feat: allow to inject configuration from env var

You can use an env var to pass the configuration instead of using the
configuration file in the root folder `config.toml`

```
TORRUST_TRACKER_CONFIG=$(cat config.toml)
TORRUST_TRACKER_CONFIG=`cat config.toml` cargo run
```

This allow the applciation to be executed in dockerized environments
whithout needing to mount a file or volume for the configuration.

Author: josecelano

src/config.rs

@@ -1,10 +1,10 @@
 use std::collections::HashMap;
-use std::fs;
 use std::net::IpAddr;
 use std::path::Path;
 use std::str::FromStr;
+use std::{env, fs};
 
-use config::{Config, ConfigError, File};
+use config::{Config, ConfigError, File, FileFormat};
 use serde::{Deserialize, Serialize};
 use serde_with::{serde_as, NoneAsEmptyString};
 use {std, toml};
@@ -164,7 +164,7 @@ impl Configuration {
             let config = Configuration::default();
             config.save_to_file(path)?;
             return Err(Error::Message(
-                "Please edit the config.TOML in the root folder and restart the tracker.".to_string(),
+                "Please edit the config.TOML and restart the tracker.".to_string(),
             ));
         }
 
@@ -173,6 +173,26 @@ impl Configuration {
         Ok(torrust_config)
     }
 
+    /// # Errors
+    ///
+    /// Will return `Err` if the environment variable does not exist or has a bad configuration.
+    pub fn load_from_env_var(config_env_var_name: &str) -> Result<Configuration, Error> {
+        match env::var(config_env_var_name) {
+            Ok(config_toml) => {
+                let config_builder = Config::builder()
+                    .add_source(File::from_str(&config_toml, FileFormat::Toml))
+                    .build()
+                    .map_err(Error::ConfigError)?;
+                let config = config_builder.try_deserialize().map_err(Error::ConfigError)?;
+                Ok(config)
+            }
+            Err(_) => Err(Error::Message(format!(
+                "No environment variable for configuration found: {}",
+                &config_env_var_name
+            ))),
+        }
+    }
+
     /// # Errors
     ///
     /// Will return `Err` if `filename` does not exist or the user does not have

src/main.rs

@@ -1,3 +1,4 @@
+use std::env;
 use std::sync::Arc;
 
 use log::info;
@@ -7,7 +8,8 @@ use torrust_tracker::{ephemeral_instance_keys, logging, setup, static_time, trac
 
 #[tokio::main]
 async fn main() {
-    const CONFIG_PATH: &str = "config.toml";
+    const CONFIG_PATH: &str = "./config.toml";
+    const CONFIG_ENV_VAR_NAME: &str = "TORRUST_TRACKER_CONFIG";
 
     // Set the time of Torrust app starting
     lazy_static::initialize(&static_time::TIME_AT_APP_START);
@@ -16,11 +18,12 @@ async fn main() {
     lazy_static::initialize(&ephemeral_instance_keys::RANDOM_SEED);
 
     // Initialize Torrust config
-    let config = match Configuration::load_from_file(CONFIG_PATH) {
-        Ok(config) => Arc::new(config),
-        Err(error) => {
-            panic!("{}", error)
-        }
+    let config = if env::var(CONFIG_ENV_VAR_NAME).is_ok() {
+        println!("Loading configuration from env var {CONFIG_ENV_VAR_NAME}");
+        Arc::new(Configuration::load_from_env_var(CONFIG_ENV_VAR_NAME).unwrap())
+    } else {
+        println!("Loading configuration from config file {CONFIG_PATH}");
+        Arc::new(Configuration::load_from_file(CONFIG_PATH).unwrap())
     };
 
     // Initialize statistics