fix(trace): set the Bugsnag user off the main thread

apps/flipcash/app/src/main/kotlin/com/flipcash/app/internal/startup/ForwardUserId.kt

@@ -0,0 +1,28 @@
+package com.flipcash.app.internal.startup
+
+import com.bugsnag.android.Bugsnag
+import com.getcode.utils.TraceManager
+import kotlinx.coroutines.CoroutineDispatcher
+import kotlinx.coroutines.Dispatchers
+
+/**
+ * Forwards every [TraceManager.userId] change to [setUser] on [dispatcher], never on the thread
+ * that changed it.
+ *
+ * `Bugsnag.setUser` takes the NDK plugin's state lock, and Bugsnag's own worker holds that lock in
+ * `NativeBridge.synchronizeState` while it waits on `DeviceDataCollector.calculateFreeDisk`. Account
+ * creation sets the user id on the main thread, so calling Bugsnag there froze the UI until the
+ * worker let go — the ANR in Bugsnag `6acec70adbd69358a81f2824`.
+ *
+ * [dispatcher] runs one change at a time, in order, so a sign-out followed by a sign-in leaves
+ * Bugsnag with the signed-in id.
+ */
+@OptIn(kotlinx.coroutines.ExperimentalCoroutinesApi::class)
+internal fun forwardUserIdOffThread(
+    dispatcher: CoroutineDispatcher = Dispatchers.IO.limitedParallelism(1),
+    setUser: (String?) -> Unit = { id -> Bugsnag.setUser(id, null, null) },
+) {
+    TraceManager.setOnUserIdChanged { id ->
+        launchBestEffort(tag = "bugsnag-user", dispatcher = dispatcher) { setUser(id) }
+    }
+}

apps/flipcash/app/src/main/kotlin/com/flipcash/app/internal/startup/TraceInitializer.kt

@@ -57,9 +57,7 @@ class TraceInitializer: Initializer<Unit> {
 
                 ErrorUtils.addReporter(BugsnagErrorReporter())
                 TraceManager.addSink(BugsnagBreadcrumbSink())
-                TraceManager.setOnUserIdChanged { id ->
-                    Bugsnag.setUser(id, null, null)
-                }
+                forwardUserIdOffThread()
 
                 stageProvider.fetchAndCache(versionCode)
                 if (stageProvider.resolvedStage == ReleaseStage.Internal) {

apps/flipcash/app/src/test/kotlin/com/flipcash/app/internal/startup/ForwardUserIdTest.kt

@@ -0,0 +1,46 @@
+package com.flipcash.app.internal.startup
+
+import com.getcode.utils.TraceManager
+import kotlinx.coroutines.ExperimentalCoroutinesApi
+import kotlinx.coroutines.test.StandardTestDispatcher
+import kotlinx.coroutines.test.advanceUntilIdle
+import kotlinx.coroutines.test.runTest
+import kotlin.test.AfterTest
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+
+@OptIn(ExperimentalCoroutinesApi::class)
+class ForwardUserIdTest {
+
+    @AfterTest
+    fun tearDown() {
+        TraceManager.setOnUserIdChanged(null)
+        TraceManager.userId = null
+    }
+
+    @Test
+    fun `does not call setUser on the thread that changed the id`() = runTest {
+        val forwarded = mutableListOf<String?>()
+        forwardUserIdOffThread(dispatcher = StandardTestDispatcher(testScheduler)) { forwarded += it }
+
+        TraceManager.userId = "user-1"
+
+        assertTrue(forwarded.isEmpty())
+        advanceUntilIdle()
+        assertEquals(listOf<String?>("user-1"), forwarded)
+    }
+
+    @Test
+    fun `forwards changes in the order they were made`() = runTest {
+        val forwarded = mutableListOf<String?>()
+        forwardUserIdOffThread(dispatcher = StandardTestDispatcher(testScheduler)) { forwarded += it }
+
+        TraceManager.userId = "user-1"
+        TraceManager.userId = null
+        TraceManager.userId = "user-2"
+        advanceUntilIdle()
+
+        assertEquals(listOf("user-1", null, "user-2"), forwarded)
+    }
+}