Respond to CR.

SignalServiceKit/src/Contacts/ContactsUpdater.m

@@ -103,7 +103,7 @@ NS_ASSUME_NONNULL_BEGIN
                         [SignalRecipient markRecipientAsRegisteredAndGet:recipientId transaction:transaction];
                     [recipients addObject:recipient];
                 } else {
-                    [SignalRecipient removeUnregisteredRecipient:recipientId transaction:transaction];
+                    [SignalRecipient markRecipientAsUnregistered:recipientId transaction:transaction];
                 }
             }
         }];

SignalServiceKit/src/Contacts/SignalRecipient.h

@@ -10,10 +10,11 @@ NS_ASSUME_NONNULL_BEGIN
 //
 // a) It serves as a cache of "known" Signal accounts.  When the service indicates
 //    that an account exists, we make sure that an instance of SignalRecipient exists
-//    for that recipient id (using mark as registered).
-//    When the service indicates that an account does not exist, we remove any
-//    SignalRecipient.
-// b) We hang the "known device list" for known signal accounts on this entity.
+//    for that recipient id (using mark as registered) and has at least one device.
+//    When the service indicates that an account does not exist, we remove any devices
+//    from that SignalRecipient - but do not remove it from the database.
+//    Note that SignalRecipients without any devices are not considered registered.
+//// b) We hang the "known device list" for known signal accounts on this entity.
 @interface SignalRecipient : TSYapDatabaseObject
 
 @property (nonatomic, readonly) NSOrderedSet *devices;
@@ -41,7 +42,7 @@ NS_ASSUME_NONNULL_BEGIN
 + (void)markRecipientAsRegistered:(NSString *)recipientId
                          deviceId:(UInt32)deviceId
                       transaction:(YapDatabaseReadWriteTransaction *)transaction;
-+ (void)removeUnregisteredRecipient:(NSString *)recipientId transaction:(YapDatabaseReadWriteTransaction *)transaction;
++ (void)markRecipientAsUnregistered:(NSString *)recipientId transaction:(YapDatabaseReadWriteTransaction *)transaction;
 
 @end
 

SignalServiceKit/src/Contacts/SignalRecipient.m

@@ -141,7 +141,7 @@ NS_ASSUME_NONNULL_BEGIN
     OWSAssertDebug(devicesToAdd.count > 0 || devicesToRemove.count > 0);
 
     // Add before we remove, since removeDevicesFromRecipient:...
-    // can removeUnregisteredRecipient:... if the recipient has
+    // can markRecipientAsUnregistered:... if the recipient has
     // no devices left.
     if (devicesToAdd.count > 0) {
         [self addDevicesToRegisteredRecipient:[NSSet setWithArray:devicesToAdd] transaction:transaction];
@@ -192,6 +192,12 @@ NS_ASSUME_NONNULL_BEGIN
 
 - (void)removeWithTransaction:(YapDatabaseReadWriteTransaction *)transaction
 {
+    // We need to distinguish between "users we know to be unregistered" and
+    // "users whose registration status is unknown".  The former correspond to
+    // instances of SignalRecipient with no devices.  The latter do not
+    // correspond to an instance of SignalRecipient in the database (although
+    // they may correspond to an "unsaved" instance of SignalRecipient built
+    // by getOrBuildUnsavedRecipientForRecipientId.
     OWSFailDebug(@"Don't call removeWithTransaction.");
     
     [super removeWithTransaction:transaction];
@@ -256,7 +262,7 @@ NS_ASSUME_NONNULL_BEGIN
     }
 }
 
-+ (void)removeUnregisteredRecipient:(NSString *)recipientId transaction:(YapDatabaseReadWriteTransaction *)transaction
++ (void)markRecipientAsUnregistered:(NSString *)recipientId transaction:(YapDatabaseReadWriteTransaction *)transaction
 {
     OWSAssertDebug(transaction);
     OWSAssertDebug(recipientId.length > 0);

SignalServiceKit/src/Messages/OWSMessageSender.m

@@ -817,7 +817,7 @@ NSString *const OWSMessageSenderRateLimitedException = @"RateLimitedException";
             return;
         }
 
-        [SignalRecipient removeUnregisteredRecipient:recipient.recipientId transaction:transaction];
+        [SignalRecipient markRecipientAsUnregistered:recipient.recipientId transaction:transaction];
 
         [[TSInfoMessage userNotRegisteredMessageInThread:thread recipientId:recipient.recipientId]
             saveWithTransaction:transaction];