pjfanning opened a new pull request, #3651:
URL: https://github.com/apache/pekko/pull/3651
### Motivation
`ProtobufSerializer` can serialize ScalaPB generated messages and the
protobuf allow list already contains
`scalapb.GeneratedMessageCompanion`, but users had to add a
`scalapb.GeneratedMessage` serialization binding
themselves.
### Modification
- Add an optional `"scalapb.GeneratedMessage" = proto` serialization binding
to the remote `reference.conf`.
- `Serialization` only uses that binding when `scalapb.GeneratedMessage` can
be loaded, in the same way as the
optional `com.google.protobuf` binding.
- Mention the binding in the serialization docs.
- Test in `ProtobufSerializerSpec` that the binding is configured but
skipped when ScalaPB is not on the classpath.
### Result
ScalaPB messages are serialized with `ProtobufSerializer` out of the box
when ScalaPB is on the classpath.
Nothing changes for applications without ScalaPB.
### Tests
Not run locally - left to CI
### References
Ported from (Apache-2.0 since 2026-10-23):
- https://github.com/akka/akka-core/pull/32070
### Notes for reviewers
- Rolling upgrade: applications that have ScalaPB on the classpath and
relied on another binding for ScalaPB
messages (for example a binding on a shared marker trait) may now pick
`ProtobufSerializer`, because
`scalapb.GeneratedMessage` can be the more specific match (or an ambiguity
warning is logged). During a rolling
upgrade, old and new nodes could then use different serializers for the
same messages. Such applications can opt
out with `"scalapb.GeneratedMessage" = none` in
`pekko.actor.serialization-bindings`. Worth a line in the
release notes / migration guide.
- No MiMa excludes; only a private method was added to `Serialization`.
- ScalaPB is not a Pekko dependency; the positive case (ScalaPB on the
classpath) is not covered by a test.
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