Issue #60416 - Correction of BufferedReadStream 's Position

[nidaca]

Correction of Microsoft.AspNetCore.WebUtilities.BufferedReadStream Position property

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The implementation of Position's property setter for Microsoft.AspNetCore.WebUtilities.BufferedReadStream class was incorrect and changed accordingly with the proposed solution described in the issue #60416 .

Description

By trying to use Microsoft.AspNetCore.WebUtilities.MultipartReader in a way that is directly using the section streams outside the initial reading loop (made through ReadNextSectionAsync), I've come to at least one case (when dealing with more than two section streams) where the conclusion is that the Position property of BufferedReadStream has an incorrect implementation when trying to position inside the internal buffer.
The solution was to advance in the internal buffer not with the offset between old position and new position (i.e. backward innerOffset), but until that offset / distance, so in the end the new Position will reflect the desired input value when getting the property.

Fixes #60416

[halter73]

Thanks for reporting this issue and following through with the fix.

[halter73]

LGTM. Even though this is a bug fix, I don't think this meets the bar for patching given that it's unusual to seek a BufferedReadStream and it's rare for the inner stream to even be seekable which is probably why this took so long to discover.