TERM=dumb fixes

[intelfx]

• libmain/progress-bar: try harder to avoid escape sequences if !isTTY
• treewide: replace usages of isatty(STDERR_FILENO) with shouldANSI()
• treewide: shouldANSI() -> isTTY()
• libutil/terminal: cache isTTY()

Motivation

Currently, progress bar code emits control sequences (at least \r and CSI K)
regardless of whether nix allows other escape sequences in its output (as
determined by nix::shouldANSI() function.

Rework progress bar code such that !isTTY disables emission of all control
sequences, including \r.

Additionally, rename nix::shouldANSI() into nix::isTTY() to better reflect
the semantics of this function.

Context

Running nix copy with TERM=dumb and/or standard streams redirected into
a pipe still causes nix to emit several control sequences, interfering with
programmatic parsing of its output:

    "msg": [
        "",
        "\u001b[KWarning: Permanently added '<redacted>' (ED25519) to the list of known hosts.",
        "",
        "\u001b[Kcopying path '/nix/store/xdrsf841kdlf95rxd3qy6yhi57s25krr-dns-root-data-2023-11-27' from 'https://cache.nixos.org'...",
        "copying path '/nix/store/l3lhzwrfgzncjj1z4wyq6kkrp6czx5qp-gcc-12.3.0-libgcc' from 'https://cache.nixos.org'...",
        <...>

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[thufschmitt]

Thanks!

• d9fc4bf, 950b640 and c6f0407 look good.
• I don't think 8c1eeb4 is worthwhile (shouldANSI feels like a better name to me)

[intelfx]

• I don't think 8c1eeb4 is worthwhile (shouldANSI feels like a better name to me)

It's just a bit of a misnomer, because semantically it also controls whether to emit things like \r which are not ANSI control codes (thus it does not answer the question of "whether our terminal is ANSI X3.64 capable", but rather "is the thing attached to our stderr a terminal at all").

Specifically, this hunk is needed to achieve reasonable UX (we should log individual actions if and only if the progress bar is not available):

https://github.com/NixOS/nix/pull/10131/files#diff-20a8b5b2a231db80eab27840bd32ac0214aa0c4e9e923e649d3d741c3da77b48R379-R381

...but if I saw a line like if (nix::shouldANSI()) verbosity = ... in an unfamiliar codebase I'd get slightly confused because there is no obvious link between ANSI capabilities and verbosity.

---

That said, I do not feel strongly about it, if you think it's not worthwhile, I'll drop it :)

[intelfx]

@thufschmitt gentle ping — does the reasoning above make any additional sense for you, or do you still think I should drop the rename?

[thufschmitt]

Sorry for the delay @intelfx

Nah, let's keep it like that. Will merge as soon as the CI is OK