How Can I Use Custom Interfaces Effectively within Go Plugins?

Custom Interfaces in Go Plugins

Question: How can I use custom interfaces with Go plugins?

Background: Custom interfaces are essential for implementing reusable and extensible software components. However, initial attempts to use them within Go plugins resulted in the following error:

panic: interface conversion: plugin.Symbol is func() (filter.Filter, error), not func() (filter.Filter, error)

Answer:

While Go plugins cannot access types defined within the plugin itself, it is possible to use custom interfaces by following these methods:

1. Using a Common Package:

Define the interface in a package that is both imported by the plugin and the main application. This approach allows both components to use the same type definition.

2. Returning an Interface{} from Plugin:

Have the plugin function return a value of type interface{}. The main application can then define a type assertion on the returned value to use the custom interface.

Example:

Main Application:

package main

import (
    "fmt"
    "plugin"

    "github.com/fagongzi/gateway/pkg/filter"
)

func main() {
    // ...
    GetInstance(func() (filter.Filter, error)) // Error occurs
}

Plugin:

package plugin

import "github.com/fagongzi/gateway/pkg/filter"

type plgFilter struct{}

func (plgFilter) Name() string   { return "Bob" }
func (plgFilter) Age() int      { return 23 }

// Return an "empty" filter to satisfy the function signature
func GetInstance() (filter.Filter, error)         { return nil, nil }
func GetInstanceInter() (interface{}, error)       { return &plgFilter{}, nil }
func GetInstancePtr() (*plgFilter, error)      { return &plgFilter{}, nil }

Comparison:

• Option 1: Requires the presence of the interface in both the plugin and application code.
• Option 2: Allows the plugin to return values of any type, offering greater flexibility.

Additional Notes:

• Custom interfaces can only be used within plugins that have no other dependencies.
• Plugins cannot define their own interfaces that can be referenced from the main application.

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