How Can I Pass Custom Interfaces to Go 1.8 Plugins?

go 1.8 plugin use custom interface

Many Go plugins require the ability to pass complex custom interfaces into them. It is, unfortunately, not currently possible to do this normally in the original plugin implementation.

One way to avoid this problem is to make the plugin return a value of type interface{}. The main application can then define the interface it expects and use type assertion on the interface{} value returned by the plugin.

For example, the plugin code might look like this:

package main

import (
    "fmt"
)

type plgFilter struct{}

func (plgFilter) Name() string {
    return "Bob"
}

func (plgFilter) Age() int {
    return 23
}

func GetFilterIface() (f interface{}, err error) {
    f = plgFilter{}
    fmt.Printf("[plugin GetFilterIface] Returning filter: %T %v\n", f, f)
    return
}

And the main application code might look like this:

package main

import (
    "fmt"
    "plugin"
)

func main() {
    p, err := plugin.Open("pg/pg.so")
    if err != nil {
        panic(err)
    }

    GetFilterIface, err := p.Lookup("GetFilterIface")
    if err != nil {
        panic(err)
    }

    filterIface, err := GetFilterIface.(func() (interface{}, error))()
    fmt.Printf("GetFilterIface result: %T %v %v\n", filterIface, filterIface, err)

    myfilter := filterIface.(MyFilter)
    fmt.Println("\tName:", myfilter.Name())
    fmt.Println("\tAge:", myfilter.Age())
}

type MyFilter interface {
    Name() string
    Age() int
}

Another solution is to define the interface in a package outside of the plugin, and both the plugin and your application can import it and refer to it.
For example, if the interface is defined in package filter:

package filter

type Filter interface {
    Name() string
    Age() int
}

Then the plugin code might look like this:

package main

import (
    "fmt"
    "filter"
)

type plgFilter struct{}

func (plgFilter) Name() string {
    return "Bob"
}

func (plgFilter) Age() int {
    return 23
}

func GetFilter() (f filter.Filter, err error) {
    f = plgFilter{}
    fmt.Printf("[plugin GetFilter] Returning filter: %T %v\n", f, f)
    return
}

And the main application code might look like this:

package main

import (
    "fmt"
    "filter"
    "plugin"
)

func main() {
    p, err := plugin.Open("pg/pg.so")
    if err != nil {
        panic(err)
    }

    GetFilter, err := p.Lookup("GetFilter")
    if err != nil {
        panic(err)
    }

    filter, err := GetFilter.(func() (filter.Filter, error))()
    fmt.Printf("GetFilter result: %T %v %v\n", filter, filter, err)
    fmt.Println("\tName:", filter.Name())
    fmt.Println("\tAge:", filter.Age())
}

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