[geary/wip/768422-namespace-support] Further Imap.Account API tweaks.

[Michael Gratton <mjog src gnome org>]

commit e361dc649af748ccd23ea79ea7d42ff349c2c130
Author: Michael James Gratton <mike vee net>
Date:   Fri Nov 3 13:41:36 2017 +1100

    Further Imap.Account API tweaks.
    
    * src/engine/imap/api/imap-account.vala
      (Account:list_child_folders_async): Renamed to
      fetch_child_folders_async to match other remote-y methods, make returb
      value non-null since nulls suck. Fix call site.

 .../imap-engine/imap-engine-generic-account.vala   |   32 ++++++++++----------
 src/engine/imap/api/imap-account.vala              |    8 ++---
 2 files changed, 19 insertions(+), 21 deletions(-)
---
diff --git a/src/engine/imap-engine/imap-engine-generic-account.vala 
b/src/engine/imap-engine/imap-engine-generic-account.vala
index 86ae9c5..fb381d0 100644
--- a/src/engine/imap-engine/imap-engine-generic-account.vala
+++ b/src/engine/imap-engine/imap-engine-generic-account.vala
@@ -464,9 +464,9 @@ private abstract class Geary.ImapEngine.GenericAccount : Geary.Account {
         results_suspect = false;
         check_open();
         
-        Gee.List<Imap.Folder>? remote_children = null;
+        Gee.List<Imap.Folder> remote_children = null;
         try {
-            remote_children = yield remote.list_child_folders_async(parent, cancellable);
+            remote_children = yield remote.fetch_child_folders_async(parent, cancellable);
         } catch (Error err) {
             // ignore everything but I/O and IMAP errors (cancellation is an IOError)
             if (err is IOError || err is ImapError)
@@ -475,25 +475,25 @@ private abstract class Geary.ImapEngine.GenericAccount : Geary.Account {
                 (parent != null ? parent.to_string() : "root"), err.message);
             results_suspect = true;
         }
-        
+
         Gee.HashMap<FolderPath, Imap.Folder> result = new Gee.HashMap<FolderPath, Imap.Folder>();
-        if (remote_children != null) {
-            foreach (Imap.Folder remote_child in remote_children) {
-                result.set(remote_child.path, remote_child);
-                if (remote_child.properties.has_children.is_possible()) {
-                    bool recursive_results_suspect;
-                    Collection.map_set_all<FolderPath, Imap.Folder>(result,
-                        yield enumerate_remote_folders_async(
-                        remote_child.path, out recursive_results_suspect, cancellable));
-                    if (recursive_results_suspect)
-                        results_suspect = true;
-                }
+        foreach (Imap.Folder remote_child in remote_children) {
+            result.set(remote_child.path, remote_child);
+            if (remote_child.properties.has_children.is_possible()) {
+                bool recursive_results_suspect;
+                Collection.map_set_all<FolderPath, Imap.Folder>(
+                    result, yield enumerate_remote_folders_async(
+                        remote_child.path, out recursive_results_suspect, cancellable
+                    )
+                );
+                if (recursive_results_suspect)
+                    results_suspect = true;
             }
         }
-        
+
         return result;
     }
-    
+
     public override Geary.ContactStore get_contact_store() {
         return local.contact_store;
     }
diff --git a/src/engine/imap/api/imap-account.vala b/src/engine/imap/api/imap-account.vala
index 00251b5..ef115ec 100644
--- a/src/engine/imap/api/imap-account.vala
+++ b/src/engine/imap/api/imap-account.vala
@@ -187,17 +187,15 @@ private class Geary.Imap.Account : BaseObject {
      * This method will perform a pipe-lined IMAP SELECT for all
      * folders found, and hence should be used with care.
      */
-    public async Gee.List<Imap.Folder>? list_child_folders_async(FolderPath? parent, Cancellable? 
cancellable)
+    public async Gee.List<Imap.Folder> fetch_child_folders_async(FolderPath? parent, Cancellable? 
cancellable)
     throws Error {
         ClientSession session = yield claim_session_async(cancellable);
-
+        Gee.List<Imap.Folder> children = new Gee.ArrayList<Imap.Folder>();
         Gee.List<MailboxInformation> mailboxes = yield send_list_async(session, parent, true, cancellable);
         if (mailboxes.size == 0) {
-            return null;
+            return children;
         }
 
-        Gee.List<Imap.Folder> children = new Gee.ArrayList<Imap.Folder>();
-
         // Work out which folders need a STATUS and send them all
         // pipe-lined to minimise network and server latency.
         Gee.Map<MailboxSpecifier, MailboxInformation> info_map = new Gee.HashMap<