1 files changed, 93 insertions, 0 deletions
diff --git a/src/access/policy.rs b/src/access/policy.rs
new file mode 100644
index 0000000..7ddef0e
--- /dev/null
+++ b/src/access/policy.rs
@@ -0,0 +1,93 @@
+//! The swarm's write protection policy (`_continuum/access/policy`): whether every node
+//! judges received changes by rules by path before applying them (`access::screen`,
+//! `access::rules`). Absent, nobody does: what exists keeps working as it did. Written
+//! by a user who may write protocol data, as any of it.
+//!
+//! `{"version": 1, "signatures": "optional"}`: rules enforced; a change is its claimed
+//! committer's whether or not a signature proves it (`document::signatures`). A policy
+//! this node doesn't know (another version, or a setting it lacks) stops it from taking
+//! writes through the API, and it receives without judging: it can't judge as the
+//! others do, so it writes nothing they might refuse, until it is upgraded.
+
+use serde_json::Value as JsonValue;
+
+use crate::document::{ReadDoc, meta, store};
+
+/// The policy versions this node knows.
+pub(crate) const VERSION: u64 = 1;
+
+/// Where the policy is.
+pub(crate) fn path() -> Vec<String> {
+    meta::path(&[meta::ACCESS, "policy"])
+}
+
+/// What the policy says.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub(crate) enum Policy {
+    /// None: changes aren't judged.
+    Open,
+    /// Rules by path enforced.
+    Enforced,
+    /// One this node doesn't know, and why.
+    Unknown(String),
+}
+
+impl Policy {
+    pub(crate) fn enforced(&self) -> bool {
+        matches!(self, Policy::Enforced)
+    }
+}
+
+/// The policy `doc` holds.
+pub(crate) fn of(doc: &impl ReadDoc) -> Policy {
+    let value = match store::read_path(doc, &path()) {
+        Ok(Some(value)) => value,
+        Ok(None) => return Policy::Open,
+        Err(error) => return Policy::Unknown(format!("it doesn't read: {error}")),
+    };
+    let version = value.get("version").and_then(JsonValue::as_u64);
+    if version != Some(VERSION) {
+        return Policy::Unknown(format!(
+            "version {}, this node knows {VERSION}",
+            value
+                .get("version")
+                .map_or("none".to_owned(), JsonValue::to_string)
+        ));
+    }
+    match value.get("signatures").and_then(JsonValue::as_str) {
+        None | Some("optional") => Policy::Enforced,
+        Some(other) => Policy::Unknown(format!("signatures {other:?}")),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::document::Document;
+    use crate::document::testing::change;
+    use serde_json::json;
+
+    fn with(policy: JsonValue) -> Policy {
+        let mut doc = Document::new();
+        change(&mut doc, |doc| {
+            store::write_path(doc, &path(), &policy).unwrap();
+        });
+        of(&doc)
+    }
+
+    #[test]
+    fn what_the_policy_says() {
+        assert_eq!(of(&Document::new()), Policy::Open);
+        assert_eq!(with(json!({"version": 1})), Policy::Enforced);
+        assert_eq!(
+            with(json!({"version": 1, "signatures": "optional"})),
+            Policy::Enforced
+        );
+        assert!(matches!(with(json!({"version": 2})), Policy::Unknown(_)));
+        assert!(matches!(
+            with(json!({"version": 1, "signatures": "sometimes"})),
+            Policy::Unknown(_)
+        ));
+        assert!(matches!(with(json!("on")), Policy::Unknown(_)));
+    }
+}