A group with no attributes is certainly empty. But we also want to
consider a group to be empty if all its attributes are empty groups.
The built-in handlers did not handle this second case properly.
This CL fixes that.
There are two places in the implementation that we need to consider.
For Values of KindGroup, we change the GroupValue constructor to omit
Attrs that are empty groups. A Group is then empty if and only if it
has no Attrs. This avoids a recursive check for emptiness.
It does require allocation, but that doesn't worry us because Group
values should be relatively rare.
For groups established by WithGroup, we avoid opening such groups
unless the Record contains non-empty groups. As we did for values, we
avoid adding empty groups to records in the first place, so we only
need to check that the record has at least one Attr.
We are doing extra work, so we need to make sure we aren't slowing
things down unduly. Benchmarks before and after this change show
minimal differences.
Fixes #61067.
Change-Id: I684c7ca834bbf69210516faecae04ee548846fb7
Reviewed-on: https://go-review.googlesource.com/c/go/+/508436
Run-TryBot: Jonathan Amsterdam <jba@google.com> Reviewed-by: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>