Offsetwriter refers to the design of SectionReader and removes
the section parameter n.
Since the size of the written data is determined by the user,
we cannot know where the end offset of the original data is.
The offset of SeekEnd is not valid in Seek method.
Fixes #45899.
Change-Id: I9d9445aecfa0dd4fc5168f2f65e1e3055c201b45
Reviewed-on: https://go-review.googlesource.com/c/go/+/406776
Run-TryBot: Ian Lance Taylor <iant@google.com> Reviewed-by: Joedian Reid <joedian@golang.org> Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
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