This requirement is from RFC 4880 4.2.2.4.
Also simplify the partialLengthWriter loop. The old code worked but
was written in a confusing way, with a loop whose terminating condition
didn't make sense and was never true in practice.
Rewrite it to more clearly do a set of partial writes of decreasing size.
Fixesgolang/go#32474
Change-Id: Ia53ceb39a34f1d6f2ea7c60190d52948bb0db59b
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/181121
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>