&mut self) -> anyhow::Result<()> {
+        self.inner.write_u8(b'N').await?;
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::message::backend::AuthenticationOkData;
+    use crate::writer::protowriter::ProtoWriter;
+    use tokio::io::BufWriter;
+
+    #[tokio::test]
+    async fn test_message_sequence() {
+        let writer = BufWriter::new(Vec::new());
+        let mut writer = ProtoWriter::new(writer);
+
+        writer
+            .write_proto(BackendMessage::AuthenticationOk(AuthenticationOkData {
+                status: 123,
+            }))
+            .await
+            .unwrap();
+
+        writer.write_proto(BackendMessage::NoData).await.unwrap();
+
+        assert_eq!(
+            writer.inner.buffer(),
+            vec![b'R', 0, 0, 0, 8, 0, 0, 0, 123, b'n', 0, 0, 0, 4]
+        );
+    }
+
+    #[tokio::test]
+    async fn test_ssl_reject() {
+        let writer = BufWriter::new(Vec::new());
+        let mut writer = ProtoWriter::new(writer);
+
+        writer.write_ssl_reject().await.unwrap();
+
+        assert_eq!(writer.inner.buffer(), vec![b'N']);
+    }
+}

proto/src/writer/mod.rs

@@ -1,3 +1,4 @@
 
+pub mod backend;
 pub mod oneway;
 pub mod protowriter;