How to Clone a Go Context Without Cancel Propagation?

How to Clone a Go Context without Cancel Propagation

In the Go programming language, a context.Context carries metadata and cancellation signals between functions and goroutines. However, it may be desirable to create a copy of a context that retains the same values but does not inherit the cancellation status of the original.

Use Case:

This scenario arises when an HTTP request's context gets canceled after returning the response to the client, but you need to perform an asynchronous task in a separate goroutine that will likely outlive the parent context.

Solution:

1. Create a Custom Context Implementation:

Prior to Go 1.21, one approach was to create your own context.Context 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) }

Then, you can create a new context using this implementation:

ctxWithoutCancel := WithoutCancel(ctx)

2. Use WithoutCancel Function (Go 1.21 ):

As of Go 1.21, the context package includes a WithoutCancel function that simplifies this process:

ctxWithoutCancel := context.WithoutCancel(ctx)

This function returns a new context that shares the same values as the original context but is not subject to cancellation.

Example:

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

This way, doSomethingElse will continue running even after the request context has been canceled.

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