feat(gossipsub): apply max_transmit_size to the published message (#5642)

## Description

When trying to publish a message using gossipsub's `publish` method,
it should be possible to predict whether it will fit in the limit
defined by
the `max_transmit_size` config option.
If this limit applies to the final protobuf payload, it's not possible
to know
that in advance because the size of the added fields is not fixed.

This change makes the limit apply to the passed message size instead of
the final wire size.

## Notes & open questions

This is a minor version change because it changes the meaning of the
existing config option.
However, for the existing clients the limit will only become more
permissive, so it shouldn't break anything.

## Change checklist

<!-- Please add a Changelog entry in the appropriate crates and bump the
crate versions if needed. See
<https://github.com/libp2p/rust-libp2p/blob/master/docs/release.md#development-between-releases>-->

- [x] I have performed a self-review of my own code
- [x] I have made corresponding changes to the documentation
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [x] A changelog entry has been made in the appropriate crates

---------

Co-authored-by: Darius Clark <[email protected]>
Co-authored-by: João Oliveira <[email protected]>

Author: 3 people

Cargo.lock

@@ -83,7 +83,7 @@ libp2p-core = { version = "0.42.0", path = "core" }
 libp2p-dcutr = { version = "0.12.0", path = "protocols/dcutr" }
 libp2p-dns = { version = "0.42.0", path = "transports/dns" }
 libp2p-floodsub = { version = "0.45.0", path = "protocols/floodsub" }
-libp2p-gossipsub = { version = "0.47.1", path = "protocols/gossipsub" }
+libp2p-gossipsub = { version = "0.48.0", path = "protocols/gossipsub" }
 libp2p-identify = { version = "0.45.1", path = "protocols/identify" }
 libp2p-identity = { version = "0.2.9" }
 libp2p-kad = { version = "0.47.0", path = "protocols/kad" }

Cargo.toml

@@ -1,3 +1,8 @@
+## 0.48.0
+
+- Apply `max_transmit_size` to the inner message instead of the final payload.
+  See [PR 5642](https://github.com/libp2p/rust-libp2p/pull/5642).
+
 ## 0.47.1
 
 - Attempt to publish to at least mesh_n peers when flood publish is disabled.

protocols/gossipsub/CHANGELOG.md

@@ -3,7 +3,7 @@ name = "libp2p-gossipsub"
 edition = "2021"
 rust-version = { workspace = true }
 description = "Gossipsub protocol for libp2p"
-version = "0.47.1"
+version = "0.48.0"
 authors = ["Age Manning <[email protected]>"]
 license = "MIT"
 repository = "https://github.com/libp2p/rust-libp2p"

protocols/gossipsub/Cargo.toml

@@ -583,6 +583,11 @@ where
             .data_transform
             .outbound_transform(&topic, data.clone())?;
 
+        // check that the size doesn't exceed the max transmission size.
+        if transformed_data.len() > self.config.max_transmit_size() {
+            return Err(PublishError::MessageTooLarge);
+        }
+
         let raw_message = self.build_raw_message(topic, transformed_data)?;
 
         // calculate the message id from the un-transformed data
@@ -593,11 +598,6 @@ where
             topic: raw_message.topic.clone(),
         });
 
-        // check that the size doesn't exceed the max transmission size
-        if raw_message.raw_protobuf_len() > self.config.max_transmit_size() {
-            return Err(PublishError::MessageTooLarge);
-        }
-
         // Check the if the message has been published before
         if self.duplicate_cache.contains(&msg_id) {
             // This message has already been seen. We don't re-publish messages that have already

protocols/gossipsub/src/behaviour.rs

@@ -174,7 +174,8 @@ impl Config {
 
     /// The maximum byte size for each gossipsub RPC (default is 65536 bytes).
     ///
-    /// This represents the maximum size of the entire protobuf payload. It must be at least
+    /// This represents the maximum size of the published message. It is additionally wrapped
+    /// in a protobuf struct, so the actual wire size may be a bit larger. It must be at least
     /// large enough to support basic control messages. If Peer eXchange is enabled, this
     /// must be large enough to transmit the desired peer information on pruning. It must be at
     /// least 100 bytes. Default is 65536 bytes.