Polish cfgsync runtime ergonomics

cfgsync/README.md

@@ -143,6 +143,24 @@ What does not belong in cfgsync core is equally important. Generic cfgsync shoul
 
 Those belong in the adapter or in the consuming application.
 
+## Start here
+
+If you want the shortest path into the library, start with the end-to-end
+runtime example:
+
+- `cfgsync/runtime/examples/minimal_cfgsync.rs`
+
+It shows the full loop:
+
+- define a snapshot materializer
+- serve cfgsync
+- register a node
+- fetch artifacts
+- write them locally
+
+After that, the only concepts you usually need to learn are the ones in the
+next section.
+
 ## Minimal integration path
 
 For a new application, the shortest sensible path is:
@@ -155,18 +173,6 @@ For a new application, the shortest sensible path is:
 
 That gives you the main value of the library without forcing extra application logic into cfgsync itself.
 
-## Minimal example
-
-A minimal standalone example lives in:
-
-- `cfgsync/runtime/examples/minimal_cfgsync.rs`
-
-It shows the smallest complete path:
-
-- registration snapshot materializer
-- cfgsync server
-- node artifact generation
-
 ## Code sketch
 
 Typed registration payload:
@@ -241,9 +247,10 @@ Fetching and writing artifacts:
 use cfgsync_runtime::{Client, OutputMap};
 
 # async fn run(registration: cfgsync_core::NodeRegistration) -> anyhow::Result<()> {
-let outputs = OutputMap::new()
-    .route("/config.yaml", "/node-data/node-1/config.yaml")
-    .route("deployment-settings.yaml", "/node-data/shared/deployment-settings.yaml");
+let outputs = OutputMap::config_and_shared(
+    "/node-data/node-1/config.yaml",
+    "/node-data/shared",
+);
 
 Client::new("http://127.0.0.1:4400")
     .fetch_and_write(&registration, &outputs)

cfgsync/runtime/src/client.rs

@@ -21,6 +21,13 @@ const FETCH_RETRY_DELAY: Duration = Duration::from_millis(250);
 #[derive(Debug, Clone, Default)]
 pub struct OutputMap {
     routes: HashMap<String, PathBuf>,
+    fallback: Option<FallbackRoute>,
+}
+
+#[derive(Debug, Clone)]
+enum FallbackRoute {
+    Under(PathBuf),
+    Shared { dir: PathBuf },
 }
 
 impl OutputMap {
@@ -41,12 +48,62 @@ impl OutputMap {
         self
     }
 
+    /// Writes payload files under `root`, preserving each artifact path.
+    ///
+    /// For example, `/config.yaml` is written to `<root>/config.yaml` and
+    /// `shared/deployment-settings.yaml` is written to
+    /// `<root>/shared/deployment-settings.yaml`.
+    #[must_use]
+    pub fn under(root: impl Into<PathBuf>) -> Self {
+        Self {
+            routes: HashMap::new(),
+            fallback: Some(FallbackRoute::Under(root.into())),
+        }
+    }
+
+    /// Writes the node config to `config_path` and all other files under
+    /// `shared_dir`, preserving their relative artifact paths.
+    #[must_use]
+    pub fn config_and_shared(
+        config_path: impl Into<PathBuf>,
+        shared_dir: impl Into<PathBuf>,
+    ) -> Self {
+        let config_path = config_path.into();
+        let shared_dir = shared_dir.into();
+
+        Self::default()
+            .route("/config.yaml", config_path.clone())
+            .route("config.yaml", config_path)
+            .with_fallback(FallbackRoute::Shared { dir: shared_dir })
+    }
+
     fn resolve_path(&self, file: &NodeArtifactFile) -> PathBuf {
         self.routes
             .get(&file.path)
             .cloned()
+            .or_else(|| {
+                self.fallback
+                    .as_ref()
+                    .map(|fallback| fallback.resolve(&file.path))
+            })
             .unwrap_or_else(|| PathBuf::from(&file.path))
     }
+
+    fn with_fallback(mut self, fallback: FallbackRoute) -> Self {
+        self.fallback = Some(fallback);
+        self
+    }
+}
+
+impl FallbackRoute {
+    fn resolve(&self, artifact_path: &str) -> PathBuf {
+        let relative = artifact_path.trim_start_matches('/');
+
+        match self {
+            FallbackRoute::Under(root) => root.join(relative),
+            FallbackRoute::Shared { dir } => dir.join(relative),
+        }
+    }
 }
 
 /// Runtime-oriented cfgsync client that handles registration, fetch, and local