Why Does Returning an Interface as a Go Plugin Symbol Fail, and How Can It Be Fixed?

Binding Plugins to Interfaces: A Conundrum

In Go, it's possible to create plugins that implement specific interfaces. However, there's a peculiar behaviour when returning an interface as a plugin symbol. Let's delve into this issue and explore a solution.

The Interface and Plugin:

We define an interface IPlugin with various methods and create a plugin containing a struct testpl that implements it. Initially, we have three different getPlugin functions:

• The first returns a testpl struct directly, which works as expected.
• The second returns testpl as an IPlugin interface, but it fails when binding the plugin symbol.
• The third returns a generic interface{}, which also fails.

The Problem with Lookup:

The solution lies in understanding how Plugin.Lookup() works. It returns a pointer to the symbol being looked up. In the case where the plugin exposes a variable (such as Greeter), the value stored in sym is a pointer to the variable.

The problem arises when we try to bind the plugin symbol to an interface. A value of a pointer type to an interface never satisfies any interfaces except the empty interface. Therefore, attempting to type-assert iface.IPlugin type from a value of type iface.IPlugin will always fail.

The Solution:

To resolve this issue, we need to either:

• Return a function from the plugin that returns the greeter (Greeter() iface.IPlugin). This way, when we lookup the Greeter symbol, it will be of type func() iface.IPlugin, which doesn't require the plugin package to return a pointer to the value.
• Dereference the plugin symbol pointer to obtain a value of type iface.IPlugin. This can be done as follows:

pgPtr, ok := sym.(*iface.IPlugin)
if !ok {
    panic(errors.New("error binding plugin to interface"))
}
pg := *pgPtr

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