[fractal/fractal-next] auth_dialog: Always prefer supported stage

[Julian Sparber <jsparber src gnome org>]

commit af8e512c45e5e8421164d8807a1698ce1c46bfb4
Author: Kai A. Hiller <V02460 gmail com>
Date:   Sat Feb 12 10:04:09 2022 +0100

    auth_dialog: Always prefer supported stage
    
    Changes the logic of choosing the next auth stage. Instead of using the
    first stage matching the current prefix (which is always the first stage
    in the list for the empty starting prefix), it now checks and prefers
    those stages that are actually supported by Fractal.

 src/components/auth_dialog.rs | 69 ++++++++++++++++++++++++++++++-------------
 1 file changed, 48 insertions(+), 21 deletions(-)
---
diff --git a/src/components/auth_dialog.rs b/src/components/auth_dialog.rs
index 12666569..763ac2ad 100644
--- a/src/components/auth_dialog.rs
+++ b/src/components/auth_dialog.rs
@@ -2,7 +2,12 @@ use std::{cell::Cell, fmt::Debug, future::Future};
 
 use adw::subclass::prelude::*;
 use gtk::{
-    gdk, gio::prelude::*, glib, glib::clone, prelude::*, subclass::prelude::*, CompositeTemplate,
+    gdk,
+    gio::prelude::*,
+    glib::{self, clone},
+    prelude::*,
+    subclass::prelude::*,
+    CompositeTemplate,
 };
 use matrix_sdk::{
     ruma::{
@@ -252,30 +257,52 @@ impl AuthDialog {
 
             self.show_auth_error(&uiaa_info.auth_error);
 
-            // Find the first flow that matches the completed flow
-            let flow = uiaa_info
+            let stage_nr = uiaa_info.completed.len();
+            let possible_stages: Vec<&AuthType> = uiaa_info
                 .flows
                 .iter()
-                .find(|flow| flow.stages.starts_with(&uiaa_info.completed))
-                .ok_or(AuthError::NoStageToChoose)?;
-
-            match flow.stages[uiaa_info.completed.len()] {
-                AuthType::Password => {
-                    auth_data = Some(self.perform_password_stage(uiaa_info.session).await?);
-                    continue;
-                }
-                // TODO implement other authentication types
-                // See: https://gitlab.gnome.org/GNOME/fractal/-/issues/835
-                _ => {
-                    let session = uiaa_info.session.ok_or(AuthError::MalformedResponse)?;
-                    let first_stage = flow.stages.first();
-                    let stage = first_stage.ok_or(AuthError::NoStageToChoose)?;
-                    auth_data = Some(self.perform_fallback(session, stage).await?);
-
-                    continue;
-                }
+                .filter(|flow| flow.stages.starts_with(&uiaa_info.completed))
+                .flat_map(|flow| flow.stages.get(stage_nr))
+                .collect();
+
+            let session = uiaa_info.session;
+            auth_data = Some(self.perform_next_stage(&session, &possible_stages).await?);
+        }
+    }
+
+    /// Performs the most preferred one of the given stages.
+    ///
+    /// Stages that Fractal actually implements are preferred.
+    async fn perform_next_stage(
+        &self,
+        session: &Option<String>,
+        stages: &[&AuthType],
+    ) -> Result<AuthData, AuthError> {
+        // Default to first stage if non is supported.
+        let a_stage = stages.first().ok_or(AuthError::NoStageToChoose)?;
+        for stage in stages {
+            if let Some(auth_result) = self.try_perform_stage(session, stage).await {
+                return auth_result;
             }
         }
+        let session = session.clone().ok_or(AuthError::MalformedResponse)?;
+        self.perform_fallback(session, a_stage).await
+    }
+
+    /// Tries to perform the given stage.
+    ///
+    /// Returns None if the stage is not implemented by Fractal.
+    async fn try_perform_stage(
+        &self,
+        session: &Option<String>,
+        stage: &AuthType,
+    ) -> Option<Result<AuthData, AuthError>> {
+        match stage {
+            AuthType::Password => Some(self.perform_password_stage(session.clone()).await),
+            // TODO implement other authentication types
+            // See: https://gitlab.gnome.org/GNOME/fractal/-/issues/835
+            _ => None,
+        }
     }
 
     /// Performs the password stage.