Category Archives: Go

Go, without package scoped variables

This is a thought experiment, what would Go look like if we could no longer declare variables at the package level? What would be the impact of removing package scoped variable declarations, and what could we learn about the design of Go programs?

I’m only talking about expunging var, the other five top level declarations would still be permitted as they are effectively constant at compile time. You can, of course, continue to declare variables at the function or block scope.

Why are package scoped variables bad?

But first, why are package scoped variables bad? Putting aside the problem of globally visible mutable state in a heavily concurrent language, package scoped variables are fundamentally singletons, used to smuggle state between unrelated concerns, encourage tight coupling and makes the code that relies on them hard to test.

As Peter Bourgon wrote recently:

tl;dr: magic is bad; global state is magic → [therefore, you want] no package level vars; no func init.

Removing package scoped variables, in practice

To put this idea to the test I surveyed the most popular Go code base in existence; the standard library, to see how package scoped variables were used, and assessed the effect applying this experiment would have.

Errors

One of the most frequent uses of public package level var declarations are errors; io.EOF,
sql.ErrNoRows, crypto/x509.ErrUnsupportedAlgorithm, and so on. Removing the use of package scoped variables would remove the ability to use public variables for sentinel error values. But what could be used to replace them?

I’ve written previously that you should prefer behaviour over type or identity when inspecting errors. Where that isn’t possible, declaring error constants removes the potential for modification which retaining their identity semantics.

The remaining error variables are private declarations which give a symbolic name to an error message. These error values are unexported so they cannot be used for comparison by callers outside the package. Declaring them at the package level, rather than at the point they occur inside a function negates the opportunity to add additional context to the error. Instead I recommend using something like pkg/errors to capture a stack trace at the point the error occurs.

Registration

A registration pattern is followed by several packages in the standard library such as net/http, database/sql, flag, and to a lesser extent log. It commonly involves a package scoped private map or struct which is mutated by a public function—a textbook singleton.

Not being able to create a package scoped placeholder for this state would remove the side effects in the image, database/sql, and crypto packages to register image decoders, database drivers and cryptographic schemes. However, this is precisely the magic that Peter is referring to–importing a package for the side effect of changing some global state of your program is truly spooky action at a distance.

Registration also promotes duplicated business logic. The net/http/pprof package registers itself, via a side effect with net/http.DefaultServeMux, which is both a potential security issue—other code cannot use the default mux without exposing the pprof endpoints—and makes it difficult to convince the net/http/pprof package to register its handlers with another mux.

If package scoped variables were no longer used, packages like net/http/pprof could provide a function that registers routes on a supplied http.ServeMux, rather than relying on side effects to altering global state.

Removing the ability to apply the registry pattern would also solve the issues encountered when multiple copies of the same package are imported in the final binary and try to register themselves during startup.

Interface satisfaction assertions

The interface satisfaction idiom

var _ SomeInterface = new(SomeType)

occurred at least 19 times in the standard library. In my opinion these assertions are tests. They don’t need to be compiled, only to be eliminated, every time you build your package. Instead they should be moved to the corresponding _test.go file. But if we’re prohibiting package scoped variables, this prohibition also applies to tests, so how can we keep this test?

One option is to move the declaration from package scope to function scope, which will still fail to compile if SomeType stop implementing SomeInterface

func TestSomeTypeImplementsSomeInterface(t *testing.T) {
       // won't compile if SomeType does not implement SomeInterface
       var _ SomeInterface = new(SomeType)
}

But, as this is actually a test, it’s not hard to rewrite this idiom as a standard Go test.

func TestSomeTypeImplementsSomeInterface(t *testing.T) {
       var i interface{} = new(SomeType)
       if _, ok := i.(SomeInterface); !ok {
               t.Fatalf("expected %t to implement SomeInterface", i)
       }
}

As a side note, because the spec says that assignment to the blank identifier must fully evaluate the right hand side of the expression, there are probably a few suspicious package level initialisation constructs hidden in those var declarations.

It’s not all beer and skittles

The previous sections showed that avoiding package scoped variables might be possible, but there are some areas of the standard library which have proved more difficult to apply this idea.

