internal/poll: revise the determination about [handled] and improve the code readability for SendFile
There were a bit too many conditional branches in the old code,
resulting in a poor readability. It could be more concise by reducing
and consolidating some of the conditions.
Furthermore, how we've determined whether or not the data transimission
was handled by sendfile(2) seems inappropriate, because it marked the
operation as unhandled whenever any non-retryable error occurs from
calling sendfile(2), it doesn't look like a right approach, at least
this is an inconsistent behavior with what we've done in Splice.
Related to #64044
Change-Id: Ieb65e0879a8841654d0e64a1263a4e43179df1ba
Reviewed-on: https://go-review.googlesource.com/c/go/+/537275
TryBot-Result: Gopher Robot <gobot@golang.org>
Commit-Queue: Ian Lance Taylor <iant@golang.org>
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Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
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