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What I Don’t Like About Error Handling in Go, and How to Work Around It
======================
More often than not, people who write Go have some sort of opinion on its error handling model. Depending on your experience with other languages, you may be used to different approaches. That’s why I’ve decided to write this article, as despite being relatively opinionated, I think drawing on my experiences can be useful in the debate. The main issues I wanted to cover are that it is difficult to force good error handling practice, that errors don’t have stack traces, and that error handling itself is too verbose. However I’ve looked at some potential workarounds for these problems which could help negate the issues somewhat.
### Quick Comparison to Other Languages
[In Go, all errors are values][1]. Because of this, a fair amount of functions end up returning an `error`, looking something like this:
```
func (s *SomeStruct) Function() (string, error)
```
As a result of this, the calling code will regularly have `if` statements to check for them:
```
bytes, err := someStruct.Function()
if err != nil {
// Process error
}
```
Another approach is the `try-catch` model that is used in other languages such as Java, C#, Javascript, Objective C, Python etc. You could see the following Java code as synonymous to the previous Go examples, declaring `throws` instead of returning an `error`:
```
public String function() throws Exception
```
And then doing `try-catch` instead of `if err != nil`:
```
try {
String result = someObject.function()
// continue logic
}
catch (Exception e) {
// process exception
}
```
Of course, there are more differences than this. For example, an `error` can’t crash your program, whereas an `Exception` can. There are others as well, and I want to focus on them in this article.
### Implementing Centralised Error Handling
Taking a step back, let’s look at why and how we might want to have a centralised place for handling errors.
An example most people would be familiar with is a web service – if some unexpected server-side error were to happen, we would generate a 5xx error. At a first pass in Go you might implement this:
```
func init() {
http.HandleFunc("/users", viewUsers)
http.HandleFunc("/companies", viewCompanies)
}
func viewUsers(w http.ResponseWriter, r *http.Request) {
user // some code
if err := userTemplate.Execute(w, user); err != nil {
http.Error(w, err.Error(), 500)
}
}
func viewCompanies(w http.ResponseWriter, r *http.Request) {
companies = // some code
if err := companiesTemplate.Execute(w, companies); err != nil {
http.Error(w, err.Error(), 500)
}
}
```
This isn’t a good solution, as we would have to repeat the same error handling across all of our handler functions. It would be much better to do it all in one place for maintainability purposes. Fortunately, there is [an alternative by Andrew Gerrand on the Go blog][2] which works quite nicely. We can create a Type which does http error handling:
```
type appHandler func(http.ResponseWriter, *http.Request) error
func (fn appHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
if err := fn(w, r); err != nil {
http.Error(w, err.Error(), 500)
}
}
```
And then it can be used as a wrapper to decorate our handlers:
```
func init() {
http.Handle("/users", appHandler(viewUsers))
http.Handle("/companies", appHandler(viewCompanies))
}
```
Then all we need to do is change the signature of handler functions so they return `errors`. This works nicely, as we have been able to apply the [dry][3] principle and not re-use code unnecessarily – now we return default errors in a single place.
### Error Context
In the previous example, there are many potential errors which we could receive, all of which could be generated in many parts of the call stack. This is when things start to get tricky.
To demonstrate this, we can expand on our handler. It’s more likely to look like this as the template execution is not the only place where an error could occur:
```
func viewUsers(w http.ResponseWriter, r *http.Request) error {
user, err := findUser(r.formValue("id"))
if err != nil {
return err;
}
return userTemplate.Execute(w, user);
}
```
The call chain could get quite deep, and throughout it, all sorts of errors could be instantiated in different places. This post by [Russ Cox][4] explains the best practice to prevent this from being too much of a problem:
> Part of the intended contract for error reporting in Go is that functions include relevant available context, including the operation being attempted (such as the function name and its arguments)
The example given is a call to the OS package:
```
err := os.Remove("/tmp/nonexist")
fmt.Println(err)
```
Which prints the output:
```
remove /tmp/nonexist: no such file or directory
```
To summarise, you are outputting the method called, the arguments given, and the specific thing that went wrong, immediately after doing it. When creating an `Exception` message in another language you would also follow this practice. So if we stuck to these rules in our `viewUsers` handler, it could almost always be clear what the cause of an error is.
