How to discover all package types at runtime?

Question

As far as I'm aware (see here, and here) there is no type discovery mechanism in the reflect package, which expects that you already have an instance of the type or value you want to inspect.

Is there any other way to discover all exported types (especially the structs) in a running go package?

Here's what I wish I had (but it doesn't exist):

import "time"
import "fmt"

func main() {
    var types []reflect.Type
    types = reflect.DiscoverTypes(time)
    fmt.Println(types)
}

The end goal is to be able to discover all the structs of a package that meet certain criteria, then be able to instantiate new instances of those structs.

BTW, a registration function that identifies the types is not a valid approach for my use case.

---

Whether you think it's a good idea or not, here's why I want this capability (because I know you're going to ask):

I've written a code generation utility that loads go source files and builds an AST to scan for types that embed a specified type. The output of the utility is a set of go test functions based on the discovered types. I invoke this utility using go generate to create the test functions then run go test to execute the generated test functions. Every time the tests change (or a new type is added) I must re-run go generate before re-running go test. This is why a registration function is not a valid option. I'd like to avoid the go generate step but that would require my utility to become a library that is imported by the running package. The library code would need to somehow scan the running namespace during init() for types that embed the expected library type.

Answer 1

In Go 1.5, you can use the new package types and importer to inspect binary and source packages. For example:

package main

import (
    "fmt"
    "go/importer"
)

func main() {
    pkg, err := importer.Default().Import("time")
    if err != nil {
        fmt.Printf("error: %s\n", err.Error())
        return
    }
    for _, declName := range pkg.Scope().Names() {
        fmt.Println(declName)
    }
}

You can use the package go/build to extract all the packages installed. Or you can configure the Lookup importer to inspect binaries outside the environment.

Before 1.5, the only no-hacky way is to use the package ast to compile the source code.

Answer 2

(see bottom for 2019 update)

Warning: untested and hacky. Can break whenever a new version of Go is released.

It is possible to get all types the runtime knows of by hacking around Go's runtime a little. Include a small assembly file in your own package, containing:

TEXT yourpackage·typelinks(SB), NOSPLIT, $0-0
    JMP reflect·typelinks(SB)

In yourpackage, declare the function prototype (without body):

func typelinks() []*typeDefDummy

Alongside a type definition:

type typeDefDummy struct {
    _      uintptr           // padding
    _      uint64            // padding
    _      [3]uintptr        // padding
    StrPtr *string           
}

Then just call typelinks, iterate over the slice and read each StrPtr for the name. Seek those starting with yourpackage. Note that if there are two packages called yourpackage in different paths, this method won't work unambiguously.

can I somehow hook into the reflect package to instantiate new instances of those names?

Yeah, assuming d is a value of type *typeDefDummy (note the asterisk, very important):

t := reflect.TypeOf(*(*interface{})(unsafe.Pointer(&d)))

Now t is a reflect.Type value which you can use to instantiate reflect.Values.

---

Edit: I tested and executed this code successfully and have uploaded it as a gist.

Adjust package names and include paths as necessary.

Update 2019

A lot has changed since I originally posted this answer. Here's a short description of how the same can be done with Go 1.11 in 2019.

$GOPATH/src/tl/tl.go

package tl

import (
    "unsafe"
)

func Typelinks() (sections []unsafe.Pointer, offset [][]int32) {
    return typelinks()
}

func typelinks() (sections []unsafe.Pointer, offset [][]int32)

func Add(p unsafe.Pointer, x uintptr, whySafe string) unsafe.Pointer {
    return add(p, x, whySafe)
}

func add(p unsafe.Pointer, x uintptr, whySafe string) unsafe.Pointer

$GOPATH/src/tl/tl.s

TEXT tl·typelinks(SB), $0-0
    JMP reflect·typelinks(SB)

TEXT tl·add(SB), $0-0
    JMP reflect·add(SB)

main.go

package main

import (
    "fmt"
    "reflect"
    "tl"
    "unsafe"
)

func main() {
    sections, offsets := tl.Typelinks()
    for i, base := range sections {
        for _, offset := range offsets[i] {
            typeAddr := tl.Add(base, uintptr(offset), "")
            typ := reflect.TypeOf(*(*interface{})(unsafe.Pointer(&typeAddr)))
            fmt.Println(typ)
        }
    }
}

Happy hacking!