fix(config): read an empty document as the defaults

Cargo.toml

@@ -98,7 +98,7 @@ serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 # The config file and the sections an embedder hands over as YAML. Safe Rust;
 # deserialization only.
-serde-saphyr = { version = "2.0", default-features = false, features = ["deserialize"] }
+serde-saphyr = { version = "2.1", default-features = false, features = ["deserialize"] }
 # The key path (`runtime.worker_threads`) a config error names; already in the
 # tree through axum.
 serde_path_to_error = "0.1"

src/config.rs

@@ -340,8 +340,45 @@ pub fn from_yaml<T: serde::de::DeserializeOwned>(yaml: &str) -> anyhow::Result<T
 /// boolean where text is expected is an error, not the text it spells, so
 /// `name: true` does not start a service called "true". Errors stay on one
 /// line, without a snippet of the document, so a log record holds the whole
-/// message.
+/// message. One well-formed document without any node (empty, only comments,
+/// a bare `---`) sets nothing: a type that refuses it is read from an empty
+/// mapping. An explicit `null` is a node and keeps its meaning; two
+/// documents or a broken directive stay errors.
 pub(crate) fn parse_yaml<T: serde::de::DeserializeOwned>(yaml: &str) -> Result<T, YamlError> {
+    match parse_yaml_document(yaml) {
+        Err(refused) if is_one_empty_document(yaml) => {
+            parse_yaml_document("{}").map_err(|_| refused)
+        }
+        read => read,
+    }
+}
+
+/// Whether `yaml` is a single document the reader accepts and that holds no
+/// node: the reader checks the syntax and the document count, the text that
+/// nothing in it is an explicit `null`.
+fn is_one_empty_document(yaml: &str) -> bool {
+    has_no_node(yaml)
+        && matches!(
+            parse_yaml_document::<Option<serde::de::IgnoredAny>>(yaml),
+            Ok(None)
+        )
+}
+
+/// Whether `yaml` holds no node: blank and comment lines, document markers
+/// (`---`, `...`) with at most a comment after them, and directives.
+fn has_no_node(yaml: &str) -> bool {
+    yaml.lines().all(|line| {
+        let line = line.trim();
+        let rest = line
+            .strip_prefix("---")
+            .or_else(|| line.strip_prefix("..."))
+            .unwrap_or(line)
+            .trim_start();
+        rest.is_empty() || rest.starts_with('#') || line.starts_with('%')
+    })
+}
+
+fn parse_yaml_document<T: serde::de::DeserializeOwned>(yaml: &str) -> Result<T, YamlError> {
     let mut path = None;
     serde_saphyr::with_deserializer_from_str_with_options(
         yaml,

src/config/tests.rs

@@ -917,6 +917,46 @@ fn an_empty_document_is_the_default_config() {
     }
 }
 
+#[test]
+fn an_explicit_null_document_keeps_its_meaning() {
+    // `null` is a node, unlike an empty document: an optional reads it as
+    // absent and an untyped value as null, never as an empty mapping.
+    for yaml in ["null", "~", "--- null\n"] {
+        assert_eq!(
+            parse_yaml::<Option<String>>(yaml).unwrap(),
+            None,
+            "{yaml:?}"
+        );
+        assert_eq!(
+            parse_yaml::<serde_json::Value>(yaml).unwrap(),
+            serde_json::Value::Null,
+            "{yaml:?}"
+        );
+    }
+    // A config is a mapping; an explicit null is not one.
+    assert!(ProxyConfig::from_yaml_str("null").is_err());
+}
+
+#[test]
+fn an_empty_document_keeps_an_optional_absent() {
+    // The empty-mapping reading applies only when the type refuses no node.
+    for yaml in ["", "# nothing\n", "---\n"] {
+        assert_eq!(
+            parse_yaml::<Option<String>>(yaml).unwrap(),
+            None,
+            "{yaml:?}"
+        );
+    }
+}
+
+#[test]
+fn a_malformed_empty_looking_document_is_refused() {
+    // Two documents or a broken directive are errors, not an empty config.
+    for yaml in ["---\n---\n", "%YAML\n---\n"] {
+        assert!(ProxyConfig::from_yaml_str(yaml).is_err(), "{yaml:?}");
+    }
+}
+
 #[test]
 fn a_repeated_key_is_refused() {
     // Which of two values would win is not something to guess for a config.