[folks] folks: Check UID components are non-empty

[Philip Withnall <pwithnall src gnome org>]

commit f56e1f7852c72575087c8ec0dfb239a7a07fc7e1
Author: Philip Withnall <philip tecnocode co uk>
Date:   Mon Nov 14 00:10:43 2016 +0000

    folks: Check UID components are non-empty
    
    When building a UID, check that the three components are non-empty. Add
    preconditions to build_uid() which check this. All of the existing
    callers of that method are guaranteed to not pass in empty strings, so
    this is not a behaviour change.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=773011

 folks/persona.vala |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)
---
diff --git a/folks/persona.vala b/folks/persona.vala
index 6c54686..265669f 100644
--- a/folks/persona.vala
+++ b/folks/persona.vala
@@ -300,7 +300,8 @@ public abstract class Folks.Persona : Object
   /**
    * Build a UID from the given components.
    *
-   * Each component is escaped before the UID is built.
+   * Each component is escaped before the UID is built. All components must be
+   * non-empty strings.
    *
    * @param backend_name the {@link Backend.name}
    * @param persona_store_id the {@link PersonaStore.id}
@@ -311,6 +312,9 @@ public abstract class Folks.Persona : Object
    */
   public static string build_uid (string backend_name,
       string persona_store_id, string persona_id)
+        requires (backend_name != "")
+        requires (persona_store_id != "")
+        requires (persona_id != "")
     {
       return "%s:%s:%s".printf (Persona._escape_uid_component (backend_name),
           Persona._escape_uid_component (persona_store_id),