Real singletons

While I think that the singleton pattern is generally overplayed, especially in its registration form, there are always some real singleton values in every program. A good example of this is  os.Stdout and friends.

package os 

var (
        Stdin  = NewFile(uintptr(syscall.Stdin), "/dev/stdin")
        Stdout = NewFile(uintptr(syscall.Stdout), "/dev/stdout")
        Stderr = NewFile(uintptr(syscall.Stderr), "/dev/stderr")
)

There are a few problems with this declaration. Firstly Stdin, Stdout, and Stderr are of type *os.File, not their respective io.Reader or io.Writer interfaces. This makes replacing them with alternatives problematic. However the notion of replacing them is exactly the kind of magic that this experiment seeks to avoid.

As the previous constant error example showed, we can retain the singleton nature of the standard IO file descriptors, such that packages like log and fmt can address them directly, but avoid declaring them as mutable public variables with something like this:

package main

import (
        "fmt"
        "syscall"
)

type readfd int

func (r readfd) Read(buf []byte) (int, error) {
        return syscall.Read(int(r), buf)
}

type writefd int

func (w writefd) Write(buf []byte) (int, error) {
        return syscall.Write(int(w), buf)
}

const (
        Stdin  = readfd(0)
        Stdout = writefd(1)
        Stderr = writefd(2)
)

func main() {
        fmt.Fprintf(Stdout, "Hello world")
}

Caches

The second most common use of unexported package scoped variables are caches. These come in two forms; real caches made out of maps (see the registration pattern above) and sync.Pool, and quasi constant variables that ameliorate the cost of a compilation.

As an example the crypto/ecsda package has a zr type whose Read method zeros any buffer passed to it. The package keeps a single instance of zr around because it is embedded in other structs as an io.Reader, potentially escaping to the heap each time it is instantiated.

package ecdsa 

type zr struct {
        io.Reader
}

// Read replaces the contents of dst with zeros.
func (z *zr) Read(dst []byte) (n int, err error) {
        for i := range dst {
                dst[i] = 0
        }
        return len(dst), nil
}

var zeroReader = &zr{}

However zr doesn’t embed an io.Reader, it is an io.Reader, so the unused zr.Reader field could be eliminated, giving zr a width of zero. In my testing this modified type can be created directly where it is used without performance regression.

        csprng := cipher.StreamReader{
                R: zr{},
                S: cipher.NewCTR(block, []byte(aesIV)),
        }

Perhaps some of the caching decision could be revisited as the inlining and escape analysis options available to the compiler have improved significantly since the standard library was first written.

Tables

The last major use of  common use of private package scoped variables is for tables, as seen in the unicode, crypto/*, and math packages. These tables either encode constant data in the form of arrays of integer types, or less commonly simple structs and maps.

Replacing package scoped variables with constants would require a language change along the lines of #20443. So, fundamentally, providing there is no way to modify those tables at run time, they are probably a reasonable exception to this proposal.

A bridge too far

Even though this post was just a thought experiment, it’s clear that forbidding all package scoped variables is too draconian to be workable as a language precept. Addressing the bespoke uses of private var usage may prove impractical from a performance standpoint, would be akin to pinning a “kick me” sign to ones back and inviting all the Go haters to take a free swing.

However, I believe there are a few concrete recommendations that can be drawn from this exercise, without going to the extreme of changing the language spec.

  • Firstly, public var declarations should be eschewed. This is not a controversial conclusion and not one that is unique to Go. The singleton pattern is discouraged, and an unadorned public variable that can be changed at any time by any party that knows its name should be a design, and concurrency, red flag.
  • Secondly, where public package var declarations are used, the type of those variables should be carefully constructed to expose as little surface area as possible. It should not be the default to take a type expected to be used on a per instance basis, and assign it to a package scoped variable.

Private variable declarations are more nuanced, but certain patterns can be observed:

  • Private variables with public setters, which I labelled registries, have the same effect on the overall program design as their public counterparts. Rather than registering dependencies globally, they should instead be passed in during declaration using a constructor function, compact literal, config structure, or option function.
  • Caches of []byte vars can often be expressed as consts at no performance cost.  Don’t forget the compiler is pretty good at avoiding string([]byte) conversions where they don’t escape the function call.
  • Private variables that hold tables, like the unicode package, are an unavoidable consequence of the lack of a constant array type. As long as they are unexported, and do not expose any way to mutate them, they can be considered effectively constant for the purpose of this discussion.

