### Backwards compatibility in go

2015-07-29

tl;dr: There are next to no "backwards compatible API changes" in go. You should explicitely name your compatibility-guarantees.

I really love go, I really hate vendoring and up until now I didn't really get, why anyone would think go should need something like that. After all, go seems to be predestined to be used with automatically checked semantic versioning. You can enumerate all possible changes to an API in go and the list is quite short. By looking at vcs-tags giving semantic versions and diffing the API, you can automatically check that you never break compatibility (the go compiler and stdlib actually do something like that). Heck, in theory you could even write a package manager that automatically (without any further annotations) determines the latest version of a package that still builds all your stuff or gives you the minimum set of packages that need changes to reconcile conflicts.

This thought lead me to contemplate what makes an API change a breaking change. After a bit of thought, my conclusion is that almost every API change is a breaking change, which might surprise you.

For this discussion we first need to make some assumptions about what constitutes breakage. We will use the go1 compatibility promise. The main gist is: Stuff that builds before is guaranteed to build after. Notable exceptions (apart from necessary breakages due to security or other bugs) are unkeyed struct literals and dot-imports.

 I should clarify, that whenever I talk about an API-change, I mean your exported API as defined by Code (as opposed to comments/documentation). This includes the public identifiers exported by your package, including type information. It excludes API-requirements specified in documentation, like on io.Writer. These are just too complex to talk about in a meaningful way and must be dealt with separately anyway. [/edit]

So, given this definition of breakage, we can start enumerating all the possible changes you could do to an API and check whether they are breaking under the definition of the go1 compatibility promise:

#### Adding func/type/var/const at package scope

This is the only thing that seems to be fine under the stability guarantee. It turns out the go authors thought about this one and put the exception of dot-imports into the compatibility promise, which is great.

dot-imports are imports of the form . import "foo". They import every package-level identifier of package foo into the scope of the current file.

Absence of dot-imports means, every identifier at your package scope must be referenced with a selector-expression (i.e. foo.Bar) which can't be redeclared by downstream. It also means that you should never use dot-imports in your packages (which is a bad idea for other reasons too). Treat dot-imports as a historic artifact which is completely deprecated. An exception is the need to use a separate foo_test package for your tests to break dependency cycles. In that case it is widely deemed acceptable to . import "foo" to save typing and add clarity.

#### Removing func/type/var/const at package scope

Downstream might use the removed function/type/variable/constant, so this is obviously a breaking change.

#### Adding a method to an interface

Downstream might want to create an implementation of your interface and try to pass it. After you add a method, this type doesn't implement your interface anymore and downstreams code will break.

#### Removing a method from an interface

Downstream might want to call this method on a value of your interface type, so this is obviously a breaking change.

#### Adding a field to a struct

This is perhaps surprising, but adding a field to a struct is a breaking change. The reason is, that downstream might embed two types into a struct. If one of them has a field or method Bar and the other is a struct you added the Field Bar to, downstream will fail to build (because of an ambiguous selector expression).

So, e.g.:

// foo/foo.go
package foo

type Foo struct {
Foo string
Bar int // Added after the fact
}

// bar/bar.go
package bar

type Baz struct {
Bar int
}

type Spam struct {
foo.Foo
Baz
}

func Eggs() {
var s Spam
s.Bar = 42 // ambiguous selector s.Bar
}


This is what the compatibility might refer to with the following quote:

Code that uses unkeyed struct literals (such as pkg.T{3, "x"}) to create values of these types would fail to compile after such a change. However, code that uses keyed literals (pkg.T{A: 3, B: "x"}) will continue to compile after such a change. We will update such data structures in a way that allows keyed struct literals to remain compatible, although unkeyed literals may fail to compile. (There are also more intricate cases involving nested data structures or interfaces, but they have the same resolution.)

(emphasis is mine). By "the same resolution" they might refer to only accessing embedded Fields via a keyed selector (so e.g. s.Baz.Bar in above example). If so, that is pretty obscure and it makes struct-embedding pretty much useless. Every usage of a field or method of an embedded type must be explicitly Keyed, which means you can just not embed it after all. You need to write the selector and wrap every embedded method anyway.

I hope we all agree that type embedding is awesome and shouldn't need to be avoided :)

#### Removing a field from a struct

Downstream might use the now removed field, so this is obviously a breaking change.

#### Adding a method to a type

The argument is pretty much the same as adding a field to a struct: Downstream might embed your type and suddenly get ambiguities.

#### Removing a method from a type

Downstream might call the now removed method, so this is obviously a breaking change.

#### Changing a function/method signature

Most changes are obviously breaking. But as it turns out you can't do any change to a function or method signature. This includes adding a variadic argument which looks backwards compatible on the surface. After all, every call site will still be correct, right?

The reason is, that downstream might save your function or method in a variable of the old type, which will break because of nonassignable types.

#### Conclusion

It looks to me like anything that isn't just adding a new Identifier to the package-scope will potentially break some downstream. This severely limits the kind of changes you can do to your API if you want to claim backwards compatibility.

This of course doesn't mean that you should never ever make any changes to your API ever. But you should think about it and you should clearly document, what kind of compatibility guarantees you make. When you do any changes named in this document, you should check your downstreams, whether they are affected by it. If you claim a similar level of compatibility as the go standard library, you should definitely be aware of the implications and what you can and can't do.

We, the go community, should probably come up with some coherent definition of what changes we deem backwards compatible and which we don't. A tool to automatically looks up all your (public) importerts on godoc.org, downloads the latest version and tries to build them with your changes should be fairly simple to write in go (and may even already exist). We should make it a standard check (like go vet and golint) for upstream package authors to do that kind of thing before push to prevent frustrated downstreams.

Of course there is still the possibility, that my reading of the go1 compatibility promise is wrong or inaccurate. I would welcome comments on that, just like on everything else in this post :)