wip(mysql): add MySqlOutput, unsure on exact API

sqlx-mysql/src/output.rs

@@ -1,11 +1,47 @@
 // https://dev.mysql.com/doc/internals/en/com-stmt-execute.html
 
+use crate::MySqlTypeId;
+
 // 'x: single execution
 pub struct MySqlOutput<'x> {
     buffer: &'x mut Vec<u8>,
+    null_offset: usize,
+    ty_offset: usize,
+    index: usize,
 }
 
 impl<'x> MySqlOutput<'x> {
+    pub(crate) fn new(buffer: &'x mut Vec<u8>, params: usize) -> Self {
+        // reserve space for the NULL bitmap
+        let null_offset = buffer.len();
+        let null_len = (params + 7) / 8;
+        buffer.resize(null_offset + null_len, 0);
+
+        // let MySQL know we are sending parameter types
+        buffer.push(1);
+
+        // reserve space for the parameter types
+        let ty_offset = buffer.len();
+        let ty_len = params * 2;
+        buffer.resize(ty_offset + ty_len, 0);
+
+        Self { buffer, null_offset, ty_offset, index: 0 }
+    }
+
+    // for use in protocol::Execute
+    // do NOT use in the types/ module
+    pub(crate) fn declare(&mut self, ty: MySqlTypeId) {
+        self.buffer[self.ty_offset] = ty.ty();
+        self.buffer[self.ty_offset + 1] = ty.flags();
+
+        self.ty_offset += 2;
+        self.index += 1;
+    }
+
+    pub(crate) fn null(&mut self) {
+        self.buffer[self.null_offset + (self.index / 8)] |= (1 << (self.index % 8)) as u8;
+    }
+
     pub(crate) fn buffer(&mut self) -> &mut Vec<u8> {
         self.buffer
     }