Consolidate audit file_path from Option<String> to String

- Change AuditConfig.file_path to non-optional String with a default path
- Remove redundant unwrap_or_default call in state validation
- Update test to pass empty string explicitly instead of relying on None
- Gracefully fall back to memory backend when file_path is empty in IPC audit

Author: developerworks

src/config/audit.rs

@@ -24,8 +24,8 @@ pub struct AuditConfig {
 
     /// File path for file backend. Required when backend is "file".
     #[schemars(!default)]
-    #[serde(default)]
-    pub file_path: Option<String>,
+    #[serde(default = "default_file_path")]
+    pub file_path: String,
 
     /// Failure strategy when audit backend is unavailable.
     /// - "fail_closed": reject write commands when audit cannot be written.
@@ -47,7 +47,7 @@ impl Default for AuditConfig {
         Self {
             enabled: true,
             backend: "memory".into(),
-            file_path: None,
+            file_path: "/tmp/rust-supervisor-demo/audit.jsonl".to_string(),
             failure_strategy: "fail_closed".into(),
             max_defer_queue: 1000,
         }
@@ -64,6 +64,10 @@ fn default_audit_backend() -> String {
     "memory".into()
 }
 
+fn default_file_path() -> String {
+    "/tmp/rust-supervisor-demo/audit.jsonl".to_string()
+}
+
 /// Serde default helper: returns "fail_closed".
 fn default_fail_closed() -> String {
     "fail_closed".into()

src/config/state.rs

@@ -858,7 +858,7 @@ fn validate_audit(audit: &AuditConfig) -> Result<(), crate::error::types::Superv
     match audit.backend.as_str() {
         "memory" => {}
         "file" => {
-            let path = audit.file_path.as_deref().unwrap_or_default().trim();
+            let path = audit.file_path.trim();
             if path.is_empty() {
                 return Err(crate::error::types::SupervisorError::fatal_config(
                     "audit.file_path is required when audit.backend is file",

src/config/tests/yaml_config_test.rs

@@ -217,10 +217,15 @@ fn yaml_config_rejects_invalid_supervision_strategy() {
     assert!(result.is_err());
 }
 
-/// Verifies that JSON Lines audit storage requires an explicit file path.
+/// Verifies that JSON Lines audit storage rejects an empty file path.
 #[test]
 fn yaml_config_rejects_file_audit_without_path() {
-    let yaml = valid_yaml().replace("backend: memory", "backend: file");
+    let yaml = valid_yaml()
+        .replace("backend: memory", "backend: file")
+        .replace(
+            "failure_strategy: fail_closed",
+            "file_path: \"\"\n  failure_strategy: fail_closed",
+        );
     let result = parse_config_state(&yaml);
 
     assert!(result.is_err());

src/ipc/security/audit.rs

@@ -140,21 +140,25 @@ impl AuditBackend {
     /// when no file path is provided.
     pub fn from_config(config: &AuditConfig) -> Self {
         match config.backend.as_str() {
-            "file" => match &config.file_path {
-                Some(p) => match AuditBackend::new_file(p.as_str().to_owned()) {
-                    Ok(backend) => backend,
-                    Err(error) => {
-                        tracing::error!(
-                            target: "rust_supervisor::ipc::security::audit",
-                            path = %p.as_str(),
-                            ?error,
-                            "failed to open file audit backend, falling back to memory"
-                        );
-                        AuditBackend::new_memory(4096)
+            "file" => {
+                let path = config.file_path.trim();
+                if path.is_empty() {
+                    AuditBackend::new_memory(4096)
+                } else {
+                    match AuditBackend::new_file(path.to_owned()) {
+                        Ok(backend) => backend,
+                        Err(error) => {
+                            tracing::error!(
+                                target: "rust_supervisor::ipc::security::audit",
+                                path = %path,
+                                ?error,
+                                "failed to open file audit backend, falling back to memory"
+                            );
+                            AuditBackend::new_memory(4096)
+                        }
                     }
-                },
-                None => AuditBackend::new_memory(4096),
-            },
+                }
+            }
             _ => AuditBackend::new_memory(4096),
         }
     }