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The Compiler Rewrite in Go

When the Go compiler was first written, it was written in C because the core Go team has a background in Plan 9 and C was its lingua franca. However as a result of either it being written in C or the design around all the tools it was shelling out to, it wasn’t easy to cross compile Go programs. If you were building windows programs on a Mac you needed to do a separate install of Go from source with other targets enabled. This worked, it wasn’t the default though and eventually the Go compiler rewrite in Go changed this so that Go could cross compile natively with no effort required.

This has been such an amazingly productive part of the Go toolchain that I was shocked that Go didn’t have this out of the gate at version 1. Most people that use Go today don’t know that there was a point where Go didn’t have the easy to use cross-compiling superpower it currently has, and I think that is a more sure marker of success than anything else.

Go Modules

In Go's dependency model, you have a folder that contains all your Go code called the GOPATH. The GOPATH has a few top level folders that have a well-known meaning in the Go ecosystem:

bin: binary files made by go install or go get go here
pkg: intermediate compiler state goes here
src: Go packages go here

GOPATH has one major advantage: it is ruthlessly easy to understand the correlation between the packages you import in your code to their locations on disk.

If you need to see what within.website/ln is doing, you go to GOPATH/src/within.website/ln. The files you are looking for are somewhere in there. You don’t have to really understand how the package manager works (mostly because there isn’t one). If you want to hack something up you just go to the folder and add the changes you want to see.

You can delete all of the intermediate compiler state easily in one fell swoop. Just delete the pkg folder and poof, it’s all gone. This was great when you needed to free up a bunch of disk space really quickly because over months the small amount of incremental compiler state can really add up.

The go compiler would fetch any missing packages from the internet at build time so things Just Worked™️. This makes it utterly trivial to check out a project and then build/run it. That combined with go get to automatically just figure things out and install them made installing programs written in Go so easy that it’s almost magic.

It was conceptually simple to reason about. Go code goes in the GOPATH. The best place for it was in the GOPATH. There's no reason to put it anywhere else. Everything was organized into its place and it was lovely.

This wasn’t perfect though. There were notable flaws in this setup that were easy to run into in practice.

There wasn't a good way to make sure that everyone was using the same copies of every library. People did add vendoring tools later to check that everyone was using the same copies of every package, but this also introduced problems when one project used one version of a dependency and another project used another in ways that were mutually incompatible.

The process to get the newest version of a dependency was to grab the latest commit off of the default branch of that git repo. There was support for SVN, mercurial and fossil, but in practice Git was the most used one so it’s almost not worth mentioning the other version control systems. This also left you at the mercy of other random people having good code security sense and required you to audit your dependencies, but this is fairly standard across ecosystems.

Dependency names were case sensitive on Linux but not on Windows or macOS. Arguably this is a "Windows and macOS are broken for backwards compatibility reasons" thing, but this did bite me at random times without warning.

The default location for the GOPATH created a folder in your home directory.

Yeah, yeah, this default was added later but still people complained about having to put the GOPATH somewhere at first. Having to choose a place to put all the Go code they would use seemed like a big choice that people really wanted solid guidance and defaults on. After a while they changed this to default to ~/go (with an easy to use command to influence the defaults without having to set an environment variable). I don't personally understand the arguments people have for wanting to keep their home directory "clean", but the arguments are valid regardless.

If the wrong random people deleted their GitHub repos, there's a chance your builds could break unless your GOPATH had the packages in it already. Then you could share that with your coworkers or the build machine somehow, maybe even uploading those packages to a git repository to soft-fork it.

Overall I think GOPATH was a net good thing for Go. It had its downsides, but as far as these things go it was a very opinionated place to start from. This is something typical to Go (much to people's arguments), but the main thing that it focused on was making Go conceptually simple. There's not a lot going on there. You have code in the folder and then that's where the Go compiler looks for other code. It's a very lightweight approach to things that a lot of other languages could learn a lot from. It's great for monorepos because it basically treats all your Go code as one big monorepo. So many other languages don’t really translate well to working in a monorepo context like Go does.

That making sure everyone had the same versions of everything problem ended up becoming a big problem in practice. I'm assuming that the original intent of the GOPATH was to be similar to how Google's internal monorepo worked, where everyone clones and deals with the entire GOPATH in source control. You'd then have to do GOPATH juggling between monorepos, but the intent was to have everything in one big monorepo anyways, so this wasn't thought of as much of a big deal in practice. It turns out that people in fact did not want to treat Go code this way, in practice this conflicted with the dependency model that Go encouraged people to use with how people consume libraries from GitHub or other such repository hosting sites.

