libstore-c: add more derivation functions

[RossComputerGuy]

Motivation

Upstreaming more C API exposure, specifically more derivation related things.

Context

Comes from DeterminateSystems#210

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[xokdvium]

Suggested change

	typedef struct nix_derivation_output nix_derivatio_noutput;
	typedef struct nix_derivation_output nix_derivation_output;

[Ericson2314]

This one and associated functions is not needed if the consumer uses the JSON

[RossComputerGuy]

Fixed

[Ericson2314]

I have a preference that consumers should achieve these goals by deserializing the JSON themselves. Then we can make do with a smaller interface which will be easier to maintain and more stable.

[Ericson2314]

This should just be the opt paths, which is useful for the FOD case. The outputs themselves can be gotten from the JSON.

[RossComputerGuy]

Why?

[Ericson2314]

This is me elaborating what I wrote in #14031 (comment)

[RossComputerGuy]

I have a preference that consumers should achieve these goals by deserializing the JSON themselves. Then we can make do with a smaller interface which will be easier to maintain and more stable.

How do I get the JSON myself? All I have is a store path.

[Ericson2314]

1. Use your new function to read into memory derivation in store at given path
2. Use my function (recently added) to dump JSON from in memory drv

[RossComputerGuy]

Isn't there increased overhead if everything is being accessed through JSON? Wouldn't some operations make sense to do without deserializing?

[Ericson2314]

I would say optimization should come later. Let's see evidence of there being a bottleneck in practice before we make the interface bigger.

[Ericson2314]

1. Good to put from and to JSON next to each other
2. I still think we can get rid of nix_derivation entirely.
3. I want to make the output path setting easier in C++ too. I might try to land a patch for that.

[Ericson2314]

Ah does anything use the BuildResult JSON today? Hopefully that is one I can fix as part of my big JSON rework next :)

[RossComputerGuy]

I'm looking at our usage and it looks like there's not much usage of it.

[Ericson2314]

this should take a nix_derivation, not parse the JSON itself.

[RossComputerGuy]

Then we'll have to remove the verify function to even be able to get a nix_derivation.

[Ericson2314]

hmm? I am not sure what you mean. You mean to make an input-addressed derivation?

[RossComputerGuy]

We'd have to remove this:

nix/src/libstore-c/nix_api_store.cc

Line 190 in 772a380

drv.checkInvariants(*store->ptr, drvPath);

The checkInvariants function failing with a mismatch outpath hash error can prevent a nix_derivation from returning. If nix_derivation_make_outputs needs a nix_derivation and you can't get one without getting the right outpath hashes, now we have a catch-22 and it'll be impossible to add derivations.

[Ericson2314]

I am going to bed, but we can look into this soon, I think we should be able to untangle it perhaps simpler than you think