The bottom line; think long and hard about adding package scoped variables that are mutated during the operation of your program. It may be a sign that you’ve introduced magic global state.

If a map isn’t a reference variable, what is it?

In my previous post I showed that Go maps are not reference variables, and are not passed by reference. This leaves the question, if maps are not references variables, what are they?

For the impatient, the answer is:

A map value is a pointer to a runtime.hmap structure.

If you’re not satisfied with this explanation, read on.

What is the type of a map value?

When you write the statement

m := make(map[int]int)

The compiler replaces it with a call to runtime.makemap, which has the signature

// makemap implements a Go map creation make(map[k]v, hint)
// If the compiler has determined that the map or the first bucket
// can be created on the stack, h and/or bucket may be non-nil.
// If h != nil, the map can be created directly in h.
// If bucket != nil, bucket can be used as the first bucket.
func makemap(t *maptype, hint int64, h *hmap, bucket unsafe.Pointer) *hmap

As you see, the type of the value returned from runtime.makemap is a pointer to a runtime.hmap structure. We cannot see this from normal Go code, but we can confirm that a map value is the same size as a uintptr–one machine word.

package main

import (
	"fmt"
	"unsafe"
)

func main() {
	var m map[int]int
	var p uintptr
	fmt.Println(unsafe.Sizeof(m), unsafe.Sizeof(p)) // 8 8 (linux/amd64)
}

If maps are pointers, shouldn’t they be *map[key]value?

It’s a good question that if maps are pointer values, why does the expression make(map[int]int) return a value with the type map[int]int. Shouldn’t it return a *map[int]int? Ian Taylor answered this recently in a golang-nuts thread1.

In the very early days what we call maps now were written as pointers, so you wrote *map[int]int. We moved away from that when we realized that no one ever wrote `map` without writing `*map`.

Arguably renaming the type from *map[int]int to map[int]int, while confusing because the type does not look like a pointer, was less confusing than a pointer shaped value which cannot be dereferenced.

Conclusion

Maps, like channels, but unlike slices, are just pointers to runtime types. As you saw above, a map is just a pointer to a runtime.hmap structure.

Maps have the same pointer semantics as any other pointer value in a Go program. There is no magic save the rewriting of map syntax by the compiler into calls to functions in runtime/hmap.go.

Notes

  1. If you look far enough back in the history of Go repository, you can find examples of maps created with the new operator.

There is no pass-by-reference in Go

My post on pointers provoked a lot of debate about maps and pass by reference semantics. This post is a response to those debates.

To be clear, Go does not have reference variables, so Go does not have pass-by-reference function call semantics.

What is a reference variable?

In languages like C++ you can declare an alias, or an alternate name to an existing variable. This is called a reference variable.

#include <stdio.h>

int main() {
        int a = 10;
        int &b = a;
        int &c = b;

        printf("%p %p %p\n", &a, &b, &c); // 0x7ffe114f0b14 0x7ffe114f0b14 0x7ffe114f0b14
        return 0;
}

You can see that a, b, and c all refer to the same memory location. A write to a will alter the contents of b and c. This is useful when you want to declare reference variables in different scopes–namely function calls.

Go does not have reference variables

Unlike C++, each variable defined in a Go program occupies a unique memory location.

package main

import "fmt"

func main() {
        var a, b, c int
        fmt.Println(&a, &b, &c) // 0x1040a124 0x1040a128 0x1040a12c
}

It is not possible to create a Go program where two variables share the same storage location in memory. It is possible to create two variables whose contents point to the same storage location, but that is not the same thing as two variables who share the same storage location.

package main

import "fmt"

func main() {
        var a int
        var b, c = &a, &a
        fmt.Println(b, c)   // 0x1040a124 0x1040a124
        fmt.Println(&b, &c) // 0x1040c108 0x1040c110
}

In this example, b and c hold the same value–the address of a–however, b and c themselves are stored in unique locations. Updating the contents of b would have no effect on c.

But maps and channels are references, right?

Wrong. Maps and channels are not references. If they were this program would print false.

package main

import "fmt"

func fn(m map[int]int) {
        m = make(map[int]int)
}

func main() {
        var m map[int]int
        fn(m)
        fmt.Println(m == nil)
}

If the map m was a C++ style reference variable, the m declared in main and the m declared in fn would occupy the same storage location in memory. But, because the assignment to m inside fn has no effect on the value of m in main, we can see that maps are not reference variables.

