Context without cancel propagation

Issue

How can I create a copy (a clone if you will) of a Go context that contains all of the values stored in the original, but does not get canceled when the original does?

It does seem like a valid use case to me. Say I have an http request and its context is canceled after the response is returned to a client and I need to run an async task in the end of this request in a separate goroutine that will most likely outlive the parent context.

func Handler(ctx context.Context) (interface{}, error) {
        result := doStuff(ctx)
        newContext := howDoICloneYou(ctx)
        go func() {
                doSomethingElse(newContext)
        }()
        return result
}

Can anyone advice how this is supposed to be done?

Of course I can keep track of all the values that may be put into the context, create a new background ctx and then just iterate through every possible value and copy… But that seems tedious and is hard to manage in a large codebase.

Solution

Since context.Context is an interface, you can simply create your own implementation that is never canceled:

import (
    "context"
    "time"
)

type noCancel struct {
    ctx context.Context
}

func (c noCancel) Deadline() (time.Time, bool)       { return time.Time{}, false }
func (c noCancel) Done() <-chan struct{}             { return nil }
func (c noCancel) Err() error                        { return nil }
func (c noCancel) Value(key interface{}) interface{} { return c.ctx.Value(key) }

// WithoutCancel returns a context that is never canceled.
func WithoutCancel(ctx context.Context) context.Context {
    return noCancel{ctx: ctx}
}

Answered By – Peter

Answer Checked By – Pedro (GoLangFix Volunteer)

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