Check if candidate uuid does not already exist before returning it

util/uuid.lua

@@ -1,9 +1,34 @@
 -- source: https://gist.github.com/jrus/3197011
 local random = math.random
+
+local function is_uuid_unexisting(uuid)
+    for p, _ in minetest.get_auth_handler().iterate() do
+        local e = mail.get_storage_entry(p)
+        for _, m in ipairs(e.inbox) do
+            if m.id == uuid then return false end
+        end
+        for _, m in ipairs(e.outbox) do
+            if m.id == uuid then return false end
+        end
+        for _, m in ipairs(e.drafts) do
+            if m.id == uuid then return false end
+        end
+        for _, m in ipairs(e.trash) do
+            if m.id == uuid then return false end
+        end
+    end
+    return true
+end
+
 function mail.new_uuid()
     local template ='xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'
-    return string.gsub(template, '[xy]', function (c)
+    local candidate_uuid = ""
+    repeat
+        candidate_uuid = string.gsub(template, '[xy]',
+            function (c)
                 local v = (c == 'x') and random(0, 0xf) or random(8, 0xb)
                 return string.format('%x', v)
             end)
+    until is_uuid_unexisting(candidate_uuid)
+    return candidate_uuid
 end