TINKERPOP-3237 Add custom type serializer API for gremlin-go

[DR1N0]

Summary

This PR adds support for serializing custom types in gremlin-go, bringing it to feature parity with the Java driver and completing the custom type support that currently only includes deserialization. This implements the official GraphBinary specification for custom types, enabling full round-trip support for custom graph database types.

Motivation

While gremlin-go supports deserializing custom types via RegisterCustomTypeReader(), it lacks the corresponding serialization capability. This creates an asymmetry where users can read custom types from server responses but cannot write them in requests. The Java driver already has full custom type support (both CustomTypeSerializer reader and writer), and this PR brings gremlin-go to the same level.

Changes

New API Functions

• RegisterCustomTypeWriter(valueType reflect.Type, typeName string, writer CustomTypeWriter) - Registers a custom serializer for a specific type
• UnregisterCustomTypeWriter(valueType reflect.Type) - Unregisters a custom serializer

New Types

• CustomTypeWriter - Function type for user-provided serialization functions
• CustomTypeInfo - Struct holding type metadata (name and writer function)

Implementation Details

• Added customTypeWriter() function to handle GraphBinary custom type format (type_code=0x00, type_name, value)
• Modified getType() to detect registered custom types before checking built-in types
• Modified write() to properly handle custom type format, matching the Java implementation in GraphBinaryWriter.java:

  if (serializer instanceof CustomTypeSerializer) {
      buffer.writeBytes(customTypeCodeBytes);  // 0x00
      writeValue(customTypeSerializer.getTypeName(), buffer, false);
      customTypeSerializer.write(value, buffer, this);
  }

GraphBinary Format Compliance

The implementation follows the exact format used by Java's GraphBinaryWriter:

• Type code: 0x00 (DataType.CUSTOM)
• Type name: UTF-8 string with length prefix
• Value: Custom-serialized data

This ensures cross-driver compatibility - custom types serialized by gremlin-go can be deserialized by Java drivers and vice versa.

Updated Files

• gremlin-go/driver/serializer.go - Added type definitions and registration functions
• gremlin-go/driver/graphBinary.go - Added custom type writer implementation and modified type detection
• gremlin-go/driver/serializer_test.go - Added test cases following existing patterns
• CHANGELOG.asciidoc - Added changelog entry

Test Coverage

Tests follow the same pattern as existing custom type reader tests:

• Success case in TestSerializer: "test serialized request message w/ custom type"
• Failure case in TestSerializerFailures: "test unregistered custom type writer failure"

All tests pass:

=== RUN   TestSerializer/test_serialized_request_message_w/_custom_type
--- PASS: TestSerializer/test_serialized_request_message_w/_custom_type (0.00s)
=== RUN   TestSerializerFailures/test_unregistered_custom_type_writer_failure
--- PASS: TestSerializerFailures/test_unregistered_custom_type_writer_failure (0.00s)

Use Case

This feature enables users to serialize custom graph database types (e.g., JanusGraph's RelationIdentifier) when sending requests to the server:

import (
    "bytes"
    "encoding/binary"
    "reflect"
)

// Define custom type writer
customWriter := func(value interface{}, buffer *bytes.Buffer) error {
    // Custom serialization logic matching server expectations
    return nil
}

// Register the writer
RegisterCustomTypeWriter(
    reflect.TypeOf((*MyCustomType)(nil)),
    "janusgraph.RelationIdentifier",  // Must match server-side type name
    customWriter
)

// Now MyCustomType instances can be serialized in requests
client.Submit("g.E().has('id', rid)", map[string]interface{}{"rid": customValue})

Benefits

1. Feature Parity: Brings gremlin-go to the same level as the Java driver
2. GraphBinary Compliance: Implements the official specification
3. Cross-Driver Compatibility: Custom types work across Java, Go, and other GraphBinary drivers
4. Complete Custom Type Support: Enables full round-trip custom type handling

Backward Compatibility

This change is fully backward compatible - it only adds new functionality without modifying existing APIs or behavior.

References

• Java implementation: gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/structure/io/binary/GraphBinaryWriter.java
• GraphBinary specification for custom types (DataType.CUSTOM = 0x00)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@            Coverage Diff             @@
##             3.8-dev    #3335   +/-   ##
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  Coverage           ?   76.46%           
  Complexity         ?    14890           
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  Files              ?     1159           
  Lines              ?    72135           
  Branches           ?     8057           
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  Hits               ?    55159           
  Misses             ?    14047           
  Partials           ?     2929           

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