When reading less than wanted characters, "read" does not detect NUL bytes

[Davide Brini]

Best explained with an example:

$ printf 'a\x00\x00bc' | { while IFS= read -d '' -n 1 var; do echo "read: $var, 
length: ${#var}"; done; }
read: a, length: 1
read: , length: 0
read: , length: 0
read: b, length: 1
read: c, length: 1

This is as expected, and allows detecting a NUL in the input when the
length of $var is 0  (let's not talk about EOF and non-newline terminated
inputs here, this is not the issue).

However, if trying to read 2 (or more) characters at a time, it's no longer
true:

$ printf 'a\x00\x00bc' | { while IFS= read -d '' -n 2 var; do echo "read: $var, 
length: ${#var}"; done; }
read: a, length: 1
read: , length: 0
read: bc, length: 2

I would expect there to be another read of length 0 between the "a" and the
"bc".

Apologies if this has been already discussed and/or if I'm missing
something.

$ bash --version
GNU bash, version 4.4.23(1)-release (x86_64-unknown-linux-gnu)

-- 
D.