bug#71299: Investigation

[Richard Sent]

Hi Guix!

I started looking into this problem and unfortunately it looks like it
might be more complicated than I thought.

My initial thought was having two progress-reporters and selecting on
based on a simple conditional if current-error-port was a tty or not.
This would be in guix/scripts/substitute. Unfortunately, this might be a
bit more challenging.

Unlike the other scripts, guix/scripts/substitute is invoked by the
daemon directly. Furthermore, it isn't passed a file descriptor to the
invokers stderr. Instead, it goes through the build daemon. See
nix/libstore/local-store.cc:LocalStore::getLineFromSubstituter.

>From my testing calling istty? on that error port always returns #f,
ergo the "simple" progress-reporter is always used and we lose the
interactive progress bars.

Testing this is also a pain as well because it involves changing the
build daemon. In my experience the easiest way is:

1. Change the commit field for the guix package in
package-management.scm to the commit you want to check

2. Update the url field for the guix package to
"file:///path/to/guix/clone"

3. Forcefully disable authentication in guix/build-system/channel.scm

4. Generate an installer image via $ ./pre-inst-env guix system image
gnu/system/install.scm --image-type=iso9660, then boot from that image
and observe the output.

So, that leaves two options that I can see:

1. The daemon stops capturing stderr from the substituter and merely
passes it the daemon's stderr.

  1. Challenging to get right and this may have unforseen consequences.
  Two processes writing freely to the same output at once is a bad idea.

2. The daemon has a isatty? check and sets a flag for the build agent
which is passed along to scripts/substitute.

-- 
Take it easy,
Richard Sent
Making my computer weirder one commit at a time.