The problem comes from people not following this best practice, and quite often in third party Go libraries you will see messages like:
```
Oh no I broke
```
Which is just not helpful – you don’t know anything about the context which makes it really hard to debug. Even worse is when these sorts of errors are ignored or returned really far back up the stack until they are handled:
```
if err != nil {
return err
}
```
This means that when they happened isn’t communicated.
It should be noted that all these mistakes can be made in an `Exception` driven model – poor error messages, swallowing exceptions etc. So why do I think that model is more helpful?
If we’re dealing with a poor exception message, _we are still able to know where it occurred in the call stack_. This is because of stack traces, which raises something I don’t get about Go – you have the concept of `panic` in Go which contains a stack trace, but an `error` which does not. I think the reasoning is that a `panic` can crash your program, so requires a stack trace, whereas a handled error does not as you are supposed to do something about it where it occurs.
So let’s go back to our previous example – a third party library with a poor error message, which just gets propagated all the way up the call chain. Do you think debugging would be easier if you had this?
```
panic: Oh no I broke
[signal 0xb code=0x1 addr=0x0 pc=0xfc90f]
goroutine 1103 [running]:
panic(0x4bed00, 0xc82000c0b0)
/usr/local/go/src/runtime/panic.go:481 +0x3e6
github.com/Org/app/core.(_app).captureRequest(0xc820163340, 0x0, 0x55bd50, 0x0, 0x0)
/home/ubuntu/.go_workspace/src/github.com/Org/App/core/main.go:313 +0x12cf
github.com/Org/app/core.(_app).processRequest(0xc820163340, 0xc82064e1c0, 0xc82002aab8, 0x1)
/home/ubuntu/.go_workspace/src/github.com/Org/App/core/main.go:203 +0xb6
github.com/Org/app/core.NewProxy.func2(0xc82064e1c0, 0xc820bb2000, 0xc820bb2000, 0x1)
/home/ubuntu/.go_workspace/src/github.com/Org/App/core/proxy.go:51 +0x2a
github.com/Org/app/core/vendor/github.com/rusenask/goproxy.FuncReqHandler.Handle(0xc820da36e0, 0xc82064e1c0, 0xc820bb2000, 0xc5001, 0xc820b4a0a0)
/home/ubuntu/.go_workspace/src/github.com/Org/app/core/vendor/github.com/rusenask/goproxy/actions.go:19 +0x30
```
I think this might be something which has been overlooked in the design of Go – not that things aren’t overlooked in all languages.
If we use Java as an arbitrary example, one of the silliest mistakes people make is not logging the stack trace:
```
LOGGER.error(ex.getMessage()) // Doesn't log stack trace
LOGGER.error(ex.getMessage(), ex) // Does log stack trace
```
But Go seems to not have this information by design.
In terms of getting context information – Russ also mentions the community are talking about some potential interfaces for stripping out error contexts. It would be interesting to hear more about this.
### Solution to the Stack Trace Problem
Fortunately, after doing some searching, I found this excellent [Go Errors][5] library which helps solves the problem, by adding stack traces to errors:
```
if errors.Is(err, crashy.Crashed) {
fmt.Println(err.(*errors.Error).ErrorStack())
}
```
However, I’d think it would be an improvement for this feature to have first class citizenship in the language, so you wouldn’t have to fiddle around with types. Also, if we are working with a third party library like in the previous example then it is probably not using `crashy` – we still have the same problem.
### What Should We Do with an Error?
We also have to think about what should happen when an error occurs. [It’s definitely useful that they can’t crash your program][6], and it’s also idiomatic to handle them immediately:
```
err := method()
if err != nil {
// some logic that I must do now in the event of an error!