The main disconnect between importing from a GOPATH monorepo and a Go library off of GitHub is that when you import from a monorepo with a GOPATH in it, you need to be sure to import the repository path and not the path used inside the repository. This sounds weird but this is the difference between importing github.com/Xe/x/src/github.com/Xe/x/markov and github.com/Xe/x/markov. This means that things need to be extracted out of monorepos and reformatted into “flat” repos so that you can only grab the one package you need. This became tedious in practice.

In Go 1.5 (the one where they rewrote the compiler in Go) they added support for vendoring code into your repo. The idea here was to make it easy to get closer to the model that the Go authors envisioned for how people should use Go. Go code should all be in one big happy repo and everything should have its place in your GOPATH. This combined with other tools people made allowed you to vendor all of your dependencies into a vendor folder and then you could do whatever you wanted from there.

One of the big advantages of the vendor folder was that you could clone your git repo, create a new process namespace and then run tests without a network stack. Everything would work offline and you wouldn't have to worry about external state leaking in. Not to mention removing the angle of someone deleting their GitHub repos causing a huge problem for your builds.

Save tests that require internet access or a database engine!

This worked for a very long time. People were able to vendor their code into their repos and everything was better for people using Go. However the most critical oversight with the vendor folder approach was that the Go team didn't create an official tool to manage that vendor folder. They wanted to let tools like godep and glide handle that. This is kind of a reasonable take, Go comes from a very Google culture where this kind of problem doesn't happen, so as a result they probably won't be able to come up with something that meets the needs of the outside world very easily.

I can't speak for how godep or glide works, I never really used them enough to have a solid opinion. I do remember using vendor in my own projects though. That had no real dependency resolution algorithm to speak of.

After a while the Go team worked with people in the community to come up with an "official experiment" in tracking dependencies called dep. dep was a tool that used some more fancy computer science maths to help developers declare dependencies for projects in a way like you do in other ecosystems. When dep was done thinking, it emitted a bunch of files in vendor and a lockfile in your repository. This worked really well and when I was working at Heroku this was basically our butter and bread for how to deal with Go code.

It probably helped that my manager was on the team that wrote dep.

One of the biggest advantages of dep over other tools was the way that it solved versioning. It worked by having each package declare constraints in the ranges of versions that everything requires. This allowed it to do some fancy dependency resolution math similar to how the solvers in npm or cargo work.

This worked fantastically in the 99% case. There were some fairly easy to accidentally get yourself in cases where you could make the solver loop infinitely though, as well as ending up in a state where you have mutually incompatible transient dependencies without any real way around it.

npm and cargo work around this by letting you use multiple versions of a single dependency in a project.

However these cases were really really rare, only appearing in much, much larger repositories. I don't think I practically ran into this, but I'm sure someone reading this right now found themselves in dep hell and probably has a hell of a war story around it.

This lead the Go team to come up with a middle path between the unrestricted madness of GOPATH and something more maximal like dep. They eventually called this Go modules and the core reasons for it are outlined in this series of technical posts. Apparently the development of Go modules came out as a complete surprise, even to the core developer team of dep. I'm fairly sure this lead my manager to take up woodworking as his main non work side hobby, I can only wonder about the kind of resentment this created for other parts of the dep team.

The dep team was as close as we've gotten for having people in the actual industry using Go in production outside of Google having a real voice in how Go is used in the real world. I fear that we will never have this kind of thing happen again.

Go modules does solve one very critical problem for the Go ecosystem though: it allows you to have the equivalent of the GOPATH but with multiple versions of dependencies in it. It allows you to have within.website/ln@v0.7 and within.website/ln@0.9 as dependencies for two different projects without having to vendor source code or do advanced GOPATH manipulation between projects. It also adds cryptographic checksumming for each Go module that you download from the internet. This allows you to avoid having to shell out to git every time you fetch a module that someone else has fetched before. Companies could run their own Go module proxy and then use that to provide offline access to Go code fetched from the internet.

Wait, couldn't this allow Google to see the source code of all of your Go dependencies? How would this intersect with private repositories that shouldn't ever be on anything but work machines?

Yeah, this was one of the big privacy disadvantages out of the gate with Go modules. I think that in practice the disadvantages are limited, but still the fact that it defaults to phoning home to Google every time you run a Go build without all the dependencies present locally is kind of questionable. They did make up for this with the checksum verification database a little, but it's still kinda sus.

