Static plugins

[shameekganguly]

🎉 New feature

Summary

This change adds support to register plugins statically and load them from a library that is directly linked into the main program. Two new macros are added for static plugin registration: GZ_ADD_STATIC_PLUGIN and GZ_ADD_STATIC_PLUGIN_ALIAS which are functionally identical to their runtime-loading equivalents GZ_ADD_PLUGIN and GZ_ADD_PLUGIN_ALIAS respectively. Users can use this feature by invoking the static registration macros to register their plugin, and force-linking the plugin library into the program where they wish to load the plugin. The existing Loader class can be used to load plugins that have been linked in statically (see test/integration/static_plugins.cc for example).

Note that this change refactors the implementation of the following:

• register/include/gz/plugin/detail/Register.hh to share code with register/include/gz/plugin/detail/RegisterStatic.hh, and
• loader/src/Loader.cc to share code between querying and instantiating plugins from libraries which are loaded from files and linked-in libraries. A Registry class is introduced which acts as a store for a list of plugin Infos. A singleton StaticRegistry class is derived from Registry to maintain the store of statically registered plugin Infos.

Test it

Checklist

• Signed all commits for DCO
• Added tests
• codecheck passed (See contributing)
• All tests passed (See test coverage)

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #97 (87012c8) into main (9ddd731) will decrease coverage by 1.09%.
The diff coverage is 96.73%.

@@            Coverage Diff             @@
##             main      #97      +/-   ##
==========================================
- Coverage   99.35%   98.25%   -1.10%     
==========================================
  Files          17       23       +6     
  Lines         617      745     +128     
==========================================
+ Hits          613      732     +119     
- Misses          4       13       +9     

Impacted Files	Coverage Δ
core/include/gz/plugin/PluginPtr.hh	100.00% <ø> (ø)
loader/include/gz/plugin/detail/StaticRegistry.hh	33.33% <33.33%> (ø)
core/src/Plugin.cc	97.70% <71.42%> (-2.30%)	⬇️
loader/src/detail/StaticRegistry.cc	90.62% <90.62%> (ø)
loader/include/gz/plugin/detail/Loader.hh	93.75% <90.90%> (-6.25%)	⬇️
loader/src/detail/Registry.cc	99.09% <99.09%> (ø)
core/include/gz/plugin/detail/PluginPtr.hh	100.00% <100.00%> (ø)
loader/include/gz/plugin/detail/Registry.hh	100.00% <100.00%> (ø)
loader/src/Loader.cc	100.00% <100.00%> (ø)
register/include/gz/plugin/detail/Common.hh	100.00% <100.00%> (ø)
... and 2 more

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[mjcarroll]

I am continuing to investigate the windows failures here in a local VM.

[shameekganguly]

Thanks for the clean up and Windows fixes Michael!

[mjcarroll]

I'm happy with this. Thanks for following style as much as possible.

I think this will be a very helpful improvement, interested to try it specifically with testing in gz-sim testing plugins.

[mjcarroll]

I made a sketch of how this can be used in gz-sim, to be fleshed out after feature freeze (for first Garden minor release): gazebosim/gz-sim#1623