Conclusion

Go does not have pass-by-reference semantics because Go does not have reference variables.

Next: If a map isn’t a reference variable, what is it?

Understand Go pointers in less than 800 words or your money back

This post is for programmers coming to Go who are unfamiliar with the idea of pointers or a pointer type in Go.

What is a pointer?

Simply put, a pointer is a value which points to the address of another. This is the textbook explanation, but if you’re coming from a language that doesn’t let you talk about address of a variable, it could very well be written in Cuneiform.

Let’s break this down.

What is memory?

Computer memory, RAM, can be thought of as a sequence of boxes, placed one after another in a line. Each box, or cell, is labeled with a unique number, which increments sequentially; this is the address of the cell, its memory location.

Each cell holds a single value. If you know the memory address of a cell, you can go to that cell and read its contents. You can place a value in that cell; replacing anything that was in there previously.

That’s all there is to know about memory. Everything the CPU does is expressed as fetching and depositing values into memory cells.

What is a variable?

To write a program that retrieves the value stored in memory location 200, multiples it by 3 and deposits the result into memory location 201, we could write something like this in pseudocode:

  • retrieve the value stored in address 200 and place it in the CPU.
  • multiple the value stored in the CPU by 3.
  • deposit the value stored in the CPU into memory location 201.


This is exactly how early programs were written; programmers would keep a list of memory locations, who used it, when, and what the value stored there represented.

Obviously this was tedious and error prone, and meant every possible value stored in memory had to be assigned an address during the construction of the program. Worse, this arrangement made it difficult to allocate storage to variables dynamically as the program ran– just imagine if you had to write large programs using only global variables.

To address this, the notion of a variable was created. A variable is just a convenient, alphanumeric pseudonym for a memory location; a label, or nickname.

Now, rather than talking about memory locations, we can talk about variables, which are convenient names we give to memory locations. The previous program can now be expressed as:

  • Retrieve the value stored in variable a and place it in the CPU.
  • multiple it by 3
  • deposit the value into the variable b.

This is the same program, with one crucial improvement–because we no longer need to talk about memory locations directly, we no longer need to keep track of them–that drudgery is left to the compiler.

Now we can write a program like

var a = 6
var b = a * 3

And the compiler will make sure that the variables a and b are assigned unique memory locations to hold their value for as long as needed.

What is a pointer?

Now that we know that memory is just a series of numbered cells, and variables are just nicknames for a memory location assigned by the compiler, what is a pointer?

A pointer is a value that points to the memory address of another variable.

The pointer points to memory address of a variable, just as a variable represents the memory address of value.

Let’s have a look at this program fragment

func main() {
        a := 200
        b := &a
        *b++
        fmt.Println(a)
}

On the first line of main we declare a new variable a and assign it the value 200.

Next we declare a variable b and assign it the address a. Remember that we don’t know the exact memory location where a is stored, but we can still store a‘s address in b.

The third line is probably the most confusing, because of the strongly typed nature of Go. b contains the address of variable a, but we want to increment the value stored in a. To do this we must dereference b, follow the pointer from b to a.

Then we add one the value, and store it back in the memory location stored in b.

The final line prints the value of a, showing that it has increased to 201.

Conclusion

If you are coming from a language with no notion of pointers, or where every variable is implicitly a pointer don’t panic, forming a mental model of how variables and pointers relate takes time and practice. Just remember this rule:

A pointer is a value that points to the memory address of another variable.

Next: There is no pass-by-reference in Go

Why Go?

A few weeks ago I was asked by a friend, “why should I care about Go”? They knew that I was passionate about Go, but wanted to know why I thought other people should care. This article contains three salient reasons why I think Go is an important programming language.

Safety

As individuals, you and I may be perfectly capable of writing a program in C that neither leaks memory or reuses it unsafely. However, with more than 40 years of experience, it is clear that programmers working in C are unable to reliably do so en masse.

Despite tools such as static code analysis, valgrind, tsan, and -Werror being available for a decades, there is scant evidence those tools have achieved widespread acknowledgement, let alone widespread adoption. In aggregate, programmers have shown they simply cannot safely manage their own memory.

