Use context.Cause when returning errors originating in context cancelation

[oakad]

Describe the feature

context.Cause was added in Go 1.20, so this SDK should be able to use it. Right now this SDK simply returns the value of context.Err() which, in case of cancellation, is always non-informative context.Canceled.

Use Case

In my particular case, I have a concurrent data pipeline which ingests very large files from S3. If error is detected somewhere mid-pipeline, the most convenient way to cancel a very long read is to cancel the context on it (as context.CancelCauseFunc/context.CancelFunc are idempotent and concurrency safe).

However, I would also like to log the exact reason as to why read was aborted, and it is not helpful, that I'm getting context.Canceled out of the reader and not a much more descriptive error I supply as a cause.

The above reasoning applies to other operations of the sort (reading/writing to S3, listening to Kinesis, anything that has to run for a long time and requires concurrent processing on the client).

Proposed Solution

Replace uses of ctx.Err() with context.Cause(ctx). The replacement should be reasonably benign in most cases.

Other Information

No response

Acknowledgements

• I may be able to implement this feature request
• This feature might incur a breaking change

AWS Go SDK V2 Module Versions Used

github.com/aws/aws-sdk-go-v2 v1.36.3
github.com/aws/aws-sdk-go-v2/config v1.29.14
github.com/aws/aws-sdk-go-v2/feature/ec2/imds v1.16.30
github.com/aws/aws-sdk-go-v2/feature/rds/auth v1.5.11
github.com/aws/aws-sdk-go-v2/feature/s3/manager v1.17.75
github.com/aws/aws-sdk-go-v2/service/cloudwatchlogs v1.50.0
github.com/aws/aws-sdk-go-v2/service/ec2 v1.218.0
github.com/aws/aws-sdk-go-v2/service/s3 v1.79.3

Go version used

1.24.3

[wty-Bryant]

It sounds reasonable, but replacing all usage of ctx.Err() with context.Cause(ctx) might bring some breaking change, consider the case when someone's error handling code depends on part of the ctx.Err() message, which could differ from the err passed to context.CancelCauseFunc.

We would not regard it as a scheduled feature for now due to backward compatibility. But I just leave this issue open in case you have some compatible workaround.

[oakad]

Unless people actively use CancelCauseFunc, Cause will return the same error as Err(). This is one of those changes which are benign early on (people don't do any breaking shenanigans because the language feature is newly introduced) and gets worse over time because people will be adding work arounds as time goes on.