The earliest version of Go modules basically was a glorified vendor folder manager. This worked out amazingly well and probably made prototyping this a hell of a lot easier. This worked well enough that we used this in production for many services at Heroku. We had no real issues with it and most of the friction was with the fact that most of the existing ecosystem had already been using dep or glide.

If they had shipped this in prod, it probably would have been a huge success. It would also let people continue to use dep, glide and godep, but just doing that would also leave the ecosystem kinda fragmented. You’d need to have code for all 4 version management systems to parse their configuration files and implement algorithms that would be compatible with the semantics of all of them. It would work and the Go team is definitely smart enough to do it, but in practice it would be a huge mess.

This also solved the case-insensitive filesystem problem with bang-casing. This allows them to encode the capital letters in a path in a way that works on macOS and Windows without having to worry about horrifying hacks that are only really in place for Photoshop to keep working.

However one of the bigger downsides that came with Go modules is what I've been calling the "v2 landmine" that Semantic Import Versioning gives you. One of the very earliest bits of Go advice was to make the import paths for version 1 of a project and version 2 of a project different so that people can mix the two to allow more graceful upgrading across a larger project. Semantic Import Versioning enforces this at the toolchain level, which means that it can be the gate between compiling your code or not.

Many people have been telling me that I’m kind of off base for thinking that this is a landmine for people, but I am using the term “landmine” to talk about this because I feel like it reflects the rough edges of unexpectedly encountering this in the wild. It kinda feels like you stepped on a landmine.

The core of this is that when you create major version 2 of a Go project, you need to adjust all your import paths everywhere in that project to import the v2 of that package or you will silently import the v1 version of that package. This can end up making large projects create circular dependencies on themselves, which is quite confusing in practice. When consumers are aware of this, then they can use that to more gradually upgrade larger codebases to the next major version of a Go module, which will allow for smaller refactors.

This also applies to consumers. Given that this kind of thing is something that you only do in Go it can come out of left field. The go router chi tried doing modules in the past and found that it lead to confusing users. Conveniently they only really found this out after the Go modules design was considered final and Semantic Import Versioning has always been a part of Go modules and the Go team is now refusing to budge on this.

My suggestion to people is to never release a version 1.x.x of a Go project to avoid the “v2 landmine”. The Go team claims that the right bit of tooling can help ease the pain, but this tooling never really made it out into the public. I bet it works great inside google3 though!

Overall though, Go modules has been a net positive for the community and for people wanting to create reliable software in Go. It’s just such a big semantic break in how the toolchain works that I almost think it would have been easier to accept if that was Go 2.

Wait, doesn’t the Go compiler have a backwards compatibility promise that any code built with Go 1.x works on go 1.(x+1)?

Yes, but that only applies to code you write, not semantics of the toolchain itself. On one hand this makes a lot of sense and on the other it feels like a cop-out. The changes in how go get now refers to adding dependencies to a project and go install now installs a binary to the system have made an entire half decade of tool installation documentation obsolete. It’s understandable why they want to make that change, but the way that it broke people’s muscle memory is quite frustrating for users that aren’t keeping on top of every single change in semantics of toolchains (this bites me constantly when I need to quick and dirty grab something outside of a Nix package). I understand why this isn’t a breaking change as far as the compatibility promise but this feels like a cop-out.

Contexts

One of Go’s major features is its co-operative threading system that it calls goroutines. Goroutines are kinda like coroutines that are scheduled by the scheduler. However there is no easy way to "kill" a goroutine. You have to add something to the invocation of the goroutine that lets you signal it to stop and then opt-in the goroutine to stop. This ended up leading to the context package being created in the standard library. A Go context lets you more easily and uniformly handle timeouts and giving up when there is no more work to be done.

Mara+hacker\ This started as something that existed inside the Google monorepo that escaped out into the world. They also claim to have an internal tool that makes context.TODO() useful (probably by showing you the callsities above that function?), but they never released that tool as open source so it’s difficult to know where to use it without that added context.

Examples of how to thread them in:
- Basic example using a select statement and a timer to poll the context timeout vs the timer (let’s pretend the timer is some important but cancellable event that takes time to process)
- HTTP request
- Handling a control-C signal and cancelling a bunch of HTTP request

This is a huge benefit to the language because of how disjointed the process of doing this before contexts was. Trying to do this before contexts usually made you create a "stop channel" where you’d feed it a bool or a struct{}{} to signal that the other side doesn't need to care anymore. Because this wasn’t in the core of the language, every single implementation was different and required learning what the library did.

