Description
The GCS service supports negative suffix length range requests:
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Range
Range: <unit>=-<suffix-length>
However, the Go client doesn't currently allow making range requests in this way. One use-case for this is fetching the last N bytes of a log file, something that is especially useful in the context of a log file for a failed build (since error messages typically are at the end of the log file).
Additionally, when specifying a suffix length, it is useful to know the absolute offset of the partial content which was returned. This is useful so one can implement "page backwards" functionality.
This information is in the Content-Range header which is already being parsed to obtain the total size of the object even though a partial response is being returned:
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Range
Content-Range: <unit> <range-start>-<range-end>/<size>
To that end, I've put forth two patches which would allow this functionality:
https://code-review.googlesource.com/c/gocloud/+/44131
https://code-review.googlesource.com/c/gocloud/+/44132
The 44131 patch extends the existing NewRangeReader() API so that instead of returning an error if the offset parameter is negative, it would interpret this to indicate that it is desired to obtain the absolute value number of bytes from the END of the object.
The 44132 patch extends the existing ReaderObjectAttrs structure with a new Offset field which would return the absolute offset of bytes that was returned. Specifically, if a positive offset is passed into NewRangeReader() API, then the ReaderObjectAttrs.Offset would reflect that exact same value. However, this becomes more useful when a negative offset is passed to ReadObjectAttrs, since this would reflect the actual absolute offset of bytes that was returned.
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