GroupMembers: Filter out members for which an account doesn't exist

None of the callers is interested in members that don't exist as account
in Gerrit.

Change-Id: I3866e85181d200c078c40bfab7f2937459902fb1
Signed-off-by: Edwin Kempin <ekempin@google.com>

java/com/google/gerrit/server/account/GroupMembers.java

@@ -17,6 +17,7 @@ package com.google.gerrit.server.account;
 import static com.google.common.collect.ImmutableSet.toImmutableSet;
 
 import com.google.common.collect.Sets;
+import com.google.common.collect.Streams;
 import com.google.gerrit.common.Nullable;
 import com.google.gerrit.reviewdb.client.Account;
 import com.google.gerrit.reviewdb.client.AccountGroup;
@@ -56,6 +57,8 @@ public class GroupMembers {
   /**
    * Recursively enumerate the members of the given group. Should not be used with the
    * PROJECT_OWNERS magical group.
+   *
+   * <p>Group members for which an account doesn't exist are filtered out.
    */
   public Set<Account> listAccounts(AccountGroup.UUID groupUUID) throws IOException {
     if (SystemGroupBackend.PROJECT_OWNERS.equals(groupUUID)) {
@@ -71,6 +74,8 @@ public class GroupMembers {
   /**
    * Recursively enumerate the members of the given group. The project should be specified so the
    * PROJECT_OWNERS magical group can be expanded.
+   *
+   * <p>Group members for which an account doesn't exist are filtered out.
    */
   public Set<Account> listAccounts(AccountGroup.UUID groupUUID, Project.NameKey project)
       throws NoSuchProjectException, IOException {
@@ -124,7 +129,8 @@ public class GroupMembers {
             .getMembers()
             .stream()
             .filter(groupControl::canSeeMember)
-            .map(accountCache::get)
+            .map(accountCache::maybeGet)
+            .flatMap(Streams::stream)
             .map(AccountState::getAccount)
             .collect(toImmutableSet());