ProtoSchemaConverter produces an unresolvable schema when an enum is referenced by multiple non-top-level messages

[erlendnils1]

Prior art: java-bigquerystorage#428 ("fix: ProtoSchemaConverter's problem when converting fields reference same enum", merged 2020-07-20) fixed the case where multiple fields reference the same enum, by introducing the global enumTypes dedup set and the synthetic _E wrapper. That fix is incomplete: the single wrapper is emitted into the scope of the first message that references the enum, so it only resolves for references reachable from there (same message, or a top-level reference). It does not cover the same enum being referenced from multiple sibling nested messages when none of them is the top-level message — the case below. No follow-up issue was filed for that gap.

Summary

ProtoSchemaConverter.convert(Descriptor) generates a self-contained DescriptorProto that the backend cannot rebuild when the same enum is referenced from more than one message and none of those references is on the top-level message. The resulting write stream fails with:

failed to create file descriptor: proto: message field "…Inner2.color" cannot resolve type: "*.example_Color_E.Color" not found

and the client retries indefinitely.

Root cause

In ProtoSchemaConverter.convertInternal, message types are hoisted to the root of the schema (rootProtoSchema.addNestedType(...)), but enums are handled differently: an enum is wrapped in a synthetic enclosing struct <flatEnumName>_E that is added to resultProto — the local scope of whichever message is currently being converted — and this is guarded by the global enumTypes dedup set introduced in #428, so the wrapper is emitted only once, for the first message that references the enum.

if (inputField.getType() == FieldDescriptor.Type.ENUM) {
    String enumFullName = inputField.getEnumType().getFullName();
    String enclosingTypeName = getNameFromFullName(enumFullName) + "_E";
    String enumName = inputField.getEnumType().getName();
    String actualEnumFullName = enclosingTypeName + "." + enumName;
    if (enumTypes.contains(enumFullName)) {
        resultField.setTypeName(actualEnumFullName);          // (B) subsequent messages: reference only
    } else {
        EnumDescriptorProto enumType = inputField.getEnumType().toProto();
        resultProto.addNestedType(                            // (A) wrapper added to LOCAL scope, not root
            DescriptorProto.newBuilder()
                .setName(enclosingTypeName)
                .addEnumType(enumType.toBuilder().setName(enumName))
                .build());
        resultField.setTypeName(actualEnumFullName);
        enumTypes.add(enumFullName);
    }
}

Consequently:

• The first message to reference the enum gets the _E wrapper nested inside its own scope, so its type_name (<flatEnumName>_E.<Enum>, a relative name) resolves.
• Every other message gets the same relative type_name but no wrapper in scope. Proto name resolution walks the referencing message's scope outward to the root and never finds the wrapper — it lives inside a sibling message. Resolution fails.

This only manifests when no reference is on the top-level message, because for the top-level message resultProto == rootProtoSchema, so a wrapper created there does land at the root and is visible to everyone.

Minimal reproduction

example.proto:

syntax = "proto3";

package example;

enum Color {
  COLOR_UNSPECIFIED = 0;
  RED = 1;
  GREEN = 2;
}

message Inner1 {
  Color color = 1;
}

message Inner2 {
  Color color = 1;
}

message Root {
  Inner1 inner1 = 1;
  Inner2 inner2 = 2;
}

ProtoSchema schema = ProtoSchemaConverter.convert(Root.getDescriptor());

// Rebuild it the way the backend does:
FileDescriptorProto file = FileDescriptorProto.newBuilder()
    .setName("__schema.proto")
    .addMessageType(schema.getProtoDescriptor())
    .build();
Descriptors.FileDescriptor.buildFrom(file, new Descriptors.FileDescriptor[] {});

Actual result

The converter emits the example_Color_E wrapper only inside example_Inner1; example_Inner2.color points at a type that is not in its scope:

name: "example_Root"
field { name: "inner1" type: TYPE_MESSAGE type_name: "example_Inner1" }
field { name: "inner2" type: TYPE_MESSAGE type_name: "example_Inner2" }
nested_type {
  name: "example_Inner1"
  field { name: "color" type: TYPE_ENUM type_name: "example_Color_E.Color" }
  nested_type {                       # <-- wrapper lives ONLY here
    name: "example_Color_E"
    enum_type { name: "Color" value {...} }
  }
}
nested_type {
  name: "example_Inner2"
  field { name: "color" type: TYPE_ENUM type_name: "example_Color_E.Color" }   # <-- unresolvable
}

buildFrom(...) throws:

example_Root.example_Inner2.color: "example_Color_E.Color" is not defined.

The real Storage Write API surfaces the same thing as a retriable UNKNOWN on the write stream (failed to create file descriptor … not found).

Expected result

The flattened schema should be resolvable regardless of how many messages reference the enum, or where. Emitting the _E wrapper once at the root (as a sibling of the hoisted messages, matching how message types are already handled) fixes it: the relative type_name then resolves from any message's scope.

Workaround

Post-process the ProtoSchema and relocate every _E enum wrapper to the root DescriptorProto before calling setWriterSchema(...).

Environment

• com.google.cloud:google-cloud-bigquerystorage 3.28.0; reproduced identically against 3.29.0.
• The enum-handling block in ProtoSchemaConverter is unchanged since We should add unit-tests for api-client based spi classes #428 (2020) and is identical in the current google-cloud-java monorepo copy.