}
```
But what happens if we want to call lots of methods which return errors, and then handle them all in the same place? Something like this:
```
err := doSomething()
if err != nil {
// handle the error here
}
func doSomething() error {
err := someMethod()
if err != nil {
return err
}
err = someOther()
if err != nil {
return err
}
someOtherMethod()
}
```
It feels a little verbose, whereas in other languages you can treat multiple statements that fail as a block:
```
try {
someMethod()
someOther()
someOtherMethod()
}
catch (Exception e) {
// process exception
}
```
Or just propagate failure in the method signature:
```
public void doSomething() throws SomeErrorToPropogate {
someMethod()
someOther()
someOtherMethod()
}
```
Personally I think both of these example achieve the same thing, only the `Exception` model is less verbose and more flexible. If anything, I find the `if err != nil` to feel like boilerplate. Maybe there is a way that it could be cleaned up?
### Treating Multiple Statements That Fail as a Block
To begin with, I did some more reading and found a relatively pragmatic solution on the [by Rob Pike on the Go Blog.][7]
He defines a struct with a method which wraps errors:
```
type errWriter struct {
w io.Writer
err error
}
func (ew *errWriter) write(buf []byte) {
if ew.err != nil {
return
}
_, ew.err = ew.w.Write(buf)
}
```
This let’s us do:
```
ew := &errWriter{w: fd}
ew.write(p0[a:b])
ew.write(p1[c:d])
ew.write(p2[e:f])
// and so on
if ew.err != nil {
return ew.err
}
```
This is also a good solution, but I still feel like something is missing – as we can’t re-use this pattern. If we wanted a method which took a string as an argument, then we’d have to change the function signature. Or what if we didn’t want to perform a write? We could try and make it more generic:
```
type errWrapper struct {
err error
}
```
```
func (ew *errWrapper) do(f func() error) {
if ew.err != nil {
return
}
ew.err = f();
}
```
But we have the same problem that it won’t compile if we want to call functions which have different arguments. However you simply wrap those function calls:
```
w := &errWrapper{}
w.do(func() error {
return someFunction(1, 2);
})
w.do(func() error {
return otherFunction("foo");
})
err := w.err
if err != nil {
// process error here
}
```
This works, but doesn’t help too much as it ends up being more verbose than the standard `if err != nil`checks. I would be interested to hear if anyone can offer any other solutions. Maybe the language itself needs some sort of way to propagate or group errors in a less bloated fashion – but it feels like it’s been specifically designed to not do that.
### Conclusion
After reading this, you might think that by picking on `errors` I’m opposed to Go. But that’s not the case, I’m just describing how it compares to my experience with the `try catch` model. It’s a great language for systems programming, and some outstanding tools have been produced by it. To name a few there is [Kubernetes][8], [Docker][9], [Terraform][10], [Hoverfly][11] and others. There’s also the advantage of your tiny, highly performant, native binary. But `errors` have been difficult to adjust to. I hope my reasoning makes sense, and also that some of the solutions and workarounds could be of help.
