refactor: get_config_bool_opt(): Return None if only default value exists

And also:
- Make it `pub(crate)`.
- Use it in `should_request_mdns()` as using `config_exists()` there isn't correct because the
  latter doesn't look at environment.

src/config.rs

@@ -446,8 +446,8 @@ impl Context {
         Ok(self.sql.get_raw_config(key.as_ref()).await?.is_some())
     }
 
-    /// Get a configuration key. Returns `None` if no value is set, and no default value found.
-    pub async fn get_config(&self, key: Config) -> Result<Option<String>> {
+    /// Get a config key value. Returns `None` if no value is set.
+    pub(crate) async fn get_config_opt(&self, key: Config) -> Result<Option<String>> {
         let env_key = format!("DELTACHAT_{}", key.as_ref().to_uppercase());
         if let Ok(value) = env::var(env_key) {
             return Ok(Some(value));
@@ -467,7 +467,12 @@ impl Context {
             Config::SysConfigKeys => Some(get_config_keys_string()),
             _ => self.sql.get_raw_config(key.as_ref()).await?,
         };
+        Ok(value)
+    }
 
+    /// Get a config key value if set, or a default value. Returns `None` if no value exists.
+    pub async fn get_config(&self, key: Config) -> Result<Option<String>> {
+        let value = self.get_config_opt(key).await?;
         if value.is_some() {
             return Ok(value);
         }
@@ -484,7 +489,16 @@ impl Context {
         Ok(val.map(|s| s.to_string()))
     }
 
-    /// Returns Some(T) if a value for the given key exists and was successfully parsed.
+    /// Returns Some(T) if a value for the given key is set and was successfully parsed.
+    /// Returns None if could not parse.
+    pub(crate) async fn get_config_opt_parsed<T: FromStr>(&self, key: Config) -> Result<Option<T>> {
+        self.get_config_opt(key)
+            .await
+            .map(|s: Option<String>| s.and_then(|s| s.parse().ok()))
+    }
+
+    /// Returns Some(T) if a value for the given key exists (incl. default value) and was
+    /// successfully parsed.
     /// Returns None if could not parse.
     pub async fn get_config_parsed<T: FromStr>(&self, key: Config) -> Result<Option<T>> {
         self.get_config(key)
@@ -512,14 +526,21 @@ impl Context {
         Ok(self.get_config_parsed(key).await?.unwrap_or_default())
     }
 
-    /// Returns boolean configuration value (if any) for the given key.
-    pub async fn get_config_bool_opt(&self, key: Config) -> Result<Option<bool>> {
-        Ok(self.get_config_parsed::<i32>(key).await?.map(|x| x != 0))
+    /// Returns boolean configuration value (if set) for the given key.
+    pub(crate) async fn get_config_bool_opt(&self, key: Config) -> Result<Option<bool>> {
+        Ok(self
+            .get_config_opt_parsed::<i32>(key)
+            .await?
+            .map(|x| x != 0))
     }
 
     /// Returns boolean configuration value for the given key.
     pub async fn get_config_bool(&self, key: Config) -> Result<bool> {
-        Ok(self.get_config_bool_opt(key).await?.unwrap_or_default())
+        Ok(self
+            .get_config_parsed::<i32>(key)
+            .await?
+            .map(|x| x != 0)
+            .unwrap_or_default())
     }
 
     /// Returns true if movebox ("DeltaChat" folder) should be watched.
@@ -547,9 +568,9 @@ impl Context {
 
     /// Returns whether MDNs should be requested.
     pub(crate) async fn should_request_mdns(&self) -> Result<bool> {
-        match self.config_exists(Config::MdnsEnabled).await? {
-            true => self.get_config_bool(Config::MdnsEnabled).await,
-            false => Ok(!self.get_config_bool(Config::Bot).await?),
+        match self.get_config_bool_opt(Config::MdnsEnabled).await? {
+            Some(val) => Ok(val),
+            None => Ok(!self.get_config_bool(Config::Bot).await?),
         }
     }
 
@@ -1017,12 +1038,14 @@ mod tests {
         let t = &TestContext::new_alice().await;
         assert!(t.should_request_mdns().await?);
         assert!(t.should_send_mdns().await?);
+        assert!(t.get_config_bool_opt(Config::MdnsEnabled).await?.is_none());
         // The setting should be displayed correctly.
         assert!(t.get_config_bool(Config::MdnsEnabled).await?);
 
         t.set_config_bool(Config::Bot, true).await?;
         assert!(!t.should_request_mdns().await?);
         assert!(t.should_send_mdns().await?);
+        assert!(t.get_config_bool_opt(Config::MdnsEnabled).await?.is_none());
         assert!(t.get_config_bool(Config::MdnsEnabled).await?);
         Ok(())
     }