Preserve null for reference default value defined as reference

Until now, a reference with a default value which is a reference that
resolves to a YAML `null` value would result in a YAML string `"None"`
when the resolution falls back on the default value.

The cause for this is that we serialized `Value::Null` to `"None"` until
now to preserve compatibility with Python's `str()` for nested reference
lookups (since Python Reclass just uses `str()` to serialize nested
references).

This commit changes the serialization of `Value::Null` to `"null"` which
is preserved as a YAML `null` when a reference default value is defined
as a nested reference that resolves to YAML `null`.

Conversely, any existing nested references that expect YAML `null` to be
serialized as `None` will break with this change.

Author: simu

src/types/value.rs

@@ -484,8 +484,10 @@ impl Value {
     pub(crate) fn raw_string(&self) -> Result<String> {
         match self {
             Value::Literal(s) => Ok(s.clone()),
-            // We serialize Null as `None` to be compatible with Python's str()
-            Value::Null => Ok("None".to_string()),
+            // We serialize Null as `null`. NOTE(sg): This isn't compatible with Python's str(),
+            // but ensures that a reclass-rs reference default value which is a reference that
+            // resolves to a Null value is preserved correctly.
+            Value::Null => Ok("null".to_string()),
             // We need custom formatting for bool instead of `format!("{b}")`, so that this
             // function returns strings which match Python's `str()` implementation.
             Value::Bool(b) => match b {

src/types/value/value_tests.rs

@@ -301,7 +301,7 @@ fn test_raw_string_literal() {
 
 #[test]
 fn test_raw_string_null() {
-    assert_eq!(Value::Null.raw_string().unwrap(), "None".to_string());
+    assert_eq!(Value::Null.raw_string().unwrap(), "null".to_string());
 }
 
 #[test]

tests/inventory-reference-default-values/nodes/n12.yml

@@ -0,0 +1,13 @@
+parameters:
+  facts:
+    level: level
+  data:
+    "null": null
+    nonestr: None
+    bool: true
+    num: 1.25
+  nullref: ${data:${facts:level}::${data:null}}
+  nulldefault: ${data:${facts:level}::~}
+  boolref: ${data:${facts:level}::${data:bool}}
+  numref: ${data:${facts:level}::${data:num}}
+  nonestrref: ${data:${facts:level}::${data:nonestr}}

tests/test_inventory_reference_default_values.py

@@ -61,3 +61,10 @@ def test_inventory_reference_default_values():
 
     n11 = inv["nodes"]["n11"]["parameters"]
     assert n11["text"] == n11["expected"]
+
+    n12 = inv["nodes"]["n12"]["parameters"]
+    assert n12["nullref"] is None
+    assert n12["nulldefault"] is None
+    assert n12["boolref"]
+    assert n12["numref"] == 1.25
+    assert n12["nonestrref"] == "None"