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StackId should support the "any" stack ("*") #104

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edmorley opened this issue Oct 12, 2021 · 1 comment · Fixed by #109
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StackId should support the "any" stack ("*") #104

edmorley opened this issue Oct 12, 2021 · 1 comment · Fixed by #109
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@schneems
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Buildpack API 0.5

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@edmorley
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edmorley commented Oct 12, 2021 •
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As of Buildpack API 0.5, a buildpack can declare support for any stack using a stack ID of "*":
https://github.com/buildpacks/rfcs/blob/main/text/0056-any-stack-buildpacks.md
https://github.com/buildpacks/spec/blob/main/buildpack.md#buildpack-implementations
https://github.com/buildpacks/spec/blob/main/platform.md#stack-id

However this isn't currently supported:

use std::str::FromStr;
use libcnb::data::buildpack::BuildpackId;

let valid = BuildpackId::from_str("*");
assert_eq!(valid.unwrap().as_str(), "*");

Gives:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: InvalidBuildpackId("*")', src/data/buildpack.rs:8:18

Due to:
https://github.com/Malax/libcnb.rs/blob/f46bdda85838c9cc21cd1de3243d4160b85afc12/src/data/buildpack.rs#L209-L224

In addition, there is no validation that mixins is not set when the any stack is used (which is forbidden by the spec).

GUS-W-10055742
@edmorley edmorley mentioned this issue Oct 12, 2021
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@hone hone added this to the Buildpack API milestone Oct 18, 2021
@schneems
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schneems commented Oct 19, 2021 •
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In this example:

use std::str::FromStr;
use libcnb::data::buildpack::BuildpackId;

let valid = BuildpackId::from_str("*");
assert_eq!(valid.unwrap().as_str(), "*");

I think you mean:

use std::str::FromStr;
use libcnb::data::buildpack::StackId;

let valid = StackId::from_str("*");
assert_eq!(valid.unwrap().as_str(), "*");

Regarding:

In addition, there is no validation that mixins is not set when the any stack is used (which is forbidden by the spec).

I think that's the harder piece. StackId is used by Stack which is used by BuildpackToml and they're mostly there for serializing/deserializing.

I found:

It looks like this is the most promising way to get the functionality today (shadow struct with a try_from):

@schneems schneems reopened this Oct 19, 2021
@schneems schneems mentioned this issue Oct 19, 2021
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schneems added a commit that referenced this issue Oct 20, 2021
@schneems
Add support for "any" stack * in StackId
034b2a2 
When a `Stack` struct has a `StackId` of `*` then it should have no mixins.

This is validated by a "shadow" struct deserializing working from https://dev.to/equalma/validate-fields-and-types-in-serde-with-tryfrom-c2n.

Based on the comment from serde-rs/serde#939 (comment)

In my own words it works like this: We create a duplicate struct and use serde's normal `derive(Deserialize)` on it. Then we specify that the `Stack` struct should be converted using the duplicate struct with `#[serde(try_from = "StackUnchecked")]. Finally we implement `try_from` and include our special logic that checks to see we don't have a `*` stack id AND any mixins.

I also hard wrapped a few comments and put backpacks around special characters in the error output to make them clearer

Close #104
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Add support for "any" stack * in StackId heroku/libcnb.rs
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@hone @schneems @edmorley