golang从插件导出所有符号

Question

Golang plugin pkg only support Lookup a symbol by name.

How can I scan all symbols and its type in a plugin?

https://golang.org/pkg/plugin/

Answer 1

The current plugin API of Go does not support discovering and using unknown plugins. You have to know the plugin and know what to expect from it (including the types of exported symbols).

One day there might be additions to the API for what you're looking, but for now (Go 1.9.2) you're stuck with Plugin.Lookup() only.

"Hack" to peek at exported symbols

A loaded plugin is represented by a value of type *plugin.Plugin. This is a pointer to a struct which contains the symbol table, but it's not exported. If you want to take a "peek" at what symbols are exported, you may use the fmt package to print it.

I will use the plugin example from this answer: go 1.8 plugin use custom interface, which is:

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
", f, f)
    return
}

It contains a single exported symbol: the GetFilter function. Loading and printing the symbol table of this plugin:

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

fmt.Printf("%+v
", p)

We can "observe" the exported GetFilter symbol in the output:

&{pluginpath:pg loaded:0xc420020120 syms:map[GetFilter:0x7f0a7fddc860]}

The associated value for this symbol is a memory address, not really talkative.

But we can do more: we can use package reflect to print its type after we looked it up:

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

fmt.Println(reflect.TypeOf(GetFilter))

And now we'll see:

func() (filter.Filter, error)

We see it's a function which has 2 return types: filter.Filter and error.