You asked why go.itabs are dupok. I believe the fundamental reason is that Go can be s

That makes perfect sense <a class="user-mention notranslate" data-hovercard-type="user

chapter2: How you can get duplicated go.itab interface definitions about go-internals HOT 2 CLOSED

teh-cmc commented on August 27, 2024 3

chapter2: How you can get duplicated go.itab interface definitions

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Comments (2)

teh-cmc commented on August 27, 2024

That makes perfect sense @siebenmann, thanks for the great explanation.

I've added an exact reproduction of what you've described here.

Layout:

$ tree
.
├── A
│   └── lib.go
├── B
│   └── lib.go
├── C
│   └── lib.go
├── D
│   └── lib.go
└── main.go

As you've described:

A declares a type Calc which implements B.Adder.

package A

type Calc struct{}

func (c *Calc) Add(a, b int32) int32 { return a + b }

B declares an Adder interface.

package B

type Adder interface{ Add(a, b int32) int32 }

C implements a function Add that instantiates an iface<B.Adder, *A.Calc> internally:

package C

import (
	"github.com/teh-cmc/go-internals/chapter2_interfaces/issue_7/A"
	"github.com/teh-cmc/go-internals/chapter2_interfaces/issue_7/B"
)

func Add(a, b int32) int32 {
	var adder B.Adder = &A.Calc{}
	return adder.Add(a, b)
}

D does the same thing as C

package D

import (
	"github.com/teh-cmc/go-internals/chapter2_interfaces/issue_7/A"
	"github.com/teh-cmc/go-internals/chapter2_interfaces/issue_7/B"
)

func Add(a, b int32) int32 {
	var adder B.Adder = &A.Calc{}
	return adder.Add(a, b)
}

Finally, the main package calls C.Add and D.Add:

package main

import (
	"fmt"

	"github.com/teh-cmc/go-internals/chapter2_interfaces/issue_7/C"
	"github.com/teh-cmc/go-internals/chapter2_interfaces/issue_7/D"
)

func main() {
	fmt.Println(C.Add(10, 32))
	fmt.Println(D.Add(10, 32))
}

Looking at the output of go build:

$ go build -x
# ...
packagefile github.com/teh-cmc/go-internals/chapter2_interfaces/issue_7/C=$HOME/.cache/go-build/ff/ffc441d2cc7fd2bd9e12722f11fd3407dc4280577c0c74f8cb5241d72792d554-d
packagefile github.com/teh-cmc/go-internals/chapter2_interfaces/issue_7/D=$HOME/.cache/go-build/7c/7cc79061754b8fa877646bcbf5f10814217ac3dc48fc66c5964f08dd823695de-d
# ...

We can see the linker importing the archives for both package C & D, as expected.

Now if we look for itabs in those archives:

$ go tool nm $HOME/.cache/go-build/ff/ffc441d2cc7fd2bd9e12722f11fd3407dc4280577c0c74f8cb5241d72792d554-d | grep 'itab\.'
     af7 R go.itab.*github.com/teh-cmc/go-internals/chapter2_interfaces/issue_7/A.Calc,github.com/teh-cmc/go-internals/chapter2_interfaces/issue_7/B.Adder

$ go tool nm $HOME/.cache/go-build/7c/7cc79061754b8fa877646bcbf5f10814217ac3dc48fc66c5964f08dd823695de-d | grep 'itab\.'
     af7 R go.itab.*github.com/teh-cmc/go-internals/chapter2_interfaces/issue_7/A.Calc,github.com/teh-cmc/go-internals/chapter2_interfaces/issue_7/B.Adder

We see that the itab for iface<B.Adder, *A.Calc> is indeed duplicated in both archives; so the linker will have to pick one.

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teh-cmc commented on August 27, 2024

I've added a link to this discussion in the relevant part of chapter 2; and will be closing this now.
Don't hesitate to add more comments even if it's closed!

Hopefully I'll take the time to update chapter 2 with all these learnings some day.

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chapter2: How you can get duplicated go.itab interface definitions about go-internals HOT 2 CLOSED

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