templates: one Jinja language (hive-template) for the notify router's flows, flows document version 2

A todo node's key and text were hand-expanded `{field}` strings. They are now Jinja templates rendered through the new hive-template crate (minijinja, an unknown variable is an error), and the controller's save check reads the variables the template uses. The flows document is version 2; a version 1 document reads as version 2 with each `{field}` converted to `{{ field }}`, and the next save writes it.

hive-template/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "hive-template"
+edition.workspace = true
+version.workspace = true
+readme = "README.md"
+
+[lints]
+workspace = true
+
+[dependencies]
+minijinja = { version = "2", default-features = false, features = [
+  "builtins",
+  "serde",
+] }
+serde.workspace = true
+
+[dev-dependencies]
+serde_json.workspace = true

hive-template/README.md

@@ -0,0 +1,14 @@
+# hive-template
+
+The one template language the hyperhive workspace renders with: Jinja, through
+[`minijinja`](https://crates.io/crates/minijinja). A value is `{{ name }}`, a
+block is `{% if cond %}...{% endif %}`, and a lone `{` or `}` stays plain text.
+
+- **`render(template, &context)`**: the text, or a message when the template
+  does not parse or names a variable the context lacks. An unknown variable is
+  an error, never an empty string.
+- **`variables(template)`**: the variables the template reads, so a caller can
+  refuse an unknown field when the template is saved instead of when it renders.
+
+Used by the notification router's flow templates (`swarm-notify-router`,
+`swarm-wire`).

hive-template/src/lib.rs

@@ -0,0 +1,89 @@
+//! The one template language the workspace renders with: Jinja, through
+//! `minijinja`, with an unknown variable an error rather than an empty string.
+//!
+//! A template is `{{ name }}` for a value and `{% if cond %}...{% endif %}` for
+//! a block; a lone `{` or `}` is plain text. Callers pass their context as any
+//! `Serialize` value and get back the text or a message naming what is wrong.
+
+use std::collections::BTreeSet;
+
+use minijinja::{Environment, UndefinedBehavior};
+use serde::Serialize;
+
+/// An environment that refuses a name the context does not hold, and keeps the
+/// template's trailing newline.
+fn environment() -> Environment<'static> {
+    let mut env = Environment::new();
+    env.set_undefined_behavior(UndefinedBehavior::Strict);
+    env.set_keep_trailing_newline(true);
+    env
+}
+
+/// `template` rendered against `context`.
+///
+/// # Errors
+///
+/// The template does not parse, or it names a variable `context` does not hold.
+pub fn render(template: &str, context: &impl Serialize) -> Result<String, String> {
+    environment()
+        .render_str(template, context)
+        .map_err(|e| e.to_string())
+}
+
+/// The variables `template` reads that it does not define itself, such as `repo`
+/// for `{{ repo }}`. Lets a caller refuse a template that names an unknown field
+/// before any event renders it.
+///
+/// # Errors
+///
+/// The template does not parse.
+pub fn variables(template: &str) -> Result<BTreeSet<String>, String> {
+    let env = environment();
+    let parsed = env.template_from_str(template).map_err(|e| e.to_string())?;
+    Ok(parsed.undeclared_variables(false).into_iter().collect())
+}
+
+#[cfg(test)]
+mod tests {
+    use serde_json::json;
+
+    use super::*;
+
+    #[test]
+    fn a_value_and_a_block_render() {
+        let ctx = json!({ "repo": "o/r", "role": "agent" });
+        let out = render(
+            "{{ repo }}{% if role == \"agent\" %} (agent){% endif %}\n",
+            &ctx,
+        );
+        assert_eq!(out.as_deref(), Ok("o/r (agent)\n"));
+    }
+
+    #[test]
+    fn single_braces_are_plain_text() {
+        assert_eq!(
+            render("{not a name} and { open", &json!({})).as_deref(),
+            Ok("{not a name} and { open")
+        );
+    }
+
+    #[test]
+    fn an_unknown_variable_is_an_error() {
+        assert!(render("{{ numbr }}", &json!({ "number": "7" })).is_err());
+    }
+
+    #[test]
+    fn variables_lists_what_the_template_reads_not_what_it_defines() {
+        let found = variables("{{ repo }}#{{ number }}{% set x = 1 %}{{ x }}").expect("parses");
+        assert_eq!(
+            found.into_iter().collect::<Vec<_>>(),
+            ["number", "repo"].map(String::from)
+        );
+    }
+
+    #[test]
+    fn a_template_that_does_not_parse_is_refused() {
+        assert!(variables("{{ repo ").is_err());
+        assert!(render("{% if %}", &json!({})).is_err());
+    }
+}