Cap number of devices on both ends

Signal/src/Loki/Onboarding/SeedVC.swift

@@ -359,7 +359,7 @@ final class SeedVC : OnboardingBaseViewController, DeviceLinkingModalDelegate {
             }.catch(on: DispatchQueue.main) { [weak self] _ in
                 TSAccountManager.sharedInstance().resetForReregistration()
                 guard let self = self else { return }
-                let alert = UIAlertController(title: "Couldn't Link Device", message: "Please check your connection and try again.", preferredStyle: .alert)
+                let alert = UIAlertController(title: NSLocalizedString("Couldn't Link Device", comment: ""), message: NSLocalizedString("Please check your connection and try again.", comment: ""), preferredStyle: .alert)
                 self.present(alert, animated: true, completion: nil)
                 self.setUserInteractionEnabled(true)
             }

Signal/src/ViewControllers/AppSettings/AppSettingsViewController.m

@@ -249,9 +249,22 @@
     [section addItem:[OWSTableItem itemWithTitle:NSLocalizedString(@"Share Public Key", @"") actionBlock:^{ [weakSelf sharePublicKey]; }]];
     [section addItem:[OWSTableItem itemWithTitle:NSLocalizedString(@"Show QR Code", @"") actionBlock:^{ [weakSelf showQRCode]; }]];
     if (isMasterDevice) {
+        NSString *userHexEncodedPublicKey = OWSIdentityManager.sharedManager.identityKeyPair.hexEncodedPublicKey;
+        __block BOOL hasLinkedDevice;
+        [OWSPrimaryStorage.sharedManager.dbReadConnection readWithBlock:^(YapDatabaseReadTransaction *transaction) {
+            NSSet<LKDeviceLink *> *deviceLinks = [LKDatabaseUtilities getDeviceLinksFor:userHexEncodedPublicKey in:transaction];
+            hasLinkedDevice = deviceLinks.count > 0;
+        }];
         [section addItem:[OWSTableItem itemWithTitle:NSLocalizedString(@"Link Device", @"") actionBlock:^{
-            [weakSelf linkDevice];
+            if (!hasLinkedDevice) {
+                [weakSelf linkDevice];
+            } else {
+                UIAlertController *alert = [UIAlertController alertControllerWithTitle:NSLocalizedString(@"Multi Device Limit Reached", @"") message:NSLocalizedString(@"It's currently not allowed to link more than one device.", @"") preferredStyle:UIAlertControllerStyleAlert];
+                [alert addAction:[UIAlertAction actionWithTitle:NSLocalizedString(@"OK", @"") style:UIAlertActionStyleDefault handler:nil]];
+                [weakSelf presentViewController:alert animated:YES completion:nil];
+            }
         }]];
+        
         [section addItem:[OWSTableItem itemWithTitle:NSLocalizedString(@"Show Seed", @"") actionBlock:^{ [weakSelf showSeed]; }]];
     }
     [section addItem:[OWSTableItem itemWithTitle:NSLocalizedString(@"Clear All Data", @"") actionBlock:^{ [weakSelf clearAllData]; }]];

Signal/translations/en.lproj/Localizable.strings

@@ -2648,3 +2648,5 @@
 "Connecting..." = "Connecting...";
 "Couldn't Connect" = "Couldn't Connect";
 "Please pick a display name that consists of only a-z, A-Z, 0-9 and _ characters" = "Please pick a display name that consists of only a-z, A-Z, 0-9 and _ characters";
+"Multi Device Limit Reached" = "Multi Device Limit Reached";
+"It's currently not allowed to link more than one device." = "It's currently not allowed to link more than one device.";