address comment

Signed-off-by: Jay Lee <BusyJayLee@gmail.com>

Author: BusyJay

src/codec.rs

@@ -6,6 +6,10 @@ use crate::error::Result;
 pub type DeserializeFn<T> = fn(MessageReader) -> Result<T>;
 pub type SerializeFn<T> = fn(&T, &mut Vec<u8>) -> Result<()>;
 
+/// According to https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2.md, grpc uses
+/// a four bytes to describe the length of a message, so it should not exceed u32::MAX.
+const MAX_MESSAGE_SIZE: usize = u32::MAX as usize;
+
 /// Defines how to serialize and deserialize between the specialized type and byte slice.
 pub struct Marshaller<T> {
     // Use function pointer here to simplify the signature.
@@ -28,17 +32,17 @@ pub struct Marshaller<T> {
 pub mod pb_codec {
     use protobuf::{CodedInputStream, Message};
 
-    use super::MessageReader;
+    use super::{MessageReader, MAX_MESSAGE_SIZE};
     use crate::error::{Error, Result};
 
     #[inline]
     pub fn ser<T: Message>(t: &T, buf: &mut Vec<u8>) -> Result<()> {
         t.write_to_vec(buf)?;
-        if buf.len() <= u32::MAX as usize {
+        if buf.len() <= MAX_MESSAGE_SIZE as usize {
             Ok(())
         } else {
             Err(Error::Codec(
-                format!("message is too large: {} > u32::MAX", buf.len()).into(),
+                format!("message is too large: {} > {}", buf.len(), MAX_MESSAGE_SIZE).into(),
             ))
         }
     }
@@ -56,17 +60,17 @@ pub mod pb_codec {
 pub mod pr_codec {
     use prost::Message;
 
-    use super::MessageReader;
+    use super::{MessageReader, MAX_MESSAGE_SIZE};
     use crate::error::{Error, Result};
 
     #[inline]
     pub fn ser<M: Message>(msg: &M, buf: &mut Vec<u8>) -> Result<()> {
         msg.encode(buf)?;
-        if buf.len() <= u32::MAX as usize {
+        if buf.len() <= MAX_MESSAGE_SIZE as usize {
             Ok(())
         } else {
             Err(Error::Codec(
-                format!("message is too large: {} > u32::MAX", buf.len()).into(),
+                format!("message is too large: {} > {}", buf.len(), MAX_MESSAGE_SIZE).into(),
             ))
         }
     }