Process 'sent update' transcripts.

SignalServiceKit/src/Devices/OWSRecordTranscriptJob.m

@@ -161,6 +161,7 @@ NS_ASSUME_NONNULL_BEGIN
     [outgoingMessage saveWithTransaction:transaction];
     [outgoingMessage updateWithWasSentFromLinkedDeviceWithUDRecipientIds:transcript.udRecipientIds
                                                        nonUdRecipientIds:transcript.nonUdRecipientIds
+                                                            isSentUpdate:NO
                                                              transaction:transaction];
     [[OWSDisappearingMessagesJob sharedJob] startAnyExpirationForMessage:outgoingMessage
                                                      expirationStartedAt:transcript.expirationStartedAt
@@ -239,6 +240,13 @@ NS_ASSUME_NONNULL_BEGIN
 
     BOOL messageFound = NO;
     for (TSOutgoingMessage *message in messages) {
+        if (!message.isFromLinkedDevice) {
+            // isFromLinkedDevice isn't always set for very old linked messages, but:
+            //
+            // a) We should never receive a "sent update" for a very old message.
+            // b) It's safe to discard suspicious "sent updates."
+            continue;
+        }
         TSThread *thread = [message threadWithTransaction:transaction];
         if (!thread.isGroupThread) {
             continue;
@@ -257,6 +265,7 @@ NS_ASSUME_NONNULL_BEGIN
 
         [message updateWithWasSentFromLinkedDeviceWithUDRecipientIds:udRecipientIds
                                                    nonUdRecipientIds:nonUdRecipientIds
+                                                        isSentUpdate:YES
                                                          transaction:transaction];
         messageFound = YES;
     }

SignalServiceKit/src/Messages/Interactions/TSOutgoingMessage.h

@@ -220,6 +220,7 @@ typedef NS_ENUM(NSInteger, TSGroupMetaMessage) {
 
 - (void)updateWithWasSentFromLinkedDeviceWithUDRecipientIds:(nullable NSArray<NSString *> *)udRecipientIds
                                           nonUdRecipientIds:(nullable NSArray<NSString *> *)nonUdRecipientIds
+                                               isSentUpdate:(BOOL)isSentUpdate
                                                 transaction:(YapDatabaseReadWriteTransaction *)transaction;
 
 // This method is used to rewrite the recipient list with a single recipient.

SignalServiceKit/src/Messages/Interactions/TSOutgoingMessage.m

@@ -753,7 +753,9 @@ NSString *NSStringForOutgoingMessageRecipientState(OWSOutgoingMessageRecipientSt
 
 - (void)updateWithWasSentFromLinkedDeviceWithUDRecipientIds:(nullable NSArray<NSString *> *)udRecipientIds
                                           nonUdRecipientIds:(nullable NSArray<NSString *> *)nonUdRecipientIds
-                                                transaction:(YapDatabaseReadWriteTransaction *)transaction {
+                                               isSentUpdate:(BOOL)isSentUpdate
+                                                transaction:(YapDatabaseReadWriteTransaction *)transaction
+{
     OWSAssertDebug(transaction);
 
     [self
@@ -788,6 +790,19 @@ NSString *NSStringForOutgoingMessageRecipientState(OWSOutgoingMessageRecipientSt
                                        recipientState.wasSentByUD = NO;
                                        recipientStateMap[recipientId] = recipientState;
                                    }
+
+                                   if (isSentUpdate) {
+                                       // If this is a "sent update", make sure that:
+                                       //
+                                       // a) "Sent updates" should never remove any recipients.  We end up with the
+                                       //    union of the existing and new recipients.
+                                       // b) "Sent updates" should never downgrade the "recipient state" for any
+                                       //    recipients.  Prefer existing "recipient state"; "sent updates" only
+                                       //    add new recipients at the "sent" state.
+                                       [recipientStateMap removeObjectsForKeys:self.recipientStateMap.allKeys];
+                                       [recipientStateMap addEntriesFromDictionary:self.recipientStateMap];
+                                   }
+
                                    [message setRecipientStateMap:recipientStateMap];
                                } else {
                                    // Otherwise assume this is a legacy message before UD was introduced, and mark