[Tianlei Wu (tianleiwu)]

Description

Adds session_options.custom_ops_libraries, a list of custom-op libraries registered in order after the existing single custom_ops_library, and exports InheritSessionOptionsFromParent so components that create auxiliary sessions (for example MTP/draft heads) can inherit the decoder's providers, provider options, config entries and custom ops.

Models whose graphs use several custom-op domains (fused attention, hyper-connection, GEMM and head ops shipped next to the model package) currently cannot name more than one library. This change is generic and has no dependency on any model type; it is the first step of the Qwen3.8-Flash static-graph backend series (see the end of this description), but stands alone and targets main.

Summary of Changes

File	Change
src/config.h, src/config.cpp	New session_options.custom_ops_libraries (JSON string array; scalar values such as "custom_ops_libraries": "ops.so" are rejected during parsing). Overlays append to custom_ops_libraries (and [] leaves existing entries unchanged). InheritSessionOptionsFromParent exposes the existing inheritance helper; custom_ops_library and custom_ops_libraries are inherited together when the child sets neither (note that "custom_ops_libraries": [] leaves the vector empty and therefore still inherits from the parent unless custom_ops_library is set).
src/models/model.cpp	custom_ops_library and then every custom_ops_libraries entry go through the same path-traversal and allow-list checks, with duplicate resolved paths registered once. All paths are validated against the allow-list before loading starts; validated libraries are then loaded and registered sequentially in order (so a later load/registration error can still leave earlier libraries registered on session_options).
test/CMakeLists.txt, test/cpp/engine/custom_ops_test_lib/custom_ops_test_lib.cpp, test/cpp/engine/custom_ops_libraries_tests.cpp	Builds two small test custom-op shared libraries (custom_ops_test_first and custom_ops_test_second) and tests parsing/ordering, overlay rules, rejection of non-string entries and scalar values, inheritance (including [] and CreateMtpDecoderConfig), allow-list validation, and actual library loading, ordering, and deduplication.

Library Location Requirement

Every entry in custom_ops_library and custom_ops_libraries must resolve to an existing file inside one of the allowed directories:

1. The model folder (alongside genai_config.json),
2. A registered Execution Provider (EP) library directory, or
3. The current working directory.

Previously, an unresolved bare library name in custom_ops_library was passed directly to RegisterCustomOpsLibrary and could be resolved through the platform loader search path (LD_LIBRARY_PATH / PATH). Unresolved entries are now rejected before loading so the directory allow-list cannot be bypassed; users should place custom-op libraries in one of the allowed directories above (typically the model folder).

Testing

• CPU-only Release build (-DUSE_CUDA=OFF) on main: engine_unit_tests 929 passed, 0 failed, 2 skipped (including all 9 tests in CustomOpsLibrariesConfigTest, which load real test custom-op shared libraries to verify registration, ordering, and deduplication).
• lintrunner, clang-format (20.1.8) and git diff --check are clean.

Motivation and Context

Found while hosting a model that uses seven custom-op libraries. Registering them through one custom_ops_library entry was not possible, and auxiliary sessions did not inherit the decoder's custom ops, so head graphs failed to load without repeating the whole session configuration.

This is the first PR of a stack that adds an Engine-hosted CUDA-graph MTP backend for Qwen3.8-Flash-Next (greedy only, one resident request, Linux/CUDA, no prefix cache). The follow-ups are drafts stacked on kvaishnavi/qwen-38-flash-run, so this commit is also carried as the first commit of the next PR (#2726) until this one merges. The stack: #2726 (CUDA data plane library) -> #2727 (model type and device engine) -> #2728 (Engine hosting) -> #2729 (qualification tools).

Checklist

• Tests added/updated
• No breaking changes
• Documentation updated (the option is described in the PR; no dedicated config documentation exists for session_options yet)

[Copilot]

🟡 Changes recommended

Unresolved library names can bypass the intended directory allow-list.

1 open finding

• Reject unresolved entries before registering them · New

What changed in this PR

Adds ordered multi-library custom-op configuration and session-option inheritance for auxiliary sessions.

Changes:

• Parses and inherits custom_ops_libraries.
• Validates and registers multiple libraries in order.
• Adds configuration, inheritance, and allow-list tests.

File	Description
src/​config.h	Declares the new option and inheritance API.
src/​config.cpp	Parses and inherits library lists.
src/​models/​model.cpp	Validates and registers configured libraries.
test/​cpp/​engine/​custom_ops_libraries_tests.cpp	Tests parsing, inheritance, and validation.

🧠 Review effort: Balanced

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[Baiju Meswani (baijumeswani)]

The approach looks good. I checked the latest commit and tested the config behavior. The earlier missing-library issue is fixed. Please fix the wrong-type check, explain the change to old library loading, and make the list rules clear. I also suggest a few tests that actually load libraries. Details are inline. I did not run the full engine test suite.