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This patch does two things:
1. Enable line buffering for echo and yes. I found this through test
flakiness from the toybox xargs tests running in CI on devices where
"echo" is provided by toybox. For `echo y`, GNU echo does one write
of "y\n" but toybox echo was doing two writes, which makes it more
likely (4% on the heavily-loaded CI machines) for writes from the two
processes to be interleaved.
2. Fix line buffering on glibc if you're calling `toybox foo` rather
than `foo`. Otherwise we come through once and switch to unbuffered
mode, then again and switch to line buffered mode --- which doesn't
seem to actually work in glibc unless you specify a buffer (so
passing toybuf and sizeof(toybuf) works, but NULL and 0 doesn't).
I hit the second issue trying to reproduce the first issue on the desktop
rather than on Android.
(If you're scratching your head wondering "why yes(1) too, not just
echo(1)?", that represents a blind alley I went down when I mistook
which tool was in use. It seemed like the same principle should apply,
and it matches what other implementations do.)
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