NIFI-14210 Add record writer for Protobuf messages

[prankstrisse]

Summary

NIFI-14210

Changes

• StandardProtobufWriter (nifi-protobuf-services) — new SchemaRegistryService + RecordSetWriterFactory. Resolves the target Proto message either statically via a Message Name property or dynamically via a MessageNameResolver service, compiles the schema with the existing ProtobufSchemaCompiler, and serializes each Record with a new ProtobufDataSerializer. A single message is written per FlowFile, since concatenated Protobuf messages cannot be delimited on read.
• ProtobufDataSerializer — maps NiFi Record fields onto Wire Schema/MessageType descriptors and encodes the binary payload. google.protobuf.Any-typed fields are written as plain nested messages rather than re-wrapped as Any.
• WriteProtobufResultWithExternalSchema — orchestrates header + message-index + payload sequencing, mirroring the existing WriteAvroResultWithExternalSchema pattern: invokes SchemaReferenceWriter.writeHeader(...) when a Schema Reference Writer is configured, then an optional MessageIndexWriter.writeMessageIndex(...) step, then the Protobuf payload.
• MessageIndexWriter (nifi-schema-registry-service-api) — new shared interface, the write-side inverse of MessageNameResolver: given a resolved message name and schema definition, encodes the path to that message and writes it to the output stream so a reader can resolve the same name back from the encoded path.
• ConfluentProtobufMessageIndexWriter + ProtobufMessageIndexEncoder (nifi-confluent-protobuf-message-index-writer, new module) — Confluent implementation of MessageIndexWriter. Walks nested message declarations to find the target message's declaration-order path and zigzag-varint encodes it (including the [0] single-message optimization), mirroring the decoding done by ConfluentProtobufMessageNameResolver. Together with the unchanged, already-generic ConfluentEncodedSchemaReferenceWriter, this reproduces the Confluent wire format (5-byte header + message-index array + payload) on write.
• VarintUtils — relocated from the message-name-resolver module to org.apache.nifi.confluent.schema and made non-package-private so it can be shared between the resolver (read) and index writer (write) modules.
• Minor visibility/refactoring changes to ProtobufSchemaCompiler and ProtobufSchemaValidator to support reuse from the writer.

Testing

• ProtobufMessageIndexEncoderTest, ConfluentProtobufMessageIndexWriterTest — unit tests for index-path encoding, including nested messages and the single-index optimization.
• TestProtobufDataSerializer — Record-to-Protobuf field mapping and encoding.
• TestStandardProtobufWriter — controller service property/validation behavior.
• TestStandardProtobufWriterConfluentRoundTrip — end-to-end round trip: StandardProtobufWriter output (header + index + payload) is read back by StandardProtobufReader/ConfluentProtobufMessageNameResolver and compared against the original Records.

Tracking

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Issue Tracking

• Apache NiFi Jira issue created

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• Pull Request commit message starts with Apache NiFi Jira issue number, as such NIFI-00000
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Build

• Build completed using ./mvnw clean install -P contrib-check
  • JDK 21
  • JDK 25

Licensing

• New dependencies are compatible with the Apache License 2.0 according to the License Policy
• New dependencies are documented in applicable LICENSE and NOTICE files

Documentation

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