Go does not rely on the programmer to manage memory directly, instead all memory allocation is managed by the language runtime, initialized before use, and bounds checked when necessary. It’s certainly not the first mainstream language that offered these safety guarantees, Java (1995) is probably a contender for that crown. The point being, the world has no appetite for unsafe programming languages, thus Go is memory safe by default.

Developer productivity

The point at which developer time became more expensive than hardware time was crossed back in the late 1970s. Developer productivity is a sprawling topic but it boils down to this; how much time do you spend doing useful work vs waiting for the compiler or hopelessly lost in a foreign codebase.

The joke goes that Go was developed while waiting for a C++ program to compile. Fast compilation is a key feature of Go and a key recruiting tool to attract new developers. While compilation speed remains a constant battleground, it is fair to say that compilations which take minutes in other languages, take seconds in Go.

More fundamental to the question of developer productivity, Go programmers realise that code is written to be read and so place the act of reading code above the act of writing it. Go goes so far as to enforce, via tooling and custom, that all code by formatted in a specific style. This removes the friction of learning a project specific language sub-dialect and helps spot mistakes because they just look incorrect.

Due to a focus on analysis and mechanical assistance, a growing set of tools that exist to spot common coding errors have been adopted by Go developers in a way that never struck a chord with C programmers—Go developers want tools to help them keep their code clean.

Concurrency

For more than a decade, chip designers have been warning that the free lunch is over. Hardware parallelism, from the lowliest mobile phone to the most power hungry server, in the form of more, slower, cpu cores, is only available if your language can utilise them. Therefore, concurrency needs to be built into the software we write to run on today’s hardware.

Go takes a step beyond languages that expose the operating system’s multi-process or multi-threading parallelism models by offering a lightweight concurrency model based on coroutines, or goroutines as they are known in Go. Goroutines allows the programmer to eschew convoluted callback styles while the language runtime makes sure that there will be just enough threads to keep your cores active.

The rule of three

These were my three reasons for recommending Go to my friend; safety, productivity, and concurrency. Individually, there are languages that cover one, possibly two of these domains, but it is the combination of all three that makes Go an excellent choice for mainstream programmers today.

I’m speaking at GopherChina and GopherCon Singapore

In April and May I’ll be speaking at GopherChina and GopherCon Singapore, respectively. This post is a teaser for the talks that were selected by the organisers. If you’re in the area, I hope you’ll come and hear me speak.

GopherChina

GopherChina is the third event in this conference series and this year will return to Shanghai. I was lucky to attend the event in 2016 and am looking forward to 2017.

The hidden of Go

Go isn’t like C. It doesn’t have a preprocessor, it doesn’t have macros, and it certainly doesn’t have #define, but Go does have pragmas.

What are pragmas? The name come from the #pragma declaration that tells C compilers to alter their interpretation of a piece of code. Now, Go doesn’t have a #pragma directive, but it does have ways of altering the operation of the Go compiler via directive syntax hidden in comments.

This talk will explore the history of these directives, how and why they are used, and how you can, but probably shouldn’t, use them in your own code.

GopherCon Singapore

GopherCon Singapore is the latest in the GopherCon franchise, and as flight times go, relatively close to home. I’m delighted to have the opportunity to present at their inaugural conference in May.

Concurrency made Easy

In my experience, many people who come to Go do so because they have a problem where being able to run more than one task at a time in their program would be beneficial. Ruby and Python programmers come to Go because the concurrency story is much better, the same is true of Node programmers; the event loop is still inherently single threaded.

But, most programmers who stick with Go for a while tend to look back on their early efforts and say things like “wow, I really went overboard with channels” or “I went crazy with goroutines when I started writing Go. It was impossible to understand what the program did”. For people who learn Go formally from an instructor or a book, the concurrency section is always the last section they cover.

So there is a dichotomy here. Go’s headline feature is simple, lightweight concurrency. As a product the language sells itself on that feature alone. On the other hand, there is a narrative that concurrency isn’t actually that easy to use, otherwise people wouldn’t make it the last things in their books or classes, or perhaps more accurately, concurrency is not the solution to every problem.

With this as a background, I’d like to explore some strategies for using concurrency in Go without the pitfalls of convoluted code, the importance of memory ownership, and the best way to structure a Go program using goroutines.

