os: avoid nil returns from Readdirnames, Readdir, ReadDir
The refactoring of this code while adding ReadDir stopped
pre-allocating a 100-entry slice for the results.
That seemed like a good idea in general, since many
directories have nowhere near 100 entries, but it had the
side effect of returning a nil slice for an empty directory.
Some “golden” tests that are too sensitive about nil vs not
inside Google broke because Readdirnames(-1) was now
returning nil instead of []string{} on an empty directory.
It seems likely there are other such tests in the wild, and
it doesn't seem worth breaking them.
This commit restores the non-nil-ness of the old result,
without restoring the excessive preallocation.
Fixes #42367.
Change-Id: I2be72030ac703346e859a97c2d4e456fadfce9b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/267637
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Rob Pike <r@golang.org>