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context: Cause does not return Err for a custom context #62582

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ianlancetaylor opened this issue Sep 11, 2023 · 4 comments
Closed

context: Cause does not return Err for a custom context #62582

ianlancetaylor opened this issue Sep 11, 2023 · 4 comments
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NeedsFix The path to resolution is known, but the work has not been done.
Milestone
Go1.22

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@ianlancetaylor
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The documentation for context.Cause says that if a context was canceled other than via a call to CancelCauseFunc, then "Cause(c) returns the same value as c.Err()". However, that is not true today for a custom context. CC @Sajmani

Here is a test:

// customCauseContext is a custom Context used to test context.Cause.
type customCauseContext struct {
	mu   sync.Mutex
	done chan struct{}
	err  error
}

func (ccc *customCauseContext) Deadline() (deadline time.Time, ok bool) {
	return
}

func (ccc *customCauseContext) Done() <-chan struct{} {
	ccc.mu.Lock()
	defer ccc.mu.Unlock()
	return ccc.done
}

func (ccc *customCauseContext) Err() error {
	ccc.mu.Lock()
	defer ccc.mu.Unlock()
	return ccc.err
}

func (ccc *customCauseContext) Value(key any) any {
	return nil
}

func (ccc *customCauseContext) cancel() {
	ccc.mu.Lock()
	defer ccc.mu.Unlock()
	ccc.err = context.Canceled
	close(ccc.done)
}

func TestCustomContextCause(t *testing.T) {
	ccc := customCauseContext{
		done: make(chan struct{}),
	}
	ccc.cancel()
	if got := ccc.Err(); got != context.Canceled {
		t.Errorf("ccc.Err() = %v, want %v", got, context.Canceled)
	}
	if got := context.Cause(&ccc); got != context.Canceled {
		t.Errorf("Cause(ccc) = %v, want %v", got, context.Canceled)
	}
}

This fails with

--- FAIL: TestCustomContextCause (0.00s)
    foo_test.go:53: Cause(ccc) = <nil>, want context canceled
@ianlancetaylor ianlancetaylor added the NeedsFix The path to resolution is known, but the work has not been done. label Sep 11, 2023
@ianlancetaylor ianlancetaylor added this to the Go1.22 milestone Sep 11, 2023
@gopherbot
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Change https://go.dev/cl/527277 mentions this issue: context: support non-standard Context in Cause

@nightlyone
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Should that be backported to Go 1.20 where this behaviour has been introduced or is there a possible workaround for that issue that I am missing to see?

@ianlancetaylor
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It's kind of an edge case: code that has a custom Context type and calls context.Cause on that type. There isn't a lot of reason to call context.Cause on a Context that wasn't created by context.WithCancelCause, and by definition that does not return a custom context. So this issue doesn't seem to meet the requirements outlined at https://go.dev/wiki/MinorReleases. Happy to hear counter-arguments.

@colega
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colega commented Dec 20, 2023

It's kind of an edge case: code that has a custom Context type and calls context.Cause on that type.

I don't think it's such an edge case. I hit this while working on a library that does some intensive work and regularly checks for the context cancellation with if err := context.Cause(ctx); err != nil {.

The library doesn't know the type of the context that it has been provided, so in the case of a custom context it won't detect its cancellation and will keep executing.

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@colega @nightlyone @ianlancetaylor @gopherbot