BLD: Respect existing zstd target

[NuLL3rr0r]

Pull request

Summary

Respect an existing zstd::libzstd target provided by the consuming project instead of unconditionally calling find_package(zstd REQUIRED).

Databento currently searches for zstd before checking whether the expected target already exists. This can cause a conflict when a consuming project provides its own zstd target while a separate system installation is also discoverable. In this case, the system zstd package may attempt to define zstd::libzstd again.

This was encountered on FreeBSD with zstd installed under /usr/local, resulting in:

Some (but not all) targets in this export set were already defined.

Targets Defined: zstd::libzstd

Targets not yet defined: zstd::libzstd_shared, zstd::libzstd_static

The change only calls find_package(zstd REQUIRED) when zstd::libzstd does not already exist. This follows the same pattern already used for other dependencies such as nlohmann_json, httplib, and date.

Consumers that do not provide zstd::libzstd are unaffected and continue to use the existing zstd package discovery.

No new dependencies are required.

Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)

How has this change been tested?

Tested as a dependency of a consuming CMake project that builds zstd with add_subdirectory() and provides zstd::libzstd before adding Databento.

Test configuration:

• FreeBSD, Linux, and Windows
• On FreeBSD, zstd also installed system-wide under /usr/local
• consuming project provides its own zstd::libzstd target

Before this change, configuration fails on FreeBSD because the system zstd package attempts to redefine an existing target.

Checklist

• My code builds locally with no new warnings (scripts/build.sh)
• My code follows the [style guidelines](https://google.github.io/styleguide/cppguide.html)
• New and existing unit tests pass locally with my changes (scripts/test.sh)
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

Declaration

I confirm this contribution is made under an Apache 2.0 license and that I have the authority
necessary to make this contribution on behalf of its copyright owner.