fix: do not keep references to invalidated clients

SpannerImpl would keep a reference to all database clients that had been created and then invalidated because the database was deleted or did not exists in the first place. If the getDatabaseClient(String) method would be called multiple times for the same invalid database, the memory usage of the application would continue to increase. Fixes #1089

Author: olavloite

google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerImpl.java

@@ -92,9 +92,6 @@ private static String nextDatabaseClientId(DatabaseId databaseId) {
 
  private final CloseableExecutorProvider asyncExecutorProvider;
 
- @GuardedBy("this")
- private final List<DatabaseClientImpl> invalidatedDbClients = new ArrayList<>();
-
  @GuardedBy("this")
  private final Map<DatabaseId, SessionClient> sessionClients = new HashMap<>();
 
@@ -204,9 +201,8 @@ public DatabaseClient getDatabaseClient(DatabaseId db) {
  checkClosed();
  String clientId = null;
  if (dbClients.containsKey(db) && !dbClients.get(db).pool.isValid()) {
- // Move the invalidated client to a separate list, so we can close it together with the
- // other database clients when the Spanner instance is closed.
- invalidatedDbClients.add(dbClients.get(db));
+ // Close the invalidated client and remove it.
+ dbClients.get(db).closeAsync(new ClosedException());
  clientId = dbClients.get(db).clientId;
  dbClients.remove(db);
  }
@@ -253,8 +249,7 @@ void close(long timeout, TimeUnit unit) {
  checkClosed();
  closedException = new ClosedException();
  closureFutures = new ArrayList<>();
- invalidatedDbClients.addAll(dbClients.values());
- for (DatabaseClientImpl dbClient : invalidatedDbClients) {
+ for (DatabaseClientImpl dbClient : dbClients.values()) {
  closureFutures.add(dbClient.closeAsync(closedException));
  }
  dbClients.clear();