Use once_cell to keep the configuration

In order to keep the configuration globally available use
`once_cell`. Also check for a valid email address in the part which is
receiving the data from the api.

src/config.rs

@@ -1,4 +1,6 @@
-use anyhow::{bail, Error, Result};
+use anyhow::{anyhow, bail, Error, Result};
+use lettre::{message::Mailbox, Address};
+use once_cell::sync::OnceCell;
 use serde::Deserialize;
 use std::{
     env, fs,
@@ -7,6 +9,8 @@ use std::{
 };
 use tracing::{error, info, trace, warn};
 
+static CONFIG: OnceCell<Config> = OnceCell::new();
+
 fn default_database_host() -> String {
     "localhost".to_string()
 }
@@ -75,15 +79,42 @@ impl Default for Server {
     }
 }
 
+fn default_email_from() -> Mailbox {
+    Mailbox::new(
+        Some(env!("CARGO_BIN_NAME").to_string()),
+        Address::new(env!("CARGO_BIN_NAME"), "localhost").unwrap(),
+    )
+}
+
+#[derive(Debug, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct Email {
+    /// Address the notification emails are sent from
+    #[serde(default = "default_email_from")]
+    pub from: Mailbox,
+}
+
+impl Default for Email {
+    fn default() -> Self {
+        Self {
+            from: default_email_from(),
+        }
+    }
+}
+
 #[derive(Debug, Deserialize)]
 #[serde(deny_unknown_fields)]
 pub struct Config {
     /// Database configuration
     pub database: Database,
 
-    /// server configuration
+    /// Server configuration
     #[serde(default)]
     pub server: Server,
+
+    /// Email configuration
+    #[serde(default)]
+    pub email: Email,
 }
 
 impl Config {
@@ -130,4 +161,15 @@ impl Config {
             Ok(Self::try_paths()?)
         }
     }
+
+    pub fn init(file: Option<PathBuf>) -> Result<()> {
+        CONFIG
+            .set(Self::load_config(file)?)
+            .map_err(|_| anyhow!("configuration already initialized"))?;
+        Ok(())
+    }
+
+    pub fn get() -> &'static Self {
+        CONFIG.get().expect("config is not initialized")
+    }
 }