Respond to CR.

Signal/src/AppDelegate.m

@@ -1135,7 +1135,8 @@ static NSString *const kURLHostVerifyPrefix             = @"verify";
 
     [OWSBackup.sharedManager setup];
 
-    [OWSBackupLazyRestoreJob run];
+    // Resume lazy restore.
+    [OWSBackupLazyRestoreJob runAsync];
 }
 
 - (void)registrationStateDidChange

Signal/src/util/OWSBackupExportJob.m

@@ -732,15 +732,16 @@ NS_ASSUME_NONNULL_BEGIN
     [self.unsavedAttachmentExports removeLastObject];
 
     if (self.lastValidRecordNames) {
-        // Wherever possible, we do incremental backups and re-use fragments of the last backup.
+        // Wherever possible, we do incremental backups and re-use fragments of the last
+        // backup and/or restore.
         // Recycling fragments doesn't just reduce redundant network activity,
         // it allows us to skip the local export work, i.e. encryption.
         // To do so, we must preserve the metadata for these fragments.
         //
         // We check two things:
         //
-        // * That the "last known backup manifest" contains an item from which we can recover
-        //   this record's metadata.
+        // * That we already know the metadata for this fragment (from a previous backup
+        //   or restore).
         // * That this record does in fact exist in our CloudKit database.
         NSString *lastRecordName = [OWSBackupAPI recordNameForPersistentFileWithFileId:attachmentExport.attachmentId];
         OWSBackupFragment *_Nullable lastBackupFragment = [OWSBackupFragment fetchObjectWithUniqueID:lastRecordName];

Signal/src/util/OWSBackupImportJob.m

@@ -162,7 +162,7 @@ NSString *const kOWSBackup_ImportDatabaseKeySpec = @"kOWSBackup_ImportDatabaseKe
                                 }
 
                                 // Kick off lazy restore.
-                                [OWSBackupLazyRestoreJob run];
+                                [OWSBackupLazyRestoreJob runAsync];
 
                                 [weakSelf succeed];
                             }];

Signal/src/util/OWSBackupLazyRestoreJob.swift

@@ -22,15 +22,15 @@ public class OWSBackupLazyRestoreJob: NSObject {
     }
 
     @objc
-    public class func run() {
-        OWSBackupLazyRestoreJob().run()
+    public class func runAsync() {
+        OWSBackupLazyRestoreJob().runAsync()
     }
 
     public override init() {
         self.primaryStorage = OWSPrimaryStorage.shared()
     }
 
-    private func run() {
+    private func runAsync() {
         AssertIsOnMainThread()
 
         DispatchQueue.global().async {

SignalServiceKit/src/Messages/Attachments/TSAttachmentStream.m

@@ -363,6 +363,8 @@ NS_ASSUME_NONNULL_BEGIN
 
     if (![[NSFileManager defaultManager] fileExistsAtPath:self.mediaURL.path]) {
         DDLogError(@"%@ while generating thumbnail, source file doesn't exist: %@", self.logTag, self.mediaURL);
+        // If we're not lazy-restoring this message, the attachment should exist on disk.
+        OWSAssert(self.lazyRestoreFragmentId);
         return;
     }
 
@@ -649,8 +651,6 @@ NS_ASSUME_NONNULL_BEGIN
                                      [attachment setLazyRestoreFragmentId:nil];
                                  }];
     }];
-
-    [self ensureThumbnail];
 }
 
 @end

SignalServiceKit/src/Storage/TSDatabaseView.m

@@ -379,6 +379,7 @@ NSString *const TSLazyRestoreAttachmentsGroup = @"TSLazyRestoreAttachmentsGroup"
             return NSOrderedSame;
         }
 
+        // Specific ordering doesn't matter; we just need a stable ordering.
         TSAttachmentStream *attachmentStream1 = (TSAttachmentStream *)object1;
         TSAttachmentStream *attachmentStream2 = (TSAttachmentStream *)object2;
         return [attachmentStream2.creationTimestamp compare:attachmentStream1.creationTimestamp];