[contrueCT]

Purpose of the PR

Add opt-in in-process execution for REST /gremlin to remove its internal HTTP forwarding hop. gremlinserver.rest_execution_mode defaults to forward; embedded uses this Server's existing Gremlin runtime.

Builds on the merged Java 17 (#3261) and TinkerPop 3.8.1 (#3263) upgrades. Design and investigation.

Main Changes

forward:  REST -> GremlinClient -> HTTP -> GremlinExecutor
embedded: REST ------------------------> GremlinExecutor
                                         materialize / commit / serialize

• Reuse the live graph manager, workers and JSON serializer, with authentication context and transaction callbacks on the worker.
• Preserve request/error handling, blocking admission and REST/evaluation timeout behavior; validate supported local topology and wire startup/shutdown.
• Add differential contract tests and reproducible HTTP/RocksDB comparison scripts.

Readiness, new overload controls, authentication-cache consistency and global shutdown redesign remain separate tasks.

Verifying these changes

• Trivial rework / code cleanup without any test coverage. (No Need)
• Already covered by existing tests.
• Need tests and can be verified as follows:
  • Java 17: formatting, full clean compile and Apache RAT checks passed.
  • 47 focused tests passed, covering REST/Gremlin contracts, authentication context, Memory/RocksDB transactions and bootstrap.
  • Standalone RocksDB: 76 measurement groups / 316,981 responses validated, including commit/rollback/restart checks; 80,200 successful writes reconciled with stored counts.

Draft follow-up: at concurrency 32 with 1,000-result responses, the extended comparison's p95 medians were 77.94 -> 68.50 ms, while p99 medians were 90.71 -> 109.59 ms (forward -> embedded). Investigate the tail behavior at matched offered load; these were closed-loop runs on a shared host. HStore, full TinkerPop suites and real authentication-store integration remain unverified.

Does this PR potentially affect the following parts?

• Dependencies
• Modify configurations
• The public API
• Other affects (Server startup/shutdown integration)
• Nope

Documentation Status

• Doc - TODO: required documentation is pending; complete it before merging.
• Doc - Done: documentation is included here or linked below.
• Doc - No Need: no user-visible documentation is affected.

Documentation: REST Gremlin execution modes, linked from the Server README and shipped configuration.

[codecov]

Codecov Report

❌ Patch coverage is 12.41379% with 254 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.90%. Comparing base (c2ec570) to head (d1239ef).
⚠️ Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
...graph/api/gremlin/EmbeddedGremlinRestExecutor.java	0.00%	188 Missing ⚠️
...egraph/api/gremlin/GremlinRestExecutorFactory.java	5.88%	30 Missing and 2 partials ⚠️
...egraph/api/gremlin/ForwardGremlinRestExecutor.java	61.11%	6 Missing and 1 partial ⚠️
.../apache/hugegraph/api/gremlin/GremlinQueryAPI.java	12.50%	6 Missing and 1 partial ⚠️
...n/java/org/apache/hugegraph/server/RestServer.java	44.44%	4 Missing and 1 partial ⚠️
...ava/org/apache/hugegraph/dist/HugeGraphServer.java	50.00%	5 Missing ⚠️
...org/apache/hugegraph/server/ApplicationConfig.java	63.63%	3 Missing and 1 partial ⚠️
...ugegraph/server/HugeGraphWsAndHttpChannelizer.java	0.00%	4 Missing ⚠️
...a/org/apache/hugegraph/api/gremlin/GremlinAPI.java	80.00%	0 Missing and 1 partial ⚠️
...che/hugegraph/api/gremlin/GremlinRestExecutor.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3388      +/-   ##
============================================
- Coverage     55.93%   55.90%   -0.03%     
- Complexity     2277     6784    +4507     
============================================
  Files           774      779       +5     
  Lines         68534    69114     +580     
  Branches       9333     9468     +135     
============================================
+ Hits          38334    38641     +307     
- Misses        26634    26893     +259     
- Partials       3566     3580      +14     

Flag	Coverage Δ		*Carryforward flag
commons	96.37% <ø> (ø)		Carriedforward from c2ec570
hstore	35.81% <ø> (+<0.01%)	⬆️	Carriedforward from c2ec570
pd	49.15% <ø> (-0.03%)	⬇️	Carriedforward from c2ec570
server_hbase	41.83% <ø> (-0.07%)	⬇️	Carriedforward from c2ec570
server_memory	54.29% <ø> (-0.05%)	⬇️	Carriedforward from c2ec570
server_rocksdb	42.58% <12.41%> (+0.23%)	⬆️
store	31.93% <ø> (ø)		Carriedforward from c2ec570

*This pull request uses carry forward flags. Click here to find out more.

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