Every Extension Type shared library plugin includes a "GetContractInfo" function export symbol that explicitly opts into a particular version of the Agent<->ExtensionType extension contract, which includes:
- Function export symbols on the extension shared library that the Agent will call
- The expected function signature return types and function arguments for the particular contract version
The GetContractInfo symbol is defined in extension_common_export_symbols.h but the signature is documented separately for each extension type--look for GetContractInfo in:
- step handler and update manifest handler exports
- content download exports
- download handler exports
- component enumerator exports
The contract versions follow semantic versioning(i.e. back compatibility break will increment the major version, else the minor version).
The core agent will immediately lookup the GetContractInfo symbol (after loading the extension shared library DSO with dlopen(3)) using dlsym(3).
If the symbol exists, it will be called to populate the ADUC_ExtensionContractInfo struct and it will store in memory the contract version on a per-extension-type manner for use in adjusting call patterns aligned with the contract version.
If the agent does not support the contract version opted-into by the extension, then it will fail with an ExtendedResultCode matching the pattern ADUC_ERC_{ExtensionType}_UNSUPPORTED_CONTRACT_VERSION in result_codes.json or in the build-generated src/inc/aduc/result.h.
To support older custom extensions prior to GA, not including GetContractInfo symbol will be implicitly conflated with extension version 1.0 for the extension type, but eventually a version of the agent will make omitting GetContractInfo a failure, so it is strongly suggested to include it and to update older extensions.