Taking over CL 162240, the original CL hasn't been making progress.
I just took the parts that fix the immediate issue. I left the
signatslice changes out, I don't think they are necessary.

Fixes #30202

Change-Id: I5b347605f0841dd925d5a73150b8bf269fa82464
Reviewed-on: https://go-review.googlesource.com/c/go/+/183852
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>