Improve ref counting when deduplicating attachments on disk

Co-authored-by: trevor-signal <131492920+trevor-signal@users.noreply.github.com>

ts/sql/Interface.std.ts

@@ -1444,12 +1444,20 @@ type WritableInterface = {
     plaintextHash,
     version,
     contentType,
+    messageId,
   }: {
     plaintextHash: string;
     version: number;
     contentType: MIMEType;
+    messageId: string;
   }) => ExistingAttachmentData | undefined;
-  _protectAttachmentPathFromDeletion: (path: string) => void;
+  _protectAttachmentPathFromDeletion: ({
+    path,
+    messageId,
+  }: {
+    path: string;
+    messageId: string;
+  }) => void;
   resetProtectedAttachmentPaths: () => void;
 
   removeAll: () => void;

ts/sql/Server.node.ts

@@ -2932,12 +2932,19 @@ function getAndProtectExistingAttachmentPath(
     plaintextHash,
     version,
     contentType,
-  }: { plaintextHash: string; version: number; contentType: string }
+    messageId,
+  }: {
+    plaintextHash: string;
+    version: number;
+    contentType: string;
+    messageId: string;
+  }
 ): ExistingAttachmentData | undefined {
   if (!isValidPlaintextHash(plaintextHash)) {
     logger.error('getAndProtectExistingAttachmentPath: Invalid plaintextHash');
     return;
   }
+
   if (version < 2) {
     logger.error(
       'getAndProtectExistingAttachmentPath: Invalid version',
@@ -2985,8 +2992,8 @@ function getAndProtectExistingAttachmentPath(
           (${existingData.thumbnailPath}),
           (${existingData.screenshotPath})
       )
-      INSERT OR REPLACE INTO attachments_protected_from_deletion(path)
-      SELECT path
+      INSERT OR REPLACE INTO attachments_protected_from_deletion(path, messageId)
+      SELECT path, ${messageId}
       FROM existingMessageAttachmentPaths
       WHERE path IS NOT NULL;
     `;
@@ -2997,11 +3004,13 @@ function getAndProtectExistingAttachmentPath(
 
 function _protectAttachmentPathFromDeletion(
   db: WritableDB,
-  path: string
+  { path, messageId }: { path: string; messageId: string }
 ): void {
   const [protectQuery, protectParams] = sql`
-    INSERT OR REPLACE INTO attachments_protected_from_deletion(path)
-    VALUES (${path});
+    INSERT OR REPLACE INTO attachments_protected_from_deletion
+      (path, messageId)
+    VALUES 
+      (${path}, ${messageId});
   `;
   db.prepare(protectQuery).run(protectParams);
 }

ts/sql/migrations/1660-protected-attachments-non-unique.std.ts

@@ -0,0 +1,58 @@
+// Copyright 2026 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+import type { WritableDB } from '../Interface.std.js';
+
+export default function updateToSchemaVersion1660(db: WritableDB): void {
+  db.exec(`
+    DROP TABLE attachments_protected_from_deletion;
+    
+    CREATE TABLE attachments_protected_from_deletion (
+      path TEXT NOT NULL,
+      messageId TEXT NOT NULL,
+      PRIMARY KEY (path, messageId)
+    ) STRICT;
+  `);
+
+  db.exec(`
+    DROP TRIGGER stop_protecting_attachments_after_update;
+    
+    CREATE TRIGGER stop_protecting_attachments_after_update 
+    AFTER UPDATE OF path, thumbnailPath, screenshotPath, backupThumbnailPath 
+    ON message_attachments
+    WHEN 
+      OLD.path IS NOT NEW.path OR
+      OLD.thumbnailPath IS NOT NEW.thumbnailPath OR
+      OLD.screenshotPath IS NOT NEW.screenshotPath OR
+      OLD.backupThumbnailPath IS NOT NEW.backupThumbnailPath
+    BEGIN
+      DELETE FROM attachments_protected_from_deletion 
+      WHERE 
+        messageId IS NEW.messageId 
+        AND path IN (
+          NEW.path, 
+          NEW.thumbnailPath, 
+          NEW.screenshotPath, 
+          NEW.backupThumbnailPath
+        );
+    END;
+  `);
+
+  db.exec(`
+    DROP TRIGGER stop_protecting_attachments_after_insert;
+
+    CREATE TRIGGER stop_protecting_attachments_after_insert 
+    AFTER INSERT 
+    ON message_attachments
+    BEGIN
+      DELETE FROM attachments_protected_from_deletion 
+      WHERE 
+        messageId IS NEW.messageId 
+        AND path IN (
+          NEW.path, 
+          NEW.thumbnailPath, 
+          NEW.screenshotPath, 
+          NEW.backupThumbnailPath
+        );
+    END;
+  `);
+}

ts/sql/migrations/index.node.ts

@@ -142,6 +142,7 @@ import updateToSchemaVersion1620 from './1620-sort-bigger-media.std.js';
 import updateToSchemaVersion1630 from './1630-message-pin-message-data.std.js';
 import updateToSchemaVersion1640 from './1640-key-transparency.std.js';
 import updateToSchemaVersion1650 from './1650-protected-attachments.std.js';
+import updateToSchemaVersion1660 from './1660-protected-attachments-non-unique.std.js';
 
 import { DataWriter } from '../Server.node.js';
 
@@ -1644,6 +1645,7 @@ export const SCHEMA_VERSIONS: ReadonlyArray<SchemaUpdateType> = [
   { version: 1630, update: updateToSchemaVersion1630 },
   { version: 1640, update: updateToSchemaVersion1640 },
   { version: 1650, update: updateToSchemaVersion1650 },
+  { version: 1660, update: updateToSchemaVersion1660 },
 ];
 
 export class DBVersionFromFutureError extends Error {