--------------------------------------------------------------------------------
作者简介：
Andrew is a Consultant for OpenCredo having Joined the company in 2015. Andrew has several years experience working in the across a number of industries, developing web-based enterprise applications.
--------------------------------------------------------------------------------
via: https://opencredo.com/why-i-dont-like-error-handling-in-go
作者：[Andrew Morgan][a]
译者：[译者ID](https://github.com/译者ID)
校对：[校对者ID](https://github.com/校对者ID)
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
[a]:https://opencredo.com/author/andrew/
[1]:https://blog.golang.org/errors-are-values
[2]:https://blog.golang.org/error-handling-and-go
[3]:https://en.wikipedia.org/wiki/Don't_repeat_yourself
[4]:https://research.swtch.com/go2017
[5]:https://github.com/go-errors/errors
[6]:https://davidnix.io/post/error-handling-in-go/
[7]:https://blog.golang.org/errors-are-values
[8]:https://kubernetes.io/
[9]:https://www.docker.com/
[10]:https://www.terraform.io/
[11]:http://hoverfly.io/en/latest/

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我对 Go 的错误处理有哪些不满，以及我是如何处理的
======================
写 Go 的人往往对它的错误处理模式有一定的看法。根据你对其他语言的经验，你可能习惯于不同的方法。这就是为什么我决定要写这篇文章，尽管有点固执己见，但我认为吸收我的经验在辩论中是有用的。 我想要解决的主要问题是，很难去强制良好的错误处理实践，错误没有堆栈追踪，并且错误处理本身太冗长。不过，我已经看到了一些潜在的解决方案或许能帮助解决一些问题。
### 与其他语言的快速比较
[在 Go 中，所有的错误是值][1]。因为这点，相当多的函数最后会返回一个 `error`, 看起来像这样：
```
func (s *SomeStruct) Function() (string, error)
```
由于这点，调用代码常规上会使用 `if` 语句来检查它们：
```
bytes, err := someStruct.Function()
if err != nil {
// Process error
}
```
另外一种是在其他语言中如 Java、C#、Javascript、Objective C、Python 等使用的 `try-catch` 模式。如下你可以看到与先前的 Go 示例类似的 Java 代码，声明 `throws` 而不是返回 `error`：
```
public String function() throws Exception
```
`try-catch` 而不是 `if err != nil`：
```
try {
String result = someObject.function()
// continue logic
}
catch (Exception e) {
// process exception
}
```
当然，还有其他的不同。不如，`error` 不会使你的程序崩溃，然而 `Exception` 会。还有其他的一些，我希望在在本篇中专注在这些上。
### 实现集中式错误处理
退一步，让我们看看为什么以及如何在一个集中的地方处理错误。
大多数人或许会熟悉的一个例子是 web 服务 - 如果出现了一些未预料的的服务端错误，我们会生成一个 5xx 错误。在 Go 中，你或许会这么实现：
```
func init() {
http.HandleFunc("/users", viewUsers)
http.HandleFunc("/companies", viewCompanies)
}
func viewUsers(w http.ResponseWriter, r *http.Request) {
user // some code
if err := userTemplate.Execute(w, user); err != nil {
http.Error(w, err.Error(), 500)
}
}
func viewCompanies(w http.ResponseWriter, r *http.Request) {
companies = // some code
if err := companiesTemplate.Execute(w, companies); err != nil {
http.Error(w, err.Error(), 500)
}
}
```
这并不是一个好的解决方案，因为我们不得不重复在所有的处理函数中处理错误。为了能更好地维护，最好能在一处地方处理错误。幸运的是，[在 Go 的博客中，Andrew Gerrand 提供了一个替代方法][2]可以完美地实现。我们可以错见一个处理错误的类型：
```
type appHandler func(http.ResponseWriter, *http.Request) error
func (fn appHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
if err := fn(w, r); err != nil {
