Fix for #1565. Added a couple of tests and fixed another issue with re-used placeholder IDs.

lib/diff_reader.rb

@@ -22,6 +22,18 @@ class DiffReader
     @changeset = changeset
   end
 
+  ##
+  # Reads the next element from the XML document. Checks the return value
+  # and throws an exception if an error occurred.
+  def read_or_die
+    # NOTE: XML::Reader#read returns 0 for EOF and -1 for error.
+    # we allow an EOF because we are expecting this to always happen
+    # at the end of a document.
+    if @reader.read < 0
+      raise APIBadUserInput.new("Unexpected end of XML document.")
+    end
+  end
+
   ##
   # An element-block mapping for using the LibXML reader interface. 
   #
@@ -29,15 +41,25 @@ class DiffReader
   # elements, it would be better to DRY and do this in a block. This
   # could also help with error handling...?
   def with_element
-    # skip the first element, which is our opening element of the block
-    @reader.read
-    # loop over all elements. 
-    # NOTE: XML::Reader#read returns 0 for EOF and -1 for error.
-    while @reader.read == 1
-      break if @reader.node_type == 15 # end element
-      next unless @reader.node_type == 1 # element
-      yield @reader.name
+    # if the start element is empty then don't do any processing, as
+    # there won't be any child elements to process!
+    unless @reader.empty_element?
+      # read the first element
+      read_or_die
+
+      begin
+        # because we read elements in DOM-style to reuse their DOM
+        # parsing code, we don't always read an element on each pass
+        # as the call to @reader.next in the innermost loop will take
+        # care of that for us.
+        if @reader.node_type == 1 # element
+          yield @reader.name
+        else
+          read_or_die
+        end
+      end while @reader.node_type != 15 # end element
     end
+    read_or_die
   end
 
   ##
@@ -75,14 +97,18 @@ class DiffReader
   # an exception subclassing OSM::APIError will be thrown.
   def commit
 
+    # data structure used for mapping placeholder IDs to real IDs
     node_ids, way_ids, rel_ids = {}, {}, {}
     ids = { :node => node_ids, :way => way_ids, :relation => rel_ids}
 
+    # take the first element and check that it is an osmChange element
+    @reader.read
+    raise APIBadUserInput.new("Document element should be 'osmChange'.") if @reader.name != 'osmChange'
+
     result = OSM::API.new.get_xml_doc
     result.root.name = "diffResult"
 
-    # loop at the top level, within the <osmChange> element (although we
-    # don't actually check this...)
+    # loop at the top level, within the <osmChange> element
     with_element do |action_name|
       if action_name == 'create'
         # create a new element. this code is agnostic of the element type
@@ -96,6 +122,11 @@ class DiffReader
           placeholder_id = xml['id'].to_i
           raise OSM::APIBadXMLError.new(model, xml) if placeholder_id.nil?
 
+          # check if the placeholder ID has been given before and throw
+          # an exception if it has - we can't create the same element twice.
+          model_sym = model.to_s.downcase.to_sym
+          raise OSM::APIBadUserInput.new("Placeholder IDs must be unique for created elements.") if ids[model_sym].include? placeholder_id
+
           # some elements may have placeholders for other elements in the
           # diff, so we must fix these before saving the element.
           new.fix_placeholders!(ids)
@@ -104,7 +135,7 @@ class DiffReader
           new.create_with_history(@changeset.user)
           
           # save placeholder => allocated ID map
-          ids[model.to_s.downcase.to_sym][placeholder_id] = new.id
+          ids[model_sym][placeholder_id] = new.id
 
           # add the result to the document we're building for return.
           xml_result = XML::Node.new model.to_s.downcase
@@ -122,15 +153,22 @@ class DiffReader
           new = model.from_xml_node(xml, false)
           check(model, xml, new)
 
+          # if the ID is a placeholder then map it to the real ID
+          model_sym = model.to_s.downcase.to_sym
+          is_placeholder = ids[model_sym].include? new.id
+          id = is_placeholder ? ids[model_sym][new.id] : new.id
+
           # and the old one from the database
-          old = model.find(new.id)
+          old = model.find(id)
 
           new.fix_placeholders!(ids)
           old.update_from(new, @changeset.user)
 
           xml_result = XML::Node.new model.to_s.downcase
-          xml_result["old_id"] = old.id.to_s
-          xml_result["new_id"] = new.id.to_s 
+          # oh, the irony... the "new" element actually contains the "old" ID
+          # a better name would have been client/server, but anyway...
+          xml_result["old_id"] = new.id.to_s
+          xml_result["new_id"] = id.to_s 
           # version is updated in "old" through the update, so we must not
           # return new.version here but old.version!
           xml_result["new_version"] = old.version.to_s
@@ -144,7 +182,12 @@ class DiffReader
           new = model.from_xml_node(xml, false)
           check(model, xml, new)
 
-          old = model.find(new.id)
+          # if the ID is a placeholder then map it to the real ID
+          model_sym = model.to_s.downcase.to_sym
+          is_placeholder = ids[model_sym].include? new.id
+          id = is_placeholder ? ids[model_sym][new.id] : new.id
+
+          old = model.find(id)
 
           # can a delete have placeholders under any circumstances?
           # if a way is modified, then deleted is that a valid diff?
@@ -152,7 +195,9 @@ class DiffReader
           old.delete_with_history!(new, @changeset.user)
 
           xml_result = XML::Node.new model.to_s.downcase
-          xml_result["old_id"] = old.id.to_s
+          # oh, the irony... the "new" element actually contains the "old" ID
+          # a better name would have been client/server, but anyway...
+          xml_result["old_id"] = new.id.to_s
           result.root << xml_result
         end