Generate mediaName for backed-up attachments

ts/util/attachments/getMediaNameForBackup.ts

@@ -0,0 +1,67 @@
+// Copyright 2024 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { getPlaintextHashForAttachmentOnDisk } from '../../AttachmentCrypto';
+import type { AttachmentType } from '../../types/Attachment';
+import { DAY } from '../durations';
+import * as log from '../../logging/log';
+import { isOlderThan } from '../timestamp';
+import { getCdn } from '../../textsecure/downloadAttachment';
+import * as Bytes from '../../Bytes';
+
+const TIME_IN_ATTACHMENT_TIER = 30 * DAY;
+
+// We store the plaintext hash as a hex string, but the mediaName should be
+// the base64 encoded version.
+function convertHexStringToBase64(hexString: string): string {
+  return Bytes.toBase64(Bytes.fromHex(hexString));
+}
+
+type GetMediaNameDependenciesType = {
+  getPlaintextHashForAttachmentOnDisk: (
+    path: string
+  ) => Promise<string | undefined>;
+};
+
+export async function getMediaNameForBackup(
+  attachment: AttachmentType,
+  senderAci: string,
+  messageTimestamp: number,
+  // allow optional dependency injection for testing
+  dependencies: GetMediaNameDependenciesType = {
+    getPlaintextHashForAttachmentOnDisk,
+  }
+): Promise<string | undefined> {
+  if (attachment.plaintextHash) {
+    return convertHexStringToBase64(attachment.plaintextHash);
+  }
+
+  if (attachment.path) {
+    const hashFromFileOnDisk =
+      await dependencies.getPlaintextHashForAttachmentOnDisk(
+        window.Signal.Migrations.getAbsoluteAttachmentPath(attachment.path)
+      );
+    if (!hashFromFileOnDisk) {
+      log.error(
+        'getMediaNameForBackup: no hash from attachment on disk (maybe it is empty?)'
+      );
+      return;
+    }
+    return convertHexStringToBase64(hashFromFileOnDisk);
+  }
+
+  const cdnKey = getCdn(attachment);
+  if (!cdnKey) {
+    log.error('getMediaNameForBackup: attachment has no cdnKey');
+    return;
+  }
+
+  if (isOlderThan(messageTimestamp, TIME_IN_ATTACHMENT_TIER)) {
+    log.error(
+      "getMediaNameForBackup: attachment is not downloaded but is too old; it's no longer in attachment tier."
+    );
+    return;
+  }
+
+  return `${senderAci}_${cdnKey}`;
+}