Context is for cancelation

In my previous post I suggested that the best way to break the compile time coupling between the logger and the loggee was passing in a logger interface when constructing each major type in your program. The suggestion has been floated several times that logging is context specific, so maybe a logger can be passed around via a context.Context. I think this suggestion is flawed (as are most uses of context.Value, but that’s another story). This post explains why.

context.Value() is goroutine thread local storage

Using context.Context to pass a logger into a function is a poor design pattern. In effect context.Context is being used as a conduit to arbitrarily extend the API of any method that takes a context.Context value. It’s like Python’s **kwargs, or whatever the name is for that Ruby pattern of always passing a hash. Using context.Context in this way avoids an API break by smuggling data in the unstructured bag of values attached to the context. It’s thread local storage in a cheap suit.

It’s not just that values are boxed into an interface{} inside context.WithValue that I object to. The far more serious concern is there is no schema to this data, so there is no way for a method that takes a context to ensure that it contains the specific key required to complete the operation. context.Value returns nil if the key is not found, which means any code doing the naïve

log := ctx.Value("logger").(log.Logger)
log.Warn("something you'll ignore later")

will blow up if the "logger" key is not present.

Sure, you can check that the assertion succeeded, but I feel pretty confident that if this pattern were to become popular then people would eschew the two arg form of type assertion and just expect that the key always returned a valid logger. This would be especially true as logging in error paths is rarely tested, so you’ll hit this when you need it the most.

In my opinion passing loggers inside context.Context would be the worst solution to the problem of decoupling loggers from implementations. We’d have gone from an explicit compile time dependency to an implicit run time dependency, one that could not be enforced by the compiler.

To quote @freeformz

Loggers should be injected into dependencies. Full stop.

It’s verbose, but it’s the only way to achieve decoupled design.

The package level logger anti pattern

This post is a spin-off from various conversations around improving (I’m trying not to say standardising, otherwise I’ll have to link to XKCD) the way logging is performed in Go projects.

Consider this familiar pattern for establishing a package level log variable.

package foo

import “mylogger”

var log = mylogger.GetLogger(“github.com/project/foo”)

What’s wrong with this pattern?

The first problem with declaring a package level log variable is the tight coupling between package foo and package mylogger. Package foo now depends directly on package mylogger at compile time.

The second problem is the tight coupling between package foo and package mylogger is transitive. Any package that consumes package foo is itself dependant on mylogger at compile time.

This leads to a third problem, Go projects composed of packages using multiple logging libraries, or fiefdoms of projects who can only consume packages that use their particular logging library.

Avoid source level coupling

The solution to this anti pattern is to delay the binding between the type that does the logging, and the type that needs to log, until it is needed. That is, until the variable is declared.

package foo

import "github.com/pkg/log"

type T struct {
        logger log.Logger
        // other fields
}

Now, the consumer of  type T supplies a value of type log.Logger when constructing new T‘s, and the methods on T use the logger they were provided when they want to log.

Interfaces to the rescue

The eagle eyed reader will note that the previous selection removed the package level log variable, but the coupling between package foo and package log remains.

However, this can be remedied by the consumer of the logger type declaring its own interface for the behaviour it expects.

package foo

type logger interface {
        Printf(string, ...interface{})
}

type T struct {
        logger
        // other fields
}

As long as the type assigned to foo.T.logger implements foo.logger the decision for which specific type to use can be deferred until run time in the same way that io.Copy escapes any knowledge of the io.Reader and io.Writer implementations in use until it is invoked.

It’s not just logging

Logging is a cross cutting concern, but the anti patterns associated with it also apply to other common areas like metrics, telemetry, and auditing.

Get involved

The Go 1.9 development window is opening next month. If this topic is important to you, get involved.

Never start a goroutine without knowing how it will stop

In Go, goroutines are cheap to create and efficient to schedule. The Go runtime has been written for programs with tens of thousands of goroutines as the norm, hundreds of thousands are not unexpected. But goroutines do have a finite cost in terms of memory footprint; you cannot create an infinite number of them.

Every time you use the go keyword in your program to launch a goroutine, you must know how, and when, that goroutine will exit. If you don’t know the answer, that’s a potential memory leak.

Consider this trivial code snippet:

ch := somefunction()
go func() {
        for range ch { }
}()

This code obtains a channel of int from somefunction and starts a goroutine to drain it. When will this goroutine exit? It will only exit when ch is closed. When will that occur? It’s hard to say, ch is returned by somefunction. So, depending on the state of somefunction, ch might never be closed, causing the goroutine to quietly leak.

