meta request: support for should_continue cancellation callback #467
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Add a boolean `continue_callback`, which polls a request-cancellation flag set by the user. This matches the use-case of the `ContinueRequestHandler` / `ContinueRequest` / `ShouldContinue` boolean functions used by `aws-sdk-cpp`. Passive polling is preferred over active cancellation, since the latter would require to store a reference to the s3 meta request. The polling function is evaluated at the following points during the lifetime of the meta request: 1. _Request preparation_ (`aws_s3_meta_request_prepare_request`). 2. _Request update_ (the `.update` handler invoked via `aws_s3_meta_request_update`). 3. _Http stream completion_ (`.on_complete` handler invoking `aws_s3_meta_request_send_request_finish_default`). 4. _Writing download data_ (`aws_s3_meta_request_stream_response_body_synced`). If a `continue_callback` is provided and if it returns `false` at any of these 4 evaluation times, the meta request completes with the `error_code` `AWS_ERROR_S3_CANCELED`.
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This restores an updated version of #238.
Rationale
Add a boolean
continue_callback, which polls a request-cancellation flag set by the user. This matches the use-case of theContinueRequestHandler/ContinueRequest/ShouldContinueboolean functions used byaws-sdk-cpp.Passive polling is preferred over active cancellation, since the latter would require to store a reference to the s3 meta request. The polling function is evaluated at the following points during the lifetime of the meta request:
aws_s3_meta_request_prepare_request)..updatehandler invoked viaaws_s3_meta_request_update)..on_completehandler invokingaws_s3_meta_request_send_request_finish_default).aws_s3_meta_request_stream_response_body_synced).If a
continue_callbackis provided and if it returnsfalseat any of these 4 evaluation times, the meta request completes with theerror_codeAWS_ERROR_S3_CANCELED.By using a boolean flag in the body of the function, the polling overhead can be kept minimal.
By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.