http.Error(w, err.Error(), 500)
}
}
```
这可以作为一个封装器来修饰我们的处理函数：
```
func init() {
http.Handle("/users", appHandler(viewUsers))
http.Handle("/companies", appHandler(viewCompanies))
}
```
接着我们需要做的是修改处理函数的签名来使它们返回 `errors`。这个方法很好，因为我们做到了 [dry][3] 原则，并且没有重复使用不必要的代码 - 现在我们可以在一处返回默认错误了。
### 错误上下文
在先前的例子中，我们可能会收到许多潜在的错误，它们的任何一个都可能在调用堆栈的许多部分生成。这时候事情就变得棘手了。
为了演示这点，我们可以扩展我们的处理函数。它可能看上去像这样，因为模板执行并不是唯一一处会发生错误的地方：
```
func viewUsers(w http.ResponseWriter, r *http.Request) error {
user, err := findUser(r.formValue("id"))
if err != nil {
return err;
}
return userTemplate.Execute(w, user);
}
```
调用链可能会相当深，在整个过程中，各种错误可能在不同的地方实例化。[Russ Cox][4]的这篇文章解释了如何避免遇到太多这类问题的最佳实践：
在 Go 中错误报告的部分约定是函数包含相关的上下文、包含正在尝试的操作（比如函数名和它的参数）
给出的例子是 OS 包中的一个调用：
```
err := os.Remove("/tmp/nonexist")
fmt.Println(err)
```
它会输出：
```
remove /tmp/nonexist: no such file or directory
```
总结一下，执行后，输出的是被调用的函数、给定的参数、特定的出错信息。当在其他语言中创建一个 `Exception` 消息时，你也可以遵循这个实践。如果我们在 `viewUsers` 处理中坚持这点，那么几乎总是能明确错误的原因。
问题来自于那些不遵循这个最佳实践的人，并且你经常会在第三方的 Go 库中看到这些消息：
```
Oh no I broke
```
这没什么帮助 - 你无法了解上下文，这使得调试很困难。更糟糕的是，当这些错误被忽略或返回时，这些错误会被备份到堆栈中，直到它们被处理为止：
```
if err != nil {
return err
}
```
这意味着错误发生时它们没有交流。
应该注意的是，所有这些错误都可以在 `Exception` 驱动的模型中发生 - 糟糕的错误信息、隐藏异常等。那么为什么我认为该模型更有用？
如果我们在处理一个糟糕的异常消息，_我们仍然能够了解堆栈中发生了什么_。因为堆栈跟踪，这引发了一些我对 Go 不了解的部分 - 你知道 Go 的 `panic` 包含了堆栈追踪，但是 `error` 没有。我认为推论是 `panic` 可能会使你的程序崩溃，因此需要一个堆栈追踪，而处理错误并不会，因为它会假定你在它发生的地方做一些事。
所以让我们回到之前的例子 - 一个有糟糕错误信息的第三方库，它只是输出了调用链。你认为调试会更容易吗？
```
panic: Oh no I broke
[signal 0xb code=0x1 addr=0x0 pc=0xfc90f]
goroutine 1103 [running]:
panic(0x4bed00, 0xc82000c0b0)
/usr/local/go/src/runtime/panic.go:481 +0x3e6
github.com/Org/app/core.(_app).captureRequest(0xc820163340, 0x0, 0x55bd50, 0x0, 0x0)
/home/ubuntu/.go_workspace/src/github.com/Org/App/core/main.go:313 +0x12cf
github.com/Org/app/core.(_app).processRequest(0xc820163340, 0xc82064e1c0, 0xc82002aab8, 0x1)
/home/ubuntu/.go_workspace/src/github.com/Org/App/core/main.go:203 +0xb6
github.com/Org/app/core.NewProxy.func2(0xc82064e1c0, 0xc820bb2000, 0xc820bb2000, 0x1)
/home/ubuntu/.go_workspace/src/github.com/Org/App/core/proxy.go:51 +0x2a
github.com/Org/app/core/vendor/github.com/rusenask/goproxy.FuncReqHandler.Handle(0xc820da36e0, 0xc82064e1c0, 0xc820bb2000, 0xc5001, 0xc820b4a0a0)
/home/ubuntu/.go_workspace/src/github.com/Org/app/core/vendor/github.com/rusenask/goproxy/actions.go:19 +0x30
```
我认为这可能是 Go 的设计中被忽略的东西 - 不是所有语言都不会忽视的。
如果我们使用 Java 作为一个随意的例子，其中人们犯的一个最愚蠢的错误是不记录堆栈追踪：
```
LOGGER.error(ex.getMessage()) // Doesn't log stack trace
LOGGER.error(ex.getMessage(), ex) // Does log stack trace
```
但是 Go 设计中似乎没有这个信息
在获取上下文信息方面 - Russ 还提到了社区正在讨论一些潜在的接口用于剥离上下文错误。了解更多这点或许会很有趣。
### 堆栈追踪问题解决方案
幸运的是，在做了一些查找后，我发现了这个出色的[ Go 错误][5]库来帮助解决这个问题，来给错误添加堆栈跟踪：
```
if errors.Is(err, crashy.Crashed) {
fmt.Println(err.(*errors.Error).ErrorStack())
}
```
不过，我认为这个功能能成为语言的一等公民将是一个改进，这样你就不必对类型做一些修改了。此外，如果我们像先前的例子那样使用第三方库，那就可能不必使用 `crashy` - 我们仍有相同的问题。
### 我们对错误应该做什么？
我们还必须考虑发生错误时应该发生什么。[这一定有用，它们不会让你的程序崩溃][6]，通常也会立即处理它们：
```
err := method()
if err != nil {
// some logic that I must do now in the event of an error!
