Closes #1908: AutoSave doesn't work if no sessions left

components/browser/session/src/main/java/mozilla/components/browser/session/SessionManager.kt

@@ -37,11 +37,11 @@ class SessionManager(
      * Produces a snapshot of this manager's state, suitable for restoring via [SessionManager.restore].
      * Only regular sessions are included in the snapshot. Private and Custom Tab sessions are omitted.
      *
-     * @return [Snapshot] or null if no sessions are present.
+     * @return [Snapshot] of the current session state.
      */
-    fun createSnapshot(): Snapshot? = synchronized(values) {
+    fun createSnapshot(): Snapshot = synchronized(values) {
         if (values.isEmpty()) {
-            return null
+            return Snapshot.empty()
         }
 
         // Filter out CustomTab and private sessions.
@@ -60,7 +60,7 @@ class SessionManager(
 
         // We might have some sessions (private, custom tab) but none we'd include in the snapshot.
         if (sessionStateTuples.isEmpty()) {
-            return null
+            return Snapshot.empty()
         }
 
         // We need to find out the index of our selected session in the filtered list. If we have a
@@ -530,6 +530,10 @@ class SessionManager(
             val engineSession: EngineSession? = null,
             val engineSessionState: EngineSessionState? = null
         )
+
+        companion object {
+            fun empty() = Snapshot(emptyList(), NO_SELECTION)
+        }
     }
 }
 

components/browser/session/src/main/java/mozilla/components/browser/session/storage/AutoSave.kt

@@ -108,9 +108,7 @@ class AutoSave(
 
             try {
                 val snapshot = sessionManager.createSnapshot()
-                if (snapshot != null) {
-                    sessionStorage.save(snapshot)
-                }
+                sessionStorage.save(snapshot)
             } finally {
                 val took = now() - start
                 logger.debug("Saved state to disk [${took}ms]")

components/browser/session/src/test/java/mozilla/components/browser/session/SessionManagerTest.kt

@@ -300,7 +300,7 @@ class SessionManagerTest {
     @Test
     fun `createSnapshot works when manager has no sessions`() {
         val manager = SessionManager(mock())
-        assertNull(manager.createSnapshot())
+        assertTrue(manager.createSnapshot().isEmpty())
     }
 
     @Test
@@ -309,7 +309,7 @@ class SessionManagerTest {
         val session = Session("http://mozilla.org", true)
         manager.add(session)
 
-        assertNull(manager.createSnapshot())
+        assertTrue(manager.createSnapshot().isEmpty())
     }
 
     @Test
@@ -319,7 +319,7 @@ class SessionManagerTest {
         session.customTabConfig = Mockito.mock(CustomTabConfig::class.java)
         manager.add(session)
 
-        assertNull(manager.createSnapshot())
+        assertTrue(manager.createSnapshot().isEmpty())
     }
 
     @Test
@@ -329,7 +329,7 @@ class SessionManagerTest {
         session.customTabConfig = Mockito.mock(CustomTabConfig::class.java)
         manager.add(session)
 
-        assertNull(manager.createSnapshot())
+        assertTrue(manager.createSnapshot().isEmpty())
     }
 
     @Test(expected = IllegalArgumentException::class)

components/browser/session/src/test/java/mozilla/components/browser/session/storage/SessionStorageTest.kt

@@ -287,6 +287,35 @@ class SessionStorageTest {
         }
     }
 
+    @Test
+    fun `AutoSave - when no sessions left`() {
+        runBlocking {
+            val session = Session("https://www.firefox.com")
+            val sessionManager = SessionManager(mock())
+            sessionManager.add(session)
+
+            val sessionStorage: SessionStorage = mock()
+
+            val autoSave = AutoSave(
+                    sessionManager = sessionManager,
+                    sessionStorage = sessionStorage,
+                    minimumIntervalMs = 0
+            ).whenSessionsChange()
+
+            assertNull(autoSave.saveJob)
+            verify(sessionStorage, never()).save(any())
+
+            // We didn't specify a default session lambda so this will
+            // leave us without a session
+            sessionManager.remove(session)
+            assertEquals(0, sessionManager.size)
+
+            autoSave.saveJob?.join()
+
+            verify(sessionStorage).save(any())
+        }
+    }
+
     @Test
     fun `AutoSave - when session gets selected`() {
         runBlocking {