Carry the login itself, and clean up after any failed read

Two things were conflated. Reloading the list bumped the generation, and
the import treated that as its login having changed — so a routine
refresh aborted a healthy import and told the user their account had
moved. The listing and the login are separate facts: the login that
listed a row is now carried with it, and the generation decides only
what may be marked imported.

The local playlist is also created before it is read, so anything
stopping the read left it behind empty — a login that moved, but equally
a server that did not answer. The import reports whether it completed,
and an incomplete one takes its empty playlist with it. A playlist that
genuinely had nothing to add keeps it.

Yattee/Views/Settings/ImportPlaylistsView.swift

@@ -42,9 +42,10 @@ struct ImportPlaylistsView: View {
     // Merge warning state
     @State private var showMergeWarning = false
     @State private var pendingMergePlaylist: Playlist?
-    /// The listing the pending merge was chosen from. The dialog waits on the
-    /// user, so the list can be replaced before they answer it.
+    /// The listing and login the pending merge was chosen from. The dialog
+    /// waits on the user, so both can be replaced before they answer it.
     @State private var pendingMergeGeneration = 0
+    @State private var pendingMergeCredential = ""
     @State private var existingLocalPlaylist: LocalPlaylist?
 
     // MARK: - Accessibility Identifiers
@@ -108,8 +109,14 @@ struct ImportPlaylistsView: View {
                 Button(String(localized: "import.playlists.mergeWarning.merge")) {
                     if let playlist = pendingMergePlaylist, let localPlaylist = existingLocalPlaylist {
                         let generation = pendingMergeGeneration
+                        let credential = pendingMergeCredential
                         mergeImportTask = Task {
-                            await performImport(playlist, into: localPlaylist, generation: generation)
+                            await performImport(
+                                playlist,
+                                into: localPlaylist,
+                                generation: generation,
+                                credential: credential
+                            )
                         }
                     }
                 }
@@ -339,15 +346,16 @@ struct ImportPlaylistsView: View {
     private func addPlaylist(_ playlist: Playlist) async {
         guard let appEnvironment else { return }
 
-        // The listing this row came from, taken here rather than wherever the
-        // import eventually starts: a merge waits on the user, and by then the
-        // list may have been replaced.
+        // Both taken here rather than wherever the import eventually starts: a
+        // merge waits on the user, and by then the list may have been replaced
+        // and its credential re-recorded.
         let generation = listGeneration
 
         // Checked before anything is created: a playlist made for a login that
         // has already gone would be left behind empty, because the import
         // aborts on its first write.
-        guard appEnvironment.credential(forInstance: instance) == loadedCredential else {
+        guard let credential = appEnvironment.credential(forInstance: instance),
+              credential == loadedCredential else {
             loginChangedDuringImport = true
             return
         }
@@ -358,6 +366,7 @@ struct ImportPlaylistsView: View {
             // Show merge warning
             pendingMergePlaylist = playlist
             pendingMergeGeneration = generation
+            pendingMergeCredential = credential
             existingLocalPlaylist = existing
             importingPlaylistID = playlist.id.playlistID
             showMergeWarning = true
@@ -366,34 +375,44 @@ struct ImportPlaylistsView: View {
 
         // Create new local playlist and import
         let localPlaylist = appEnvironment.dataManager.createPlaylist(title: playlist.title)
-        await performImport(playlist, into: localPlaylist, generation: generation)
+        let imported = await performImport(
+            playlist,
+            into: localPlaylist,
+            generation: generation,
+            credential: credential
+        )
 
-        // The guard above and the one inside are a suspension apart, so the
-        // login can still go in between. Nothing was read, and an empty
-        // playlist named after somebody else's is not worth keeping.
-        if loginChangedDuringImport, localPlaylist.items?.isEmpty ?? true {
+        // It is created before it is read, so anything that stops the read —
+        // a login that moved, but equally a server that did not answer —
+        // leaves it behind with nothing in it. An import that genuinely had
+        // nothing to add keeps its playlist; a failed one does not.
+        if !imported, localPlaylist.items?.isEmpty ?? true {
             appEnvironment.dataManager.deletePlaylist(localPlaylist)
         }
     }
 
-    /// - Parameter generation: the listing `playlist` was chosen from, taken at
-    ///   selection time. Reading it under a later one would fetch whatever that
-    ///   ID happens to name to whoever replaced the login.
+    /// - Parameters:
+    ///   - generation: the listing `playlist` was chosen from, taken at
+    ///     selection time. It decides only what may be marked imported — a
+    ///     reload replaces the rows, not the login.
+    ///   - credential: the login that listed `playlist`, taken at selection
+    ///     time. Reading it under any other would fetch whatever that ID
+    ///     happens to name to somebody else.
+    /// - Returns: whether the playlist was read and written in full.
+    @discardableResult
     private func performImport(
         _ playlist: Playlist,
         into localPlaylist: LocalPlaylist,
-        generation: Int
-    ) async {
-        guard let appEnvironment else { return }
+        generation: Int,
+        credential: String
+    ) async -> Bool {
+        guard let appEnvironment else { return false }
 
         // The login can go, or be replaced, between choosing this playlist and
         // reading it — the merge dialog in particular waits on the user.
         // Returning quietly would leave the row importing forever and let a
-        // batch carry on; reading it under a replacement would fetch whatever
-        // that ID means to somebody else.
-        guard generation == listGeneration,
-              let credential = appEnvironment.credential(forInstance: instance),
-              credential == loadedCredential else {
+        // batch carry on.
+        guard appEnvironment.credential(forInstance: instance) == credential else {
             loginChangedDuringImport = true
             importingPlaylistID = nil
             importProgress = nil
@@ -403,7 +422,7 @@ struct ImportPlaylistsView: View {
                 String(localized: "import.playlists.failed.title"),
                 subtitle: ImportError.loginChanged.localizedDescription
             )
-            return
+            return false
         }
 
         importingPlaylistID = playlist.id.playlistID
@@ -454,9 +473,7 @@ struct ImportPlaylistsView: View {
                 // write rather than only at the start. What has already been
                 // written stays — it was imported under the login that asked
                 // for it.
-                guard generation == listGeneration,
-                      appEnvironment.credential(forInstance: instance) == credential,
-                      credential == loadedCredential else {
+                guard appEnvironment.credential(forInstance: instance) == credential else {
                     throw ImportError.loginChanged
                 }
 
@@ -495,6 +512,8 @@ struct ImportPlaylistsView: View {
                     appEnvironment.toastManager.showSuccess(String(localized: "import.playlists.added.title"))
                 }
             }
+
+            return true
         } catch {
             await MainActor.run {
                 if error as? ImportError == .loginChanged {
@@ -507,6 +526,8 @@ struct ImportPlaylistsView: View {
                     subtitle: error.localizedDescription
                 )
             }
+
+            return false
         }
     }