Make nix-store --delete explain why a path is alive

[orivej]

Fixes #1810

The output looks like this:

$ nix-store --delete /nix/store/85i1173dllq1s23k5wnyiw9a33k808bp-perl-5.32.1-man
finding garbage collector roots...
need '/nix/store/clp069h9m0hmdhi6v86gf90n1p2gqhr1-nixos-system-orivej-21.05.git.b26b54f0563' because it's a root
need '/nix/store/0ywlmj5i7gsjm0vx9nzmkmr0zrws0lac-perl-5.32.1-env' for '/nix/store/clp069h9m0hmdhi6v86gf90n1p2gqhr1-nixos-system-orivej-21.05.git.b26b54f0563'
need '/nix/store/85i1173dllq1s23k5wnyiw9a33k808bp-perl-5.32.1-man' for '/nix/store/0ywlmj5i7gsjm0vx9nzmkmr0zrws0lac-perl-5.32.1-env'
0 store paths deleted, 0.00 MiB freed
error: cannot delete path '/nix/store/85i1173dllq1s23k5wnyiw9a33k808bp-perl-5.32.1-man' since it is still alive

$ nix-store --delete /nix/store/amfzfi6gfxnjndmmff3kz9vmkq9lzh35-perl-5.32.0-devdoc
finding garbage collector roots...
need '/nix/store/clp069h9m0hmdhi6v86gf90n1p2gqhr1-nixos-system-orivej-21.05.git.b26b54f0563' because it's a root
need '/nix/store/4xixizlfkjp0z8z9xbg76zvgqpjb7rvq-nixos-system-orivej-21.05.git.b26b54f0563.drv' for '/nix/store/clp069h9m0hmdhi6v86gf90n1p2gqhr1-nixos-system-orivej-21.05.git.b26b54f0563'
need '/nix/store/22srqan5h3vk09r0vzqkbnbv4dvb3lx2-system-path.drv' for '/nix/store/4xixizlfkjp0z8z9xbg76zvgqpjb7rvq-nixos-system-orivej-21.05.git.b26b54f0563.drv'
need '/nix/store/68iqmqk0qp6vwwwwgclgkwyr4hbfjq3v-lilypond-2.22.1.drv' for '/nix/store/22srqan5h3vk09r0vzqkbnbv4dvb3lx2-system-path.drv'
need '/nix/store/vyxhlf79jibd4pq0fxfr14fxiq49ri7a-texlive-combined-2021.drv' for '/nix/store/68iqmqk0qp6vwwwwgclgkwyr4hbfjq3v-lilypond-2.22.1.drv'
need '/nix/store/3xnyaqjgd9izsc7a5s5qfc97pi2fxlnd-texlive-combined-2021' for '/nix/store/vyxhlf79jibd4pq0fxfr14fxiq49ri7a-texlive-combined-2021.drv'
need '/nix/store/7d7s9ln9scd20v41r3ihhs7xskyx9cqy-texlive-lualibs-2.73' for '/nix/store/3xnyaqjgd9izsc7a5s5qfc97pi2fxlnd-texlive-combined-2021'
need '/nix/store/li12jji15fz0i71ia90cnnf5i8l9r1zz-texlive-lualibs-2.73.drv' for '/nix/store/7d7s9ln9scd20v41r3ihhs7xskyx9cqy-texlive-lualibs-2.73'
need '/nix/store/2fl1ypicql8gfkzx31c0nfgkps2i939a-lualibs.r57277.tar.xz.drv' for '/nix/store/li12jji15fz0i71ia90cnnf5i8l9r1zz-texlive-lualibs-2.73.drv'
need '/nix/store/cb21wchxnmjkkxlj22qv7782b2is6141-curl-7.74.0.drv' for '/nix/store/2fl1ypicql8gfkzx31c0nfgkps2i939a-lualibs.r57277.tar.xz.drv'
need '/nix/store/0fyhz66ybzb0gnyrnjbl9jma09s220yp-nghttp2-1.41.0.drv' for '/nix/store/cb21wchxnmjkkxlj22qv7782b2is6141-curl-7.74.0.drv'
need '/nix/store/1zf9plakb12amj6f6b2i29b7ssbs95ly-openssl-1.1.1i.drv' for '/nix/store/0fyhz66ybzb0gnyrnjbl9jma09s220yp-nghttp2-1.41.0.drv'
need '/nix/store/jpj7rxchjh6ql0gf2y6s01ii1rdzwlmp-perl-5.32.0.drv' for '/nix/store/1zf9plakb12amj6f6b2i29b7ssbs95ly-openssl-1.1.1i.drv'
need '/nix/store/amfzfi6gfxnjndmmff3kz9vmkq9lzh35-perl-5.32.0-devdoc' for '/nix/store/jpj7rxchjh6ql0gf2y6s01ii1rdzwlmp-perl-5.32.0.drv'
0 store paths deleted, 0.00 MiB freed
error: cannot delete path '/nix/store/amfzfi6gfxnjndmmff3kz9vmkq9lzh35-perl-5.32.0-devdoc' since it is still alive

[roberth]

I like this functionality, but I don't think printing these info messages (unconditionally) is the responsibility of libstore. Perhaps the method could return a chain of references as a std::vector<string>. The CLI can then print it.

From a practical perspective, this is to prevent that unrelated callers start spamming these messages for no good reason.

[stale]

I marked this as stale due to inactivity. → More info

[Ericson2314]

I agree with what @roberth said.

Let's return something like:

struct DeadAlready {};
struct IsRoot {};
struct RootedBy { StorePath root; };

typedef std::variant<DeadAlready, IsRoot, RootedBy> OptLiveness;

to indicate why.

state.alive can likewise be a map of:

typedef std::variant<IsRoot, RootedBy> Liveness;

Making OptLiveness be std::optional<Liveness> is fine me too.

[asymmetric]

Shut up bot.

[jakubgs]

Is this dead?

[roberth]

Is this dead?

If it's a store path, who knows ;)

I don't think anyone should feel inhibited to work on solving this problem.

To make this more actionable, I think an improved solution could perform a graph search on the retrieved dependency edges, and report the path in the thrown exception.

That's perhaps a bit easier to manage than reporting all the data internally and then still having to implement such logic in the CLI.

(fwiw I wasn't a maintainer yet when I reviewed this; this was from before the Nix team)

[jakubgs]

I would love to work on and learn more about how Nix works internally, but honestly I don't have the time.