1 files changed, 76 insertions, 0 deletions
diff --git a/src/background/translation-session.js b/src/background/translation-session.js
new file mode 100644
index 0000000..1d02974
--- /dev/null
+++ b/src/background/translation-session.js
@@ -0,0 +1,76 @@
+/**
+ * Translation session management (background).
+ *
+ * Only one translation may be active at a time (SEC-14 / Design.md §9).
+ * Starting a new request aborts the previous one. Chunks belonging to a
+ * stale requestId are discarded.
+ */
+
+import { ERROR_CODES } from '../shared/constants.js';
+
+export class TranslationSession {
+  constructor() {
+    /** @type {{requestId: string, controller: AbortController, port: object}|null} */
+    this.active = null;
+  }
+
+  /**
+   * @returns {boolean}
+   */
+  hasActive() {
+    return this.active !== null;
+  }
+
+  /**
+   * Abort the active request, if any. Notifies its port so the UI can reset.
+   * @returns {void}
+   */
+  abortActive() {
+    if (!this.active) return;
+    const { controller, port, requestId } = this.active;
+    this.active = null;
+    try {
+      controller.abort();
+    } catch {
+      // ignore abort errors
+    }
+    if (port) {
+      try {
+        port.postMessage({ type: 'error', requestId, code: ERROR_CODES.REQUEST_ABORTED, message: 'Superseded by a newer request' });
+      } catch {
+        // port may be gone
+      }
+    }
+  }
+
+  /**
+   * Register a new active request, aborting any previous one.
+   * @param {string} requestId
+   * @param {object} port runtime.Port owning the request
+   * @returns {AbortController} the controller for the new request
+   */
+  start(requestId, port) {
+    this.abortActive();
+    const controller = new AbortController();
+    this.active = { requestId, controller, port };
+    return controller;
+  }
+
+  /**
+   * @param {string} requestId
+   * @returns {boolean} true when the given id is the active request.
+   */
+  isActive(requestId) {
+    return this.active !== null && this.active.requestId === requestId;
+  }
+
+  /**
+   * Clear the active session when it matches the id.
+   * @param {string} requestId
+   */
+  clear(requestId) {
+    if (this.isActive(requestId)) {
+      this.active = null;
+    }
+  }
+}
+\ No newline at end of file