fix: Prevent preview session race conditions on adapter switch (#899)

Author: charlesvien

apps/twig/src/renderer/features/sessions/service/service.ts

@@ -98,8 +98,8 @@ export class SessionService {
       permission?: { unsubscribe: () => void };
     }
   >();
-  /** Version counter to discard stale preview session results */
-  private previewVersion = 0;
+  /** AbortController for the current in-flight preview session start */
+  private previewAbort: AbortController | null = null;
 
   /**
    * Connect to a task session.
@@ -473,29 +473,16 @@ export class SessionService {
 
   // --- Preview Session Management ---
 
-  /**
-   * Start a lightweight preview session for the task input page.
-   * This session is used solely to retrieve adapter-specific config options
-   * (models, modes, reasoning levels) without creating a real PostHog task.
-   *
-   * Uses a version counter to prevent race conditions when rapidly switching
-   * adapters — stale results from a previous start are discarded.
-   */
   async startPreviewSession(params: {
     adapter: "claude" | "codex";
     repoPath?: string;
   }): Promise<void> {
-    // Increment version to invalidate any in-flight start
-    const version = ++this.previewVersion;
+    this.previewAbort?.abort();
+    const abort = new AbortController();
+    this.previewAbort = abort;
 
-    // Cancel any existing preview session first
-    await this.cancelPreviewSession();
-
-    // Check if a newer start was requested while we were cancelling
-    if (version !== this.previewVersion) {
-      log.info("Preview session start superseded, skipping", { version });
-      return;
-    }
+    await this.cleanupPreviewSession();
+    if (abort.signal.aborted) return;
 
     const auth = this.getAuthCredentials();
     if (!auth) {
@@ -523,13 +510,7 @@ export class SessionService {
         adapter: params.adapter,
       });
 
-      // Check again after the async start — a newer call may have superseded us
-      if (version !== this.previewVersion) {
-        log.info(
-          "Preview session start superseded after agent.start, cleaning up stale session",
-          { taskRunId, version },
-        );
-        // Clean up the session we just started but is now stale
+      if (abort.signal.aborted) {
         trpcVanilla.agent.cancel
           .mutate({ sessionId: taskRunId })
           .catch((err) => {
@@ -560,26 +541,24 @@ export class SessionService {
         configOptionsCount: configOptions?.length ?? 0,
       });
     } catch (error) {
-      // Only clean up if we're still the current version
-      if (version === this.previewVersion) {
-        log.error("Failed to start preview session", { error });
-        sessionStoreSetters.removeSession(taskRunId);
-      }
+      if (abort.signal.aborted) return;
+      log.error("Failed to start preview session", { error });
+      sessionStoreSetters.removeSession(taskRunId);
     }
   }
 
-  /**
-   * Cancel and clean up the preview session.
-   * Unsubscribes and removes from store first (so nothing writes to the old
-   * session), then awaits the cancel on the main process.
-   */
   async cancelPreviewSession(): Promise<void> {
+    this.previewAbort?.abort();
+    this.previewAbort = null;
+    await this.cleanupPreviewSession();
+  }
+
+  private async cleanupPreviewSession(): Promise<void> {
     const session = sessionStoreSetters.getSessionByTaskId(PREVIEW_TASK_ID);
     if (!session) return;
 
     const { taskRunId } = session;
 
-    // Unsubscribe and remove from store first so nothing writes to the old session
     this.unsubscribeFromChannel(taskRunId);
     sessionStoreSetters.removeSession(taskRunId);
 
@@ -588,8 +567,6 @@ export class SessionService {
     } catch (error) {
       log.warn("Failed to cancel preview session", { taskRunId, error });
     }
-
-    log.info("Preview session cancelled", { taskRunId });
   }
 
   // --- Subscription Management ---
@@ -661,7 +638,8 @@ export class SessionService {
     }
 
     this.connectingTasks.clear();
-    this.previewVersion = 0;
+    this.previewAbort?.abort();
+    this.previewAbort = null;
   }
 
   private handleSessionEvent(taskRunId: string, acpMsg: AcpMessage): void {