# M1 task 05: `proto` grant files

**Branch:** `m1` (run `git switch m1`; `git status --short` must be empty, otherwise stop)
**Commit subject:** `Add Grant, Mode and Constraints to proto`

## Goal

Add the data type for a grant file. A grant is a TOML file the owner writes to allow one kind of
tool call. This task only parses grants. Deciding whether a grant matches a request is `brokerd`'s
job in a later milestone; write no matching logic.

## Context

From the design brief: "A grant is a file the owner writes: tool, argument constraints (paths,
hosts, patterns), allowed data classes, expiry, mode (`auto`, `ask`, `deny`)."

From the spec: `max_taint` is "the highest session taint under which the grant applies".
`result_class` is the data class of the tool's results and defaults to `private`. `untrusted`
says whether results may have been written by someone other than the owner and defaults to `true`.
A typing mistake in a grant file must be an error, never silently ignored: a misspelt
`max_tiant` that fell back to a default would widen what the grant allows.

A full grant file:

```toml
tool = "http_fetch"
mode = "ask"
max_taint = "secret"
result_class = "public"
untrusted = false
expires = "2026-12-31T00:00:00.000Z"
secret = "example-api-token"

[constraints]
paths = ["/home/kyle/notes/**"]
hosts = ["example.com", "api.example.com"]
patterns = ["^GET "]
```

## Files

- Copy: `crates/proto/tests/grant.rs`, `crates/proto/tests/fixtures/grant/` (6 files)
- Create: `crates/proto/src/grant.rs`
- Modify: `Cargo.toml`, `crates/proto/Cargo.toml`, `crates/proto/src/lib.rs`,
  `docs/dependencies.md`, `docs/implementer-log.md`, `Cargo.lock` (generated)

## Interfaces

Consumes from task 02: `proto::DataClass`, `proto::Timestamp`.

Produces, in `crates/proto/src/grant.rs`, re-exported from the crate root:

```rust
// TOML and JSON: "auto", "ask", "deny"
pub enum Mode { Auto, Ask, Deny }

pub struct Grant {
    pub tool: String,                 // required
    pub mode: Mode,                   // required
    pub max_taint: DataClass,         // required
    pub result_class: DataClass,      // default: DataClass::Private
    pub untrusted: bool,              // default: true
    pub expires: Option<Timestamp>,   // default: None, meaning no expiry
    pub secret: Option<String>,       // default: None
    pub constraints: Constraints,     // default: all three lists empty
}

pub struct Constraints {              // also derives Default
    pub paths: Vec<String>,           // default: empty
    pub hosts: Vec<String>,           // default: empty
    pub patterns: Vec<String>,        // default: empty
}
```

Derives: all three get `Debug, Clone, PartialEq, Eq, Serialize, Deserialize`. `Mode` also gets
`Copy`. `Constraints` also gets `Default`.

Rules the tests check: `tool`, `mode` and `max_taint` are required. Unknown keys are errors in the
grant and in `[constraints]`. An unknown mode is an error. `expires` is written as a quoted string
in the `Timestamp` spelling from task 02, not as a bare TOML date.

## API notes (verified on docs.rs, 2026-09-17)

- `toml::from_str::<T>(&str) -> Result<T, toml::de::Error>`. Only the test uses the `toml` crate,
  so it is a dev-dependency. `grant.rs` itself uses only serde.
- `#[serde(default)]` on a field uses `Default::default()` when the key is missing.
  `#[serde(default = "function_name")]` calls a function in the same module instead; use it for
  `result_class` and `untrusted`, whose defaults are not the types' `Default`.
- `#[serde(deny_unknown_fields)]` and `#[serde(rename_all = "lowercase")]` as in earlier tasks.

## Steps

- [ ] **1. Copy the test and fixtures, add the dev-dependency.**

```sh
git switch m1
cp docs/plans/M1/files/crates/proto/tests/grant.rs crates/proto/tests/
cp -r docs/plans/M1/files/crates/proto/tests/fixtures/grant crates/proto/tests/fixtures/
```

Add to `[workspace.dependencies]` in the root `Cargo.toml`:

```toml
toml = "1.1.6"
```

Add a new section at the end of `crates/proto/Cargo.toml`:

```toml
[dev-dependencies]
toml.workspace = true
```

Add this row to `docs/dependencies.md`:

```markdown
| `toml` | 1.1.6 | `proto` tests; `brokerd` and `bxctl` from M3 | Parses grant files. MIT OR Apache-2.0. |
```

- [ ] **2. See the test fail.** `cargo test -p proto --test grant`. Expected: it does not compile.

- [ ] **3. Write `grant.rs`,** and add to `lib.rs` (alphabetical order):

```rust
pub mod grant;

pub use grant::{Constraints, Grant, Mode};
```

- [ ] **4. See the test pass.** `cargo test -p proto --test grant`. Expected: `4 passed; 0 failed`.

- [ ] **5. Run the gate.** `make gate`. Expected last line: `gate: ok`. If `cargo deny` reports a
  licence that is not allowed or a duplicate crate version, stop and report; do not edit
  `deny.toml`.

- [ ] **6. Log and commit.**

```sh
git add Cargo.toml Cargo.lock crates/proto docs/dependencies.md docs/implementer-log.md
git commit
```

## Done when

- `cargo test -p proto --test grant` reports 4 passed.
- `make gate` prints `gate: ok`.
- `diff -r crates/proto/tests/fixtures/grant docs/plans/M1/files/crates/proto/tests/fixtures/grant`
  prints nothing.

## Stop and report if

- `cargo deny` rejects `toml` or one of its dependencies.