nix path-info: Don't write to std::cout directly

[edolstra]

Motivation

This interferes with the progress bar, resulting in output like

evaluating derivation 'git+file:///home/eelco/Dev/nix-master#packages.x86_64-linux.default'/nix/store/zz8v96j5md952x0mxfix12xqnvq5qv5x-nix-2.26.0pre20241114_a95f6ea.drv

Context

Priorities and Process

Add 👍 to pull requests you find important.

The Nix maintainer team uses a GitHub project board to schedule and track reviews.

[roberth]

Nice fix

[roberth]

Technically this loses the streaming property, but that doesn't seem so bad.
It would be nice to have a logger->cout that does support streaming. It'd have to be an ostream implementation that performs line buffering and then calls another stream or a callback with the line, where we pass the cout() method.
(but again, nice to have)

[edolstra]

Technically this loses the streaming property

AFAIK logger->cout() is still streaming (i.e. the line is written immediately and not buffered).

[Ericson2314]

Well I assume @roberth means get a std::ostream from the logger so we don't buffer a string explicitly, even if that string ends up being one line and would be buffered anyways.

[roberth]

It should be possible to have a cout that coordinates its timing with the progress bar to avoid clobbering, and this could be done perhaps by implementing ostream in a custom way, or using an internal pipe.