feat: clean config loading and config api (#7924)

Check the README of the `config_loader` for details

codex-rs/core/src/config_loader/state.rs

@@ -0,0 +1,128 @@
+use super::fingerprint::record_origins;
+use super::fingerprint::version_for_toml;
+use super::merge::merge_toml_values;
+use codex_app_server_protocol::ConfigLayer;
+use codex_app_server_protocol::ConfigLayerMetadata;
+use codex_app_server_protocol::ConfigLayerName;
+use serde_json::Value as JsonValue;
+use std::collections::HashMap;
+use std::path::PathBuf;
+use toml::Value as TomlValue;
+
+#[derive(Debug, Default, Clone)]
+pub struct LoaderOverrides {
+    pub managed_config_path: Option<PathBuf>,
+    #[cfg(target_os = "macos")]
+    pub managed_preferences_base64: Option<String>,
+}
+
+#[derive(Debug, Clone)]
+pub struct ConfigLayerEntry {
+    pub name: ConfigLayerName,
+    pub source: PathBuf,
+    pub config: TomlValue,
+    pub version: String,
+}
+
+impl ConfigLayerEntry {
+    pub fn new(name: ConfigLayerName, source: PathBuf, config: TomlValue) -> Self {
+        let version = version_for_toml(&config);
+        Self {
+            name,
+            source,
+            config,
+            version,
+        }
+    }
+
+    pub fn metadata(&self) -> ConfigLayerMetadata {
+        ConfigLayerMetadata {
+            name: self.name.clone(),
+            source: self.source.display().to_string(),
+            version: self.version.clone(),
+        }
+    }
+
+    pub fn as_layer(&self) -> ConfigLayer {
+        ConfigLayer {
+            name: self.name.clone(),
+            source: self.source.display().to_string(),
+            version: self.version.clone(),
+            config: serde_json::to_value(&self.config).unwrap_or(JsonValue::Null),
+        }
+    }
+}
+
+#[derive(Debug, Clone)]
+pub struct ConfigLayerStack {
+    pub user: ConfigLayerEntry,
+    pub session_flags: ConfigLayerEntry,
+    pub system: Option<ConfigLayerEntry>,
+    pub mdm: Option<ConfigLayerEntry>,
+}
+
+impl ConfigLayerStack {
+    pub fn with_user_config(&self, user_config: TomlValue) -> Self {
+        Self {
+            user: ConfigLayerEntry::new(
+                self.user.name.clone(),
+                self.user.source.clone(),
+                user_config,
+            ),
+            session_flags: self.session_flags.clone(),
+            system: self.system.clone(),
+            mdm: self.mdm.clone(),
+        }
+    }
+
+    pub fn effective_config(&self) -> TomlValue {
+        let mut merged = self.user.config.clone();
+        merge_toml_values(&mut merged, &self.session_flags.config);
+        if let Some(system) = &self.system {
+            merge_toml_values(&mut merged, &system.config);
+        }
+        if let Some(mdm) = &self.mdm {
+            merge_toml_values(&mut merged, &mdm.config);
+        }
+        merged
+    }
+
+    pub fn origins(&self) -> HashMap<String, ConfigLayerMetadata> {
+        let mut origins = HashMap::new();
+        let mut path = Vec::new();
+
+        record_origins(
+            &self.user.config,
+            &self.user.metadata(),
+            &mut path,
+            &mut origins,
+        );
+        record_origins(
+            &self.session_flags.config,
+            &self.session_flags.metadata(),
+            &mut path,
+            &mut origins,
+        );
+        if let Some(system) = &self.system {
+            record_origins(&system.config, &system.metadata(), &mut path, &mut origins);
+        }
+        if let Some(mdm) = &self.mdm {
+            record_origins(&mdm.config, &mdm.metadata(), &mut path, &mut origins);
+        }
+
+        origins
+    }
+
+    pub fn layers_high_to_low(&self) -> Vec<ConfigLayer> {
+        let mut layers = Vec::new();
+        if let Some(mdm) = &self.mdm {
+            layers.push(mdm.as_layer());
+        }
+        if let Some(system) = &self.system {
+            layers.push(system.as_layer());
+        }
+        layers.push(self.session_flags.as_layer());
+        layers.push(self.user.as_layer());
+        layers
+    }
+}