Why Do Go Plugins Fail to Return Interfaces Directly, and How Can This Be Resolved?

Plugin Symbols as Function Returns

In Go, it is possible to import a plugin that implements a common interface defined outside of both packages. When returning a struct from the plugin, the interface is correctly implemented. However, attempting to return an interface directly fails due to certain technicalities in Go's plugin system.

The reason lies in the nature of plugin symbols. When looking up a variable named Greeter from a plugin using Plugin.Lookup(), it returns a pointer to that variable. This is necessary to allow changes to be made to the variable within the plugin.

Printing the type of the value stored in sym will reveal whether it is a pointer to the interface (iface.IPlugin) or an actual interface value (iface.IPlugin). In the case of the plugin returning a struct, the value stored in sym is of type main.testpl, which implements iface.IPlugin, so the type assertion succeeds.

However, when the plugin returns iface.IPlugin, the value stored in sym is of type *iface.IPlugin. This pointer to an interface value does not satisfy any interfaces directly, including iface.IPlugin. Therefore, the type assertion fails.

To bypass this issue, the plugin can be modified to expose a function that returns the Greeter interface instead of a global variable:

func Greeter() iface.IPlugin { return testpl{} }

This will expose a symbol named Greeter with type func() iface.IPlugin. When looked up using Plugin.Lookup(), it will return a function pointer. This eliminates the need for a pointer to the interface value and allows for the correct type assertion:

greeterFunc, ok := GetFilter.(func() iface.IPlugin)
if !ok {
    panic(errors.New("not of expected type"))
}
greeter := greeterFunc()

This method provides a clearer and more consistent approach to exposing plugin functionality while avoiding the pitfalls associated with returning interfaces directly from plugins.

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