read command sometimes misses newline on timeout

[Thomas Oettli]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: x86_64-pc-linux-gnu-gcc
Compilation CFLAGS: -O2 -pipe
uname output: Linux testserver 6.6.47-gentoo #1 SMP Tue Aug 20 09:38:16 CEST 
2024 x86_64 Intel(R) Xeon(R) Gold 6242 CPU @ 2.80GHz GenuineIntel GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 5.2
Patch Level: 26
Release Status: release

Description:
        I have tried to write a bash script that asynchronously reads from a 
pipe (line by line) with the help of "read -t".
    If the timeout occurs in just the right moment, read returns the full line, 
but the return code says timeout (rc > 128).
    Therefor it is not possible to know if a full line was returned or not. 
Please see the script in the Repeat-By section that reproduces the error in 
seconds.
    

Repeat-By:
    function reader() {
      local buf line
      while :; do
        read -t .01 buf
        rc=$?
        if (( rc == 0 )); then
          line+=$buf
        elif (( rc > 128 )); then
          line+=$buf
          continue
        fi
        [[ $line != TEST ]] && echo Invalid line: $line && exit
        echo OK
        line=""
      done
    }
    reader < <(
      while :; do
        echo -n TEST
        sleep .00$(($RANDOM%10))
        echo
      done
    )