Bug#426165: apt: Could use a blocking fd to read from the pipe from dpkg

[Kamil Ignacak <acerion@wp.pl>]

Hi!

If I read the history correctly, then I see the following changes to code:

In 2005 a loop reading child process' (dpkg) output was added, together 
with a call to usleep(1000) that slows an idle loop down. This was made 
in https://salsa.debian.org/apt-team/apt/-/commit/75ef8f1417.

In 2006 this bug was reported.

In 2007 a call to select() was added in the loop to postpone read() 
calls until more data from the child process is available. This was made 
in https://salsa.debian.org/apt-team/apt/-/commit/090c656632. Notice 
that select() was later replaced with pselect().

Also in 2007 the call to usleep() call in the loop was finally removed. 
This was done in https://salsa.debian.org/apt-team/apt/-/commit/7fdafa0f34.


I didn't run reporter's test with powertop before 090c656632 or after 
7fdafa0f34 so I don't know if/how much the problem was reduced. It would 
be kind of difficult for me to install Debian's version from 2006 to 
reproduce the issue in the original apt version.

Kamil