Lock databases during backup export.

Signal/src/util/OWSBackup.m

@@ -188,6 +188,11 @@ NS_ASSUME_NONNULL_BEGIN
                    backupDirPath:backupDirPath]) {
         return;
     }
+    // Use a read/write transaction to acquire a file lock on the database files.
+    //
+    // TODO: If we use multiple database files, lock them too.
+    [TSStorageManager.sharedManager.newDatabaseConnection
+        readWriteWithBlock:^(YapDatabaseReadWriteTransaction *_Nonnull transaction) {
             if (![self copyDirectory:OWSFileSystem.appDocumentDirectoryPath
                           dstDirName:@"appDocumentDirectoryPath"
                        backupDirPath:backupDirPath]) {
@@ -198,6 +203,7 @@ NS_ASSUME_NONNULL_BEGIN
                        backupDirPath:backupDirPath]) {
                 return;
             }
+        }];
     if (![self zipDirectory:backupDirPath dstFilePath:backupZipPath]) {
         return;
     }