Handle locally missing attachments for local backups

ts/AttachmentCrypto.ts

@@ -552,6 +552,7 @@ export async function decryptAttachmentV2ToSink(
       `${logId}: Failed to decrypt attachment`,
       Errors.toLogFormat(error)
     );
+    sink.end();
     throw error;
   } finally {
     await readFd?.close();

ts/services/backups/util/filePointers.ts

@@ -3,6 +3,7 @@
 import Long from 'long';
 import { BackupLevel } from '@signalapp/libsignal-client/zkgroup';
 import { omit } from 'lodash';
+import { existsSync } from 'node:fs';
 
 import {
   APPLICATION_OCTET_STREAM,
@@ -455,9 +456,15 @@ export async function getLocalBackupFilePointerForAttachment({
       getBackupCdnInfo,
     });
 
-  // localKey is required to export to the local backup. If it's missing then fall back
-  // to the filePointer which would have been generated for a remote backup.
-  if (attachment.localKey == null) {
+  // If a file disappeared locally (maybe we downloaded it and it disappeared)
+  // or localKey is missing, then we can't export to a local backup.
+  // Fallback to the filePointer which would have been generated for a remote backup.
+  const isAttachmentMissingLocally =
+    attachment.path == null ||
+    !existsSync(
+      window.Signal.Migrations.getAbsoluteAttachmentPath(attachment.path)
+    );
+  if (isAttachmentMissingLocally || attachment.localKey == null) {
     return { filePointer: remoteFilePointer, updatedAttachment };
   }
 

ts/util/downloadAttachmentFromLocalBackup.ts

@@ -1,13 +1,12 @@
 // Copyright 2025 Signal Messenger, LLC
 // SPDX-License-Identifier: AGPL-3.0-only
 
+import { existsSync } from 'node:fs';
 import { isNumber } from 'lodash';
 import {
   type AttachmentType,
   getAttachmentIdForLogging,
 } from '../types/Attachment';
-import * as log from '../logging/log';
-import { toLogFormat } from '../types/errors';
 import {
   decryptAndReencryptLocally,
   type ReencryptedAttachmentV2,
@@ -23,15 +22,7 @@ export async function downloadAttachmentFromLocalBackup(
   const dataId = `${attachmentId}`;
   const logId = `downloadAttachmentFromLocalBackup(${dataId})`;
 
-  try {
-    return await doDownloadFromLocalBackup(attachment, { logId });
-  } catch (error) {
-    log.error(
-      `${logId}: error when copying from local backup`,
-      toLogFormat(error)
-    );
-    throw new AttachmentPermanentlyUndownloadableError();
-  }
+  return doDownloadFromLocalBackup(attachment, { logId });
 }
 
 async function doDownloadFromLocalBackup(
@@ -49,6 +40,12 @@ async function doDownloadFromLocalBackup(
   strictAssert(localBackupPath, `${logId}: missing localBackupPath`);
   strictAssert(isNumber(size), `${logId}: missing size`);
 
+  if (!existsSync(localBackupPath)) {
+    throw new AttachmentPermanentlyUndownloadableError(
+      'No file at attachment localBackupPath'
+    );
+  }
+
   return decryptAndReencryptLocally({
     type: 'local',
     ciphertextPath: localBackupPath,