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# Description This allows plugins to make calls back to the engine to get config, evaluate closures, and do other things that must be done within the engine process. Engine calls can both produce and consume streams as necessary. Closures passed to plugins can both accept stream input and produce stream output sent back to the plugin. Engine calls referring to a plugin call's context can be processed as long either the response hasn't been received, or the response created streams that haven't ended yet. This is a breaking API change for plugins. There are some pretty major changes to the interface that plugins must implement, including: 1. Plugins now run with `&self` and must be `Sync`. Executing multiple plugin calls in parallel is supported, and there's a chance that a closure passed to a plugin could invoke the same plugin. Supporting state across plugin invocations is left up to the plugin author to do in whichever way they feel best, but the plugin object itself is still shared. Even though the engine doesn't run multiple plugin calls through the same process yet, I still considered it important to break the API in this way at this stage. We might want to consider an optional threadpool feature for performance. 2. Plugins take a reference to `EngineInterface`, which can be cloned. This interface allows plugins to make calls back to the engine, including for getting config and running closures. 3. Plugins no longer take the `config` parameter. This can be accessed from the interface via the `.get_plugin_config()` engine call. # User-Facing Changes <!-- List of all changes that impact the user experience here. This helps us keep track of breaking changes. --> Not only does this have plugin protocol changes, it will require plugins to make some code changes before they will work again. But on the plus side, the engine call feature is extensible, and we can add more things to it as needed. Plugin maintainers will have to change the trait signature at the very least. If they were using `config`, they will have to call `engine.get_plugin_config()` instead. If they were using the mutable reference to the plugin, they will have to come up with some strategy to work around it (for example, for `Inc` I just cloned it). This shouldn't be such a big deal at the moment as it's not like plugins have ever run as daemons with persistent state in the past, and they don't in this PR either. But I thought it was important to make the change before we support plugins as daemons, as an exclusive mutable reference is not compatible with parallel plugin calls. I suggest this gets merged sometime *after* the current pending release, so that we have some time to adjust to the previous plugin protocol changes that don't require code changes before making ones that do. # Tests + Formatting - 🟢 `toolkit fmt` - 🟢 `toolkit clippy` - 🟢 `toolkit test` - 🟢 `toolkit test stdlib` # After Submitting I will document the additional protocol features (`EngineCall`, `EngineCallResponse`), and constraints on plugin call processing if engine calls are used - basically, to be aware that an engine call could result in a nested plugin call, so the plugin should be able to handle that. |
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