Keep the integration device ID from its first read

Sources/Shared/Settings/SettingsStore.swift

@@ -24,8 +24,14 @@ public class SettingsStore {
         }
     }
 
+    static let integrationDeviceIDKey = "integrationDeviceID"
+
+    /// The identifier the server knows this installation by. It is taken from the platform's vendor
+    /// identifier the first time it is needed and kept from then on, so a build that derives it
+    /// differently (the Mac moving from Catalyst to the native app) keeps the registration it has
+    /// rather than registering a second device.
     public var integrationDeviceID: String {
-        let baseString = Current.device.identifierForVendor() ?? deviceID
+        let baseString = persistedIntegrationDeviceID ?? deviceID
 
         switch Current.appConfiguration {
         case .debug:
@@ -44,6 +50,19 @@ public class SettingsStore {
         }
     }
 
+    /// The vendor identifier as first seen, stored alongside the other registration data in the app group.
+    /// Nothing is stored while the platform has no identifier to give, so a later read can still pick it up.
+    private var persistedIntegrationDeviceID: String? {
+        if let stored = prefs.string(forKey: Self.integrationDeviceIDKey) {
+            return stored
+        }
+        guard let current = Current.device.identifierForVendor() else {
+            return nil
+        }
+        prefs.set(current, forKey: Self.integrationDeviceIDKey)
+        return current
+    }
+
     private var seenWhatsNewReleaseIDs: Set<String> {
         Set(prefs.stringArray(forKey: seenWhatsNewReleaseIDsKey) ?? [])
     }

Tests/App/Settings/SettingsStoreIntegrationDeviceID.test.swift

@@ -0,0 +1,45 @@
+@testable import Shared
+import Testing
+
+/// The identifier the server knows the installation by is taken from the vendor identifier once and kept.
+// Serialized: the tests swap `Current.device.identifierForVendor` and the stored value, which concurrent
+// tests would race on.
+@Suite(.serialized)
+struct SettingsStoreIntegrationDeviceIDTests {
+    private func withVendorIdentifier(
+        _ identifier: @escaping () -> String?,
+        _ body: () throws -> Void
+    ) rethrows {
+        let previousIdentifier = Current.device.identifierForVendor
+        let previousStored = Current.settingsStore.prefs.string(forKey: SettingsStore.integrationDeviceIDKey)
+        defer {
+            Current.device.identifierForVendor = previousIdentifier
+            Current.settingsStore.prefs.set(previousStored, forKey: SettingsStore.integrationDeviceIDKey)
+        }
+        Current.settingsStore.prefs.removeObject(forKey: SettingsStore.integrationDeviceIDKey)
+        Current.device.identifierForVendor = identifier
+        try body()
+    }
+
+    @Test func theFirstVendorIdentifierIsKeptWhenItLaterChanges() {
+        withVendorIdentifier({ "first-vendor-id" }) {
+            let first = Current.settingsStore.integrationDeviceID
+            #expect(first.hasSuffix("first-vendor-id"))
+
+            Current.device.identifierForVendor = { "second-vendor-id" }
+
+            #expect(Current.settingsStore.integrationDeviceID == first)
+        }
+    }
+
+    @Test func nothingIsKeptWhileThereIsNoVendorIdentifier() {
+        withVendorIdentifier({ nil }) {
+            #expect(Current.settingsStore.integrationDeviceID.hasSuffix(Current.settingsStore.deviceID))
+            #expect(Current.settingsStore.prefs.string(forKey: SettingsStore.integrationDeviceIDKey) == nil)
+
+            Current.device.identifierForVendor = { "late-vendor-id" }
+
+            #expect(Current.settingsStore.integrationDeviceID.hasSuffix("late-vendor-id"))
+        }
+    }
+}

Tests/Shared/Watch/WatchDeviceRegistrar.test.swift

@@ -288,13 +288,17 @@ struct WatchDeviceRegistrarTests {
         let systemName = Current.device.systemName
         let systemVersion = Current.device.systemVersion
         let identifierForVendor = Current.device.identifierForVendor
+        // The integration device ID is kept from its first read, so the stored one is set aside for the test.
+        let storedIntegrationDeviceID = Current.settingsStore.prefs.string(forKey: SettingsStore.integrationDeviceIDKey)
         defer {
             Current.device.deviceName = deviceName
             Current.device.systemModel = systemModel
             Current.device.systemName = systemName
             Current.device.systemVersion = systemVersion
             Current.device.identifierForVendor = identifierForVendor
+            Current.settingsStore.prefs.set(storedIntegrationDeviceID, forKey: SettingsStore.integrationDeviceIDKey)
         }
+        Current.settingsStore.prefs.removeObject(forKey: SettingsStore.integrationDeviceIDKey)
         Current.device.deviceName = { "Apple Watch" }
         Current.device.systemModel = { "Watch7,1" }
         Current.device.systemName = { "watchOS" }