UndefinedFunctionError: ExternalBlobReference.decode/1 during app-state sync — possible module resolution ordering issue

[sh41]

Summary

During the :regular collection app-state resync, the Coordinator GenServer crashed with UndefinedFunctionError because ExternalBlobReference.decode/1 was called as a bare module name, but could not be resolved at runtime.
Version: 0.1.0-alpha.7
Elixir: 1.19.5-otp-28
Erlang: 28.1
Rust: 1.94.1

Error

** (UndefinedFunctionError) function ExternalBlobReference.decode/1 is undefined
   (module ExternalBlobReference is not available)
    ExternalBlobReference.decode(<<10, 32, 119, ...>>)
    (baileys_ex) lib/baileys_ex/protocol/proto/noise_messages.ex:143:
      BaileysEx.Protocol.Proto.Wire.continue_nested_bytes/5
    (baileys_ex) lib/baileys_ex/protocol/proto/message_messages.ex:17:
      BaileysEx.Protocol.Proto.MessageSupport.decode_fields/3
    (baileys_ex) lib/baileys_ex/syncd/codec.ex:1007:
      BaileysEx.Syncd.Codec.decode_collection_patch/2

Root Cause Analysis

Correction: the original issue incorrectly stated the module does not exist. It does exist. Apologies for the initial inaccuracy.
The module is defined at BaileysEx.Protocol.Proto.Syncd.ExternalBlobReference in lib/baileys_ex/protocol/proto/syncd_messages.ex (line 233). The SyncdPatch module (same file, line 155) references it in its @decode_specs module attribute (line 191) as bare ExternalBlobReference:

# syncd_messages.ex — inside defmodule BaileysEx.Protocol.Proto.Syncd do
  defmodule SyncdPatch do          # line 155
    @decode_specs %{
      # ...
      3 => {:message, 3, ExternalBlobReference},   # line 191 — bare module ref
      # ...
    }
  end
  defmodule ExternalBlobReference do  # line 233 — defined AFTER SyncdPatch
    # ...
  end

Since both modules are nested inside BaileysEx.Protocol.Proto.Syncd, the bare ExternalBlobReference reference in @decode_specs should resolve at compile time to the fully-qualified BaileysEx.Protocol.Proto.Syncd.ExternalBlobReference. However, ExternalBlobReference is defined after SyncdPatch in the same file (line 233 vs line 155). This may cause a compile-time ordering issue where the module attribute is evaluated before the sibling module is compiled, resulting in the bare atom ExternalBlobReference being stored unresolved.
At runtime, Wire.continue_nested_bytes/5 calls the module from the decoded spec, and the bare ExternalBlobReference (without namespace) is not a loaded module — hence the UndefinedFunctionError.
Note: codec.ex line 874 references the same module but uses the fully-qualified path Syncd.ExternalBlobReference.decode/1, which works correctly.

Suggested Fix

Either:

1. Use the fully-qualified module name in the decode spec (safest):

   3 => {:message, 3, BaileysEx.Protocol.Proto.Syncd.ExternalBlobReference}

2. Move the ExternalBlobReference module definition above SyncdPatch in syncd_messages.ex, so it is compiled first and the bare reference resolves correctly.
3. Use __MODULE__-relative aliasing in the decode spec:

   alias BaileysEx.Protocol.Proto.Syncd.ExternalBlobReference
   # then the bare reference in @decode_specs resolves correctly

Reproduction

1. Pair a device and establish a connection with default config (sync_full_history: true)
2. Wait for initial sync — the :regular collection app-state resync triggers
3. Coordinator crashes with the error above
   Note: It may depend on the specific app-state patch data returned by the server at sync time — if the patch contains an ExternalBlobReference-typed field, the decode path is triggered.