In your design, some goroutines may run until the program exits, for example a background goroutine watching a configuration file, or the main conn.Accept loop in your server. However, these goroutines are rare enough I don’t consider them an exception to this rule.

Every time you write the statement go in a program, you should consider the question of how, and under what conditions, the goroutine you are about to start, will end.

Thinking about $GOPATH

This is a short blog post about my thoughts on using Go in anger through several workplaces, as a developer and an advocate.

What is $GOPATH?

Back when Go was first announced we used Makefiles to compile Go code. These Makefiles referenced some shared logic stored in the Go distribution. This is where $GOROOT comes from.

Back then, if you wrote Go code, you’d probably also used these Makefiles, and while you could check out your source code anywhere, most people would put their own Go code in what today we’d call $GOROOT/src as you must’ve compiled Go from source, so this directory was always going to be present.

Towards the 1.0 release goinstall, then go get, solidified the use of domain names in import paths to provide a globally unique namespace. These tools introduced a new location into which Go code would be fetched. This location was separate from $GOROOT to make clear the distinction between code provided by the Go project, and code written by the developer. By the time Go 1.1 was released in 2013, $GOROOT was removed as a fallback option.

Why does $GOPATH exist?

$GOPATH exists for two main reasons:

  1. In Go, the import declaration references a package via its fully qualified import path. $GOPATH exist so that from any directory inside $GOPATH/src the go tool can compute the absolute import path of the package in question.1
  2. A location to store dependencies fetched by go get.

Having a per user $GOPATH environment variable also means developers could use the go tool from any directory on their system to build, test and install code, but I suspect only a minority utilise this feature.

What’s wrong with $GOPATH?

In my experience, many newcomers to Go are frustrated with the single workspace $GOPATH model. They are confused that $GOPATH doesn’t let them check out the source of a project in a directory of their choice like they are used to with other languages. Additionally, $GOPATH does not let the developer have more than one copy of a project (or its dependencies)  checked out at the same time without having to update $GOPATH constantly.

I think it is important to recognise that these issues are legitimate points of confusion for many newcomers (including those on the Go team) and act as a drag on Go adoption. As we’re on the cusp of a blessed dependency management tool for Go, I think it’s equally important to continue to question the base assumptions that this new tool will build on, namely requiring a $GOPATH.

In my opinion, any Go build tool needs to provide (in addition to actually building and testing code) a way for Go code checked out in an arbitrary location on disk to recover its intended fully qualified import path; the path other code will import it as.

The $GOPATH model answers this question by subtracting the prefix of $GOPATH/src from the path to the directory of the current package; the remainder is the package’s fully qualified import path. This is why if you check out a package outside a $GOPATH workspace, the go tool cannot figure out the packages’ fully qualified import path and everything falls apart.

What are some alternatives to $GOPATH?

I attempted to address both issues with gb, which gives developers the ability to check out a project anywhere you want, but has no solution for libraries, and gb projects were not go gettable. However gb showed that writing a new build tool that did not wrap the go tool meant it was not forced to reorganise the world to fit into the $GOPATH model allowing gb users to include the source of all their dependencies in their project without the pitfalls of the Go 1.6’s vendor/ directory.

Recently, on a suggestion from Bill Kennedy, I built an experimental build tool that recorded the expected import prefix in a manifest file. That prefix, rather than one computed by $GOPATH directory arithmetic, is used to determine the fully qualified import path.

I’m working on a similar tool (unfinished) based on a suggestion from Brad Fitzpatrick that uses the .git directory as a sentinel to determine the root of the project and hopefully infer the full import path from the git remote configuration.

While these experiments are unfinished, both demonstrate that you can avoid the $GOPATH restrictions and retain compatibility with the go get ecosystem. Potentially in the case of Kodos, even avoid a manifest file.

Conclusion

Kang and Kodos use a lot of forked code from gb, which I hope to rectify over the new years’ break. If you are interesting in contributing or better yet, building your own Go tool to explore this problem space, Kang, Kodos, and gb are permissively licensed.

Notes:

  1. This is notably different from the way imports work in scripting languages like Python and Ruby, which use directly scanning and inserting onto a global search path source code directories.