`tee` get stuck with dependency.

[Jin Li]

Hi all,

I have the following test case.

~~~
tmpdir=$(mktemp -d)
cd "$tmpdir"
mkfifo a b
seq 10 | tee a b > /dev/null &
pid=$!
set -v
x=$(wc -l a)
sed -e "s/^/b$x/" b
wait "$pid"
~~~

In the above test case, the second fifo `b` will depend on the first
fifo `a`. But it will get stuck in reading the first fifo `a`, in the
following line,
~~~
x=$(wc -l a)
~~~

Do you have some idea to resolve `tee` with dependency for two fifos?
Many thanks for your help.

Best regards,
Jin