bug#12366: [gnu-prog-discuss] Writing unwritable files

[Paul Eggert]

>> If some other process is writing F
>> while I run 'sed -i F', F is not replaced atomically.

> How not so?

For example:

echo ac >f
sed -i 's/a/b/' f &
sed -i 's/c/d/' f
wait
cat f

If 'sed' were truly atomic, then the output of this would
always be 'bd'.  But it's not.