feat: [torrust#978] add semantic validation for configration

The section [core.provate_mode] can be inlcuded in the configration TOML
file only if the tracker is running in private mode (`private = true`).

This commits adds that validation and makes it possible to add more
semantic validations in the future.

Semantic validations are validations that depend on more than one value.
Like the one added, it could be any other incompatible combination.

packages/configuration/src/lib.rs

@@ -5,6 +5,7 @@
 //!
 //! The current version for configuration is [`v2`].
 pub mod v2;
+pub mod validator;
 
 use std::collections::HashMap;
 use std::env;

packages/configuration/src/v2/core.rs

@@ -3,6 +3,7 @@ use serde::{Deserialize, Serialize};
 
 use super::network::Network;
 use crate::v2::database::Database;
+use crate::validator::{SemanticValidationError, Validator};
 use crate::{AnnouncePolicy, TrackerPolicy};
 
 #[allow(clippy::struct_excessive_bools)]
@@ -131,3 +132,13 @@ impl PrivateMode {
         true
     }
 }
+
+impl Validator for Core {
+    fn validate(&self) -> Result<(), SemanticValidationError> {
+        if self.private_mode.is_some() && !self.private {
+            return Err(SemanticValidationError::UselessPrivateModeSection);
+        }
+
+        Ok(())
+    }
+}

packages/configuration/src/v2/mod.rs

@@ -251,6 +251,7 @@ use self::health_check_api::HealthCheckApi;
 use self::http_tracker::HttpTracker;
 use self::tracker_api::HttpApi;
 use self::udp_tracker::UdpTracker;
+use crate::validator::{SemanticValidationError, Validator};
 use crate::{Error, Info, Metadata, Version};
 
 /// This configuration version
@@ -394,6 +395,12 @@ impl Configuration {
     }
 }
 
+impl Validator for Configuration {
+    fn validate(&self) -> Result<(), SemanticValidationError> {
+        self.core.validate()
+    }
+}
+
 #[cfg(test)]
 mod tests {
 

packages/configuration/src/validator.rs

@@ -0,0 +1,19 @@
+//! Trait to validate semantic errors.
+//!
+//! Errors could involve more than one configuration option. Some configuration
+//! combinations can be incompatible.
+use thiserror::Error;
+
+/// Errors that can occur validating the configuration.
+#[derive(Error, Debug)]
+pub enum SemanticValidationError {
+    #[error("Private mode section in configuration can only be included when the tracker is running in private mode.")]
+    UselessPrivateModeSection,
+}
+
+pub trait Validator {
+    /// # Errors
+    ///
+    /// Will return an error if the configuration is invalid.
+    fn validate(&self) -> Result<(), SemanticValidationError>;
+}

src/bootstrap/app.rs

@@ -14,6 +14,7 @@
 use std::sync::Arc;
 
 use torrust_tracker_clock::static_time;
+use torrust_tracker_configuration::validator::Validator;
 use torrust_tracker_configuration::Configuration;
 use tracing::info;
 
@@ -24,10 +25,18 @@ use crate::core::Tracker;
 use crate::shared::crypto::ephemeral_instance_keys;
 
 /// It loads the configuration from the environment and builds the main domain [`Tracker`] struct.
+///
+/// # Panics
+///
+/// Setup can file if the configuration is invalid.
 #[must_use]
 pub fn setup() -> (Configuration, Arc<Tracker>) {
     let configuration = initialize_configuration();
 
+    if let Err(e) = configuration.validate() {
+        panic!("Configuration error: {e}");
+    }
+
     let tracker = initialize_with_configuration(&configuration);
 
     info!("Configuration:\n{}", configuration.clone().mask_secrets().to_json());