Fix deadlock with nested calls to runBlocking.

Fixes #14460

app/src/test/java/org/thoughtcrime/securesms/backup/v2/ArchiveDatabaseExecutorTest.kt

@@ -0,0 +1,66 @@
+package org.thoughtcrime.securesms.backup.v2
+
+import android.app.Application
+import assertk.assertThat
+import assertk.assertions.isEqualTo
+import assertk.assertions.isTrue
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.annotation.Config
+import java.util.concurrent.CountDownLatch
+import java.util.concurrent.TimeUnit
+import java.util.concurrent.atomic.AtomicBoolean
+import java.util.concurrent.atomic.AtomicReference
+
+@RunWith(RobolectricTestRunner::class)
+@Config(manifest = Config.NONE, application = Application::class)
+class ArchiveDatabaseExecutorTest {
+
+  @Test
+  fun `runBlocking returns the result of the block`() {
+    val result = ArchiveDatabaseExecutor.runBlocking {
+      42
+    }
+
+    assertThat(result).isEqualTo(42)
+  }
+
+  @Test
+  fun `nested runBlocking calls do not deadlock`() {
+    val completed = AtomicBoolean(false)
+    val result = AtomicReference<String>()
+    val latch = CountDownLatch(1)
+
+    Thread {
+      try {
+        val r = ArchiveDatabaseExecutor.runBlocking {
+          ArchiveDatabaseExecutor.runBlocking {
+            ArchiveDatabaseExecutor.runBlocking {
+              "nested-result"
+            }
+          }
+        }
+        result.set(r)
+        completed.set(true)
+      } finally {
+        latch.countDown()
+      }
+    }.start()
+
+    val finishedInTime = latch.await(5, TimeUnit.SECONDS)
+
+    assertThat(finishedInTime).isTrue()
+    assertThat(completed.get()).isTrue()
+    assertThat(result.get()).isEqualTo("nested-result")
+  }
+
+  @Test
+  fun `runBlocking executes on archive-db thread`() {
+    val threadName = ArchiveDatabaseExecutor.runBlocking {
+      Thread.currentThread().name
+    }
+
+    assertThat(threadName).isEqualTo(ArchiveDatabaseExecutor.THREAD_NAME)
+  }
+}