However I wish that the documentation was a bit more clear as to what they really offer and had some more examples of how to use them. Without context as to what contexts do, its documentation can kind of read like this. This can make explaining what a context is to people kind of annoying.

If you know Lojban, some of the satire in vanbi may be lost on you. Just pretend you don’t understand any of the words in there.

I understand why they put data into the context type, but in practice I really wish they didn’t do that. This feature has been abused a lot in my experience. At Heroku a few of our production load bearing services used contexts as a dependency injection framework. This did work, but it turned a lot of things that would normally be compile time errors into runtime errors.

I say this as someone who maintains a library that uses contexts to store contextually relevant log fields as a way to make logs easier to correlate between. Arguably you could make the case that people are misusing the tool and of course this is what will happen when you do that but I don't know if this is really the right thing to tell people.

I wish contexts were in the core of the language from the beginning. I know that it is difficult to do this in practice (especially on all the targets that Go supports), but having cancellable syscalls would be so cool. It would also be really neat if contexts could be goroutine-level globals so you didn’t have to “pollute” the callsites of every function with them.

Generics

One of the biggest ticket items that Go has added is "generic types", or being able to accept types as parameters for other types. This is really a huge ticket item and I feel that in order to understand why this is a huge change I need to cover the context behind what you had before generics were added to the language.

One of the major standout features of Go is interface types. They are like Rust Traits, Java Interfaces, or Haskell Typeclasses; but the main difference is that interface types are implicit rather than explicit. When you want to meet the signature of an interface, all you need to do is implement the contract that the interface spells out. So if you have an interface like this:

type Quacker interface {
  Quack()
}

You can make a type like Duck a Quacker by defining the Duck type and a Quack method like this:

type Duck struct{}

func (Duck) Quack() { fmt.Println("Quack!") }

But this is not limited to just Ducks, you could easily make a Sheep a Quacker fairly easily:

type Sheep struct{}

func (Sheep) Quack() { fmt.Println("*confused sheep noises*") }

This allows you to deal with expected behaviors of types rather than having to have versions of functions for every concrete implementation of them. If you want to read from a file, network socket, tar archive, zip archive, the decrypted form of an encrypted stream, a TLS socket, or a HTTP/2 stream they're all io.Reader instances. With the example above we can make a function that takes a Quacker and then does something with it:

func main() {
  duck := Duck{}
  sheep := Sheep{}
  
  doSomething(duck)
  doSomething(sheep)
}

func doSomething(q Quacker) {
  q.Quack()
}

If you want to play with this example, check it out on the Go playground here. Try to make a slice of Quackers and pass it to doSomething!

You can also embed interfaces into other interfaces, which will let you create composite interfaces that assert multiple behaviours at once. For example, consider io.ReadWriteCloser. Any value that matches an io.Reader, io.Writer and an io.Closer will be able to be treated as an io.ReadWriteCloser. This allows you to assert a lot of behaviour about types even though the actual underlying types are opaque to you.

This means it’s easy to split up a net.Conn into its reader half and its writer half without really thinking about it:

conn, _ := net.Dial("tcp", "127.0.0.1:42069")

var reader io.Reader = conn
var writer io.Writer = conn

And then you can pass the writer side off to one function and the reader side off to another.

There’s also a bunch of room for "type-level middleware" like io.LimitReader. This allows you to set constraints or details around an interface type while still meeting the contract for that interface, such as an io.Reader that doesn’t let you read too much, an io.Writer that automatically encrypts everything you feed It with TLS, or even something like sending data over a Unix socket instead of a TCP one. If it fits the shape of the interface, it Just Works.

Show where that falls apart
- The container package
- Cloner
- Viewer
Introduce Go generics
- Overview of some of the types of collections it lets you make
  - Take a function with a slice Ducks or a slice of Sheep but not mixed Ducks and Sheep
- This is a huge improvement to the language

We already have Go 2. It’s just called Go 1.18 for some reason. It’s got so many improvements and fundamental changes that I believe that this is already Go 2 in spirit. I, as some random person on the internet that is not associated with the Go team, think that if there was sufficient political will that they could probably label what we have as Go 2, but I don’t think that is going to happen any time soon. Until then, we still have a very great set of building blocks that allow you to make easy to maintain production quality services, and I don’t see that changing any time soon.

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The Compiler Rewrite in Go

Go Modules

Contexts

Generics

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