[Error] Unify EnvErrorAlreadySet to error.kind

[tqchen]

Previously we relied on a special error to handle EnvErrorAlreadySet, this PR updates the behavior so that the Error being thrown can also have kind EnvErrorAlreadySet, which indicate there is an interrupt. This allows the error handling behavior to be overall simpler and more consistent, removing boliler plate in C++ exposure.

ABI impact: this change only impacts the downstream c++ usage that uses EnvErrorAlreadySet. The downstream DSLs and libraries are not impacted by this change.

The only usecase so far is tvm compiler autotuning service to propagate KeyboardInterrupt. Given the benefit and limited impact, we could consider bring it in, since it simplifies the checking and throw logics in the function and further streamlines error handling.

[gemini-code-assist]

Summary of Changes

Hello @tqchen, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request streamlines the error handling mechanism by integrating the EnvErrorAlreadySet concept directly into the general Error object's kind attribute. This change eliminates a dedicated return code and a separate exception type, leading to a more consistent and less boilerplate-heavy approach for C++ error propagation, particularly beneficial for scenarios like propagating KeyboardInterrupt from Python.

Highlights

• Unified Error Handling: The special EnvErrorAlreadySet exception and its corresponding -2 return code have been removed. Instead, EnvErrorAlreadySet is now represented as a standard Error object with its kind field set to 'EnvErrorAlreadySet'.
• Simplified C++ API: The C++ FFI layer no longer needs to explicitly catch EnvErrorAlreadySet or handle the -2 return code, simplifying the error propagation logic.
• Backward Compatibility: Python-side Cython bindings (error.pxi, function.pxi, type_info.pxi) retain checks for the -2 return code for backward compatibility, with a TODO to remove it in future versions.
• Documentation Updates: The documentation for the C API (c_api.h) and Python language guide (python_lang_guide.md) has been updated to reflect the removal of the -2 error code.

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[gemini-code-assist]

Code Review

This pull request refactors the error handling for EnvErrorAlreadySet by making it a kind of ffi::Error instead of a separate exception type. This simplifies the error handling logic across the C++ and Python FFI boundaries. The changes look good and are consistent. I've identified some code duplication in the Cython files and suggested a refactoring to improve maintainability by introducing a shared helper function for error checking.

[tqchen]

cc @guan404ming

[guan404ming]

Thanks for this @tqchen!