}
```
如果我们想要调用大量会返回错误的方法时会发生什么，在同一个地方处理它们么？看上去像这样：
```
err := doSomething()
if err != nil {
// handle the error here
}
func doSomething() error {
err := someMethod()
if err != nil {
return err
}
err = someOther()
if err != nil {
return err
}
someOtherMethod()
}
```
这感觉有点冗余，然而在其他语言中你可以将多条语句作为一个整体处理。
```
try {
someMethod()
someOther()
someOtherMethod()
}
catch (Exception e) {
// process exception
}
```
或者只要在方法签名中传递错误：
```
public void doSomething() throws SomeErrorToPropogate {
someMethod()
someOther()
someOtherMethod()
}
```
我个人认为这两个例子实现了一件事情，只有 `Exception` 模式更少冗余更加弹性。如果有什么，我发现 `if err！= nil` 感觉像样板。也许有一种方法可以清理？
### 将多条语句像一个整体那样发生错误
首先，我做了更多的阅读，并[在 Rob Pike 写的 Go 博客中][7]发现了一个比较务实的解决方案。
他定义了一个封装了错误的方法的结构体：
```
type errWriter struct {
w io.Writer
err error
}
func (ew *errWriter) write(buf []byte) {
if ew.err != nil {
return
}
_, ew.err = ew.w.Write(buf)
}
```
让我们这么做：
```
ew := &errWriter{w: fd}
ew.write(p0[a:b])
ew.write(p1[c:d])
ew.write(p2[e:f])
// and so on
if ew.err != nil {
return ew.err
}
```
这也是一个很好的方案，但是我感觉缺少了点什么 - 因为我们不能重复使用这个模式。如果我们想要一个含有字符串参数的方法，我们就不得不改变函数签名。或者如果我们不想执行写会怎样？我们可以尝试使它更通用：
```
type errWrapper struct {
err error
}
```
```
func (ew *errWrapper) do(f func() error) {
if ew.err != nil {
return
}
ew.err = f();
}
```
但是我们有一个相同的问题，如果我们想要调用含有不同参数的函数，它就无法编译了。然而你可以简单地封装这些函数调用：
```
w := &errWrapper{}
w.do(func() error {
return someFunction(1, 2);
})
w.do(func() error {
return otherFunction("foo");
})
err := w.err
if err != nil {
// process error here
}
```
这可以用，但是并没有帮助太大，因为它最后比标准的 `if err != nil` 检查带来了更多的冗余。我有兴趣听到有人能提供其他解决方案。或许语言本身需要一些方法来以不那么臃肿的方式的传递或者组合错误 - 但是感觉似乎是特意设计成不那么做。
### 总结
看完这些之后，你可能会认为我反对在 Go 中使用 `error`。但事实并非如此，我只是描述了如何将它与 `try catch` 模型的经验进行比较。它是一个用于系统编程很好的语言，并且已经出现了一些优秀的工具。仅举几例有 [Kubernetes][8]、[Docker][9]、[Terraform][10]、[Hoverfly][11] 等。还有小型、高性能、本地二进制的优点。但是，`error` 难以适应。 我希望我的推论是有道理的，而且一些方案和解决方法可能会有帮助。
--------------------------------------------------------------------------------
作者简介：
Andrew 是 OpenCredo 的顾问，于 2015 年加入公司。Andrew 在多个行业工作多年，开发基于 Web 的企业应用程序。
--------------------------------------------------------------------------------
via: https://opencredo.com/why-i-dont-like-error-handling-in-go
作者：[Andrew Morgan][a]
译者：[geekpi](https://github.com/geekpi)
校对：[校对者ID](https://github.com/校对者ID)
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
[a]:https://opencredo.com/author/andrew/
[1]:https://blog.golang.org/errors-are-values
[2]:https://blog.golang.org/error-handling-and-go
[3]:https://en.wikipedia.org/wiki/Don't_repeat_yourself
[4]:https://research.swtch.com/go2017
[5]:https://github.com/go-errors/errors
[6]:https://davidnix.io/post/error-handling-in-go/
[7]:https://blog.golang.org/errors-are-values
[8]:https://kubernetes.io/
[9]:https://www.docker.com/
[10]:https://www.terraform.io/
[11]:http://hoverfly.io/en/latest/