# Another day another incident #02

Today's incident is all about Go context.

TLDR: context got canceled, but it shouldn't.

## The problem

Imagine a chain of APIs:

- Calling API A
- Calling API B

Normally, if API A fails, API B should not be called.
But what if API A is **optional**, whether it successes or fails, API B should be called anyway.

My buggy code is like this:

```go
if err := doA(ctx); err != nil {
    log.Error(err)
    // Skip error
}

doB(ctx)
```

The problem is `doA` taking too long, so `ctx` is canceled, and the parent of `ctx` is canceled too.
So when `doB` is called with `ctx`, it will be canceled too (not what we want but sadly that what we got).

Example buggy code ([The Go Playground](https://go.dev/play/p/p4S27Su16VH)):

```go
func main() {
	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
	defer cancel()

	doA(ctx)
	doB(ctx)
}

func doA(ctx context.Context) {
	ctx, ctxCancel := context.WithTimeout(ctx, 1*time.Second)
	defer ctxCancel()

	select {
	case <-time.After(2 * time.Second):
		fmt.Println("doA")
	case <-ctx.Done():
		fmt.Println("doA", ctx.Err())
	}
}

func doB(ctx context.Context) {
	ctx, ctxCancel := context.WithTimeout(ctx, 3*time.Second)
	defer ctxCancel()

	select {
	case <-time.After(2 * time.Second):
		fmt.Println("doB")
	case <-ctx.Done():
		fmt.Println("doB", ctx.Err())
	}
}
```

The output is:

```txt
doA context deadline exceeded
doB context deadline exceeded
```

As you see both `doA` and `doB` are canceled.

## The (temporary) solution

Quick Google search leads me to [context: add WithoutCancel #40221](https://github.com/golang/go/issues/40221) and I quote:

> This is useful in multiple frequently recurring and important scenarios:
>
> - Handling of rollback/cleanup operations in the context of an event (e.g., HTTP request) that has to continue regardless of whether the triggering event is canceled (e.g., due to timeout or the client going away)
> - Handling of long-running operations triggered by an event (e.g., HTTP request) that terminates before the termination of the long-running operation

So beside waiting to upgrade to Go `1.21` to use `context.WithoutCancel`, you can use this [workaround code](https://pkg.go.dev/context@master#WithoutCancel):

```go
func DisconnectContext(parent context.Context) context.Context {
	if parent == nil {
		return context.Background()
	}

	return disconnectedContext{
		parent: parent,
	}
}

type disconnectedContext struct {
	parent context.Context
}

func (ctx disconnectedContext) Deadline() (deadline time.Time, ok bool) {
	return
}

func (ctx disconnectedContext) Done() <-chan struct{} {
	return nil
}

func (ctx disconnectedContext) Err() error {
	return nil
}

func (ctx disconnectedContext) Value(key any) any {
	return ctx.parent.Value(key)
}
```

So the buggy code becomes ([The Go Playground](https://go.dev/play/p/oIU-WxEJ_F3)):

```go
func main() {
	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
	defer cancel()
	doA(ctx)
	doB(ctx)
}

func doA(ctx context.Context) {
	ctx, ctxCancel := context.WithTimeout(ctx, 1*time.Second)
	defer ctxCancel()

	select {
	case <-time.After(2 * time.Second):
		fmt.Println("doA")
	case <-ctx.Done():
		fmt.Println("doA", ctx.Err())
	}
}

func doB(ctx context.Context) {
	ctx, ctxCancel := context.WithTimeout(DisconnectContext(ctx), 3*time.Second)
	defer ctxCancel()

	select {
	case <-time.After(2 * time.Second):
		fmt.Println("doB")
	case <-ctx.Done():
		fmt.Println("doB", ctx.Err())
	}
}
```

The output is:

```txt
doA context deadline exceeded
doB
```

As you see only `doA` is canceled, `doB` is done perfectly.
And that what we want in this case.

## Thanks

- [Using Context in Golang - Cancellation, Timeouts and Values (With Examples)](https://www.sohamkamani.com/golang/context-cancellation-and-values/)
- [Go Context timeouts can be harmful](https://uptrace.dev/blog/golang-context-timeout.html)