For mozilla-mobile#12469 - Allow for multiple PlacesApi readers.

Multiple readers allow to start and cancel history suggestions requests without
any impact on other in-progress PlacesApi queries.

components/browser/storage-sync/src/main/java/mozilla/components/browser/storage/sync/Connection.kt

@@ -29,7 +29,26 @@ const val DB_NAME = "places.sqlite"
 internal interface Connection : Closeable {
     fun registerWithSyncManager()
 
+    /**
+     * Allows to read history, bookmarks, and other data from persistent storage.
+     * All calls on the same reader are queued. Multiple readers are allowed.
+     */
     fun reader(): PlacesReaderConnection
+
+    /**
+     * Create a new reader for history, bookmarks and other data from persistent storage.
+     * Allows for disbanded calls from the default reader from [reader] and so being able to
+     * easily start and cancel any data requests without impacting others using another reader.
+     *
+     * All [PlacesApi] requests are synchronized at lower levels so even with using multiple readers
+     * all requests are ordered with concurrency not allowed.
+     */
+    fun newReader(): PlacesReaderConnection
+
+    /**
+     * Allowed to add history, bookmarks and other data to persistent storage.
+     * All calls are queued and synchronized at lower levels. Only one writer is recommended.
+     */
     fun writer(): PlacesWriterConnection
 
     // Until we get a real SyncManager in application-services libraries, we'll have to live with this
@@ -87,6 +106,12 @@ internal object RustPlacesConnection : Connection {
         return cachedReader!!
     }
 
+    override fun newReader(): PlacesReaderConnection = synchronized(this) {
+        val api = safeGetApi()
+        check(api != null) { "must call init first" }
+        return api.openReader()
+    }
+
     override fun writer(): PlacesWriterConnection {
         val api = safeGetApi()
         check(api != null) { "must call init first" }

components/browser/storage-sync/src/main/java/mozilla/components/browser/storage/sync/PlacesStorage.kt

@@ -42,14 +42,24 @@ abstract class PlacesStorage(
 
     abstract val logger: Logger
 
-    @VisibleForTesting
+    @VisibleForTesting(otherwise = VisibleForTesting.PROTECTED)
     internal open val places: Connection by lazy {
         RustPlacesConnection.init(storageDir)
         RustPlacesConnection
     }
 
+    /**
+     * Default writer for adding history, bookmarks and other data to persistent storage.
+     */
+    @VisibleForTesting(otherwise = VisibleForTesting.PROTECTED)
     internal val writer: PlacesWriterConnection by lazy { places.writer() }
-    internal val reader: PlacesReaderConnection by lazy { places.reader() }
+
+    /**
+     * Default reader for querying history, bookmarks and other data from persistent storage.
+     * Override to use a different instance if special calls synchronization / cancellation is needed.
+     */
+    @VisibleForTesting(otherwise = VisibleForTesting.PROTECTED)
+    internal open val reader: PlacesReaderConnection by lazy { places.reader() }
 
     override suspend fun warmUp() {
         logElapsedTime(logger, "Warming up places storage") {

components/browser/storage-sync/src/test/java/mozilla/components/browser/storage/sync/PlacesHistoryStorageTest.kt

@@ -598,6 +598,11 @@ class PlacesHistoryStorageTest {
                 return mock()
             }
 
+            override fun newReader(): PlacesReaderConnection {
+                fail()
+                return mock()
+            }
+
             override fun writer(): PlacesWriterConnection {
                 fail()
                 return mock()
@@ -654,6 +659,11 @@ class PlacesHistoryStorageTest {
                 return mock()
             }
 
+            override fun newReader(): PlacesReaderConnection {
+                fail()
+                return mock()
+            }
+
             override fun writer(): PlacesWriterConnection {
                 fail()
                 return mock()
@@ -702,6 +712,11 @@ class PlacesHistoryStorageTest {
                 return mock()
             }
 
+            override fun newReader(): PlacesReaderConnection {
+                fail()
+                return mock()
+            }
+
             override fun writer(): PlacesWriterConnection {
                 fail()
                 return mock()
@@ -757,6 +772,11 @@ class PlacesHistoryStorageTest {
                 return mock()
             }
 
+            override fun newReader(): PlacesReaderConnection {
+                fail()
+                return mock()
+            }
+
             override fun writer(): PlacesWriterConnection {
                 fail()
                 return mock()