GH-41919: [C++][FlightRPC] Avoid using raw pointers for auth handlers

[jcferretti]

Rationale for this change

Avoid pointer access after deletion bug explained in #41919

What changes are included in this PR?

Change SetToken argument from a raw pointer to an std::shared_ptr.

Are these changes tested?

This PR leans on existing CI tests and compile-time type checking, no additional/specific testing was performed.

Are there any user-facing changes?

No

• GitHub Issue: [C++][FlightRPC] FlightClient.authenticate is not thread safe #41919

[github-actions]

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See also:

• Other pull requests
• Contribution Guidelines - Contributing Overview

[github-actions]

⚠️ GitHub issue #41919 has been automatically assigned in GitHub to PR creator.

[jcferretti]

The AppVeyor github check failure seems unrelated to change:

error    libmamba Failed to download package from https://conda.anaconda.org/conda-forge/nomkl-1.0-h5ca1d4c_0.tar.bz2 (status 404)
Multi-download failed. Reason: Transfer finalized, status: 404 [https://conda.anaconda.org/conda-forge/nomkl-1.0-h5ca1d4c_0.tar.bz2] 568 bytes
# >>>>>>>>>>>>>>>>>>>>>> ERROR REPORT <<<<<<<<<<<<<<<<<<<<<<
    Traceback (most recent call last):
      File "C:\Miniconda38-x64\lib\site-packages\conda\exception_handler.py", line 18, in __call__
        return func(*args, **kwargs)
      File "C:\Miniconda38-x64\lib\site-packages\mamba\mamba.py", line 959, in exception_converter
        raise e
      File "C:\Miniconda38-x64\lib\site-packages\mamba\mamba.py", line 952, in exception_converter
        exit_code = _wrapped_main(*args, **kwargs)
      File "C:\Miniconda38-x64\lib\site-packages\mamba\mamba.py", line 898, in _wrapped_main
        result = do_call(parsed_args, p)
      File "C:\Miniconda38-x64\lib\site-packages\mamba\mamba.py", line 767, in do_call
        exit_code = create(args, parser)
      File "C:\Miniconda38-x64\lib\site-packages\mamba\mamba.py", line 602, in create
        return install(args, parser, "create")
      File "C:\Miniconda38-x64\lib\site-packages\mamba\mamba.py", line 558, in install
        transaction.fetch_extract_packages()
    RuntimeError: Multi-download failed. Reason: Transfer finalized, status: 404 [https://conda.anaconda.org/conda-forge/nomkl-1.0-h5ca1d4c_0.tar.bz2] 568 bytes

[kou]

+1

Could you adjust style by pre-commit run -a?

[jcferretti]

Could you adjust style by pre-commit run -a?

Done.

[lidavidm]

Thanks!

[lidavidm]

That said, this doesn't seem like a guaranteed fix. https://stackoverflow.com/a/47117985/262727

Mutating the shared_ptr while another thread is copying it is still thread-unsafe, just perhaps a little less likely to cause problems than before. I'd still want to at least document the API as thread-unsafe while perhaps allowing this PR to avoid some basic errors

[jcferretti]

That said, this doesn't seem like a guaranteed fix. https://stackoverflow.com/a/47117985/262727

Mutating the shared_ptr while another thread is copying it is still thread-unsafe

I didn't think this was a problem but it is. This fix is wrong.
https://comp.lang.cpp.narkive.com/4JTZFIE0/std-shared-ptr-thread-safety

[jcferretti]

Closing, since this is the wrong fix:
https://comp.lang.cpp.narkive.com/4JTZFIE0/std-shared-ptr-thread-safety