fix(imap): prefer login errors over connection errors

Closes #8534.

Adds a regression test for the error precedence.

src/imap.rs

@@ -146,6 +146,18 @@ pub(crate) struct ServerMetadata {
     pub app_versions: Option<String>,
 }
 
+/// Selects the most actionable error from failed IMAP connection candidates.
+///
+/// A login error proves that a connection succeeded, so it takes precedence over network errors.
+fn select_connect_error(
+    first_connection_error: Option<anyhow::Error>,
+    first_login_error: Option<anyhow::Error>,
+) -> anyhow::Error {
+    first_login_error
+        .or(first_connection_error)
+        .unwrap_or_else(|| format_err!("No IMAP connection candidates provided"))
+}
+
 struct UidGrouper<T: Iterator<Item = (i64, u32, String)>> {
     inner: Peekable<T>,
 }
@@ -314,7 +326,8 @@ impl Imap {
         self.conn_backoff_ms = max(BACKOFF_MIN_MS, self.conn_backoff_ms);
 
         let login_params = prioritize_server_login_params(&context.sql, &self.lp, "imap").await?;
-        let mut first_error = None;
+        let mut first_connection_error = None;
+        let mut first_login_error = None;
         'candidate: for lp in login_params {
             info!(context, "IMAP trying to connect to {}.", lp.connection);
             let connection_candidate = lp.connection.clone();
@@ -330,7 +343,7 @@ impl Imap {
                 Ok(client) => client,
                 Err(err) => {
                     warn!(context, "{err:#}.");
-                    first_error.get_or_insert(err);
+                    first_connection_error.get_or_insert(err);
                     continue 'candidate;
                 }
             };
@@ -410,7 +423,7 @@ impl Imap {
                     let message = stock_str::cannot_login(context, &imap_user);
 
                     warn!(context, "IMAP failed to login: {err:#}.");
-                    first_error.get_or_insert(format_err!("{message} ({err:#})"));
+                    first_login_error.get_or_insert(format_err!("{message} ({err:#})"));
 
                     // If it looks like the password is wrong, send a notification:
                     let _lock = context.wrong_pw_warning_mutex.lock().await;
@@ -446,7 +459,10 @@ impl Imap {
             }
         }
 
-        Err(first_error.unwrap_or_else(|| format_err!("No IMAP connection candidates provided")))
+        Err(select_connect_error(
+            first_connection_error,
+            first_login_error,
+        ))
     }
 
     /// Prepare a new IMAP session.

src/imap/imap_tests.rs

@@ -1,6 +1,16 @@
 use super::*;
 use crate::test_utils::TestContext;
 
+#[test]
+fn test_connect_prefers_login_error() {
+    let connection_error = format_err!("All connection attempts failed");
+    let login_error = format_err!("Cannot login, please check the password");
+
+    let error = select_connect_error(Some(connection_error), Some(login_error));
+
+    assert_eq!(error.to_string(), "Cannot login, please check the password");
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn test_set_uid_next_validity() {
     let t = TestContext::new_alice().await;