When we hit an error in /api/0.x/map, include a more helpful explanation of what's wrong in an error header (ruby's CGI stuff won't let you set it as the HTTP status message)

app/controllers/api_controller.rb

@@ -3,12 +3,15 @@ class ApiController < ApplicationController
   before_filter :authorize
   after_filter :compress_output
 
+  # The maximum area you're allowed to request, in square degrees
+  MAX_REQUEST_AREA = 0.25
+
   def map
     response.headers["Content-Type"] = 'application/xml'
     # Figure out the bbox
     bbox = params['bbox']
     unless bbox and bbox.count(',') == 3
-      render :nothing => true, :status => 400
+       report_error("The parameter bbox is required, and must be of the form min_lon,min_lat,max_lon,max_lat")
       return
     end
 
@@ -19,17 +22,42 @@ class ApiController < ApplicationController
     max_lon = bbox[2].to_f
     max_lat = bbox[3].to_f
 
+    # check the bbox is sane
+    unless min_lon <= max_lon
+       report_error("The minimum longitude must be less than the maximum longitude, but it wasn't")
+      return
+    end
+    unless min_lat <= max_lat
+       report_error("The minimum latitude must be less than the maximum latitude, but it wasn't")
+      return
+    end
+	unless min_lon >= -180 && min_lat >= -90 && max_lon <= 180 && max_lat <= 90
+       report_error("The latitudes must be between -90 and 90, and longitudes between -180 and 180")
+      return
+	end
+
+    # check the bbox isn't too large
+    requested_area = (max_lat-min_lat)*(max_lon-min_lon)
+    if requested_area > MAX_REQUEST_AREA
+       report_error("The maximum bbox size is " + MAX_REQUEST_AREA.to_s + ", and your request was too large. Either request a smaller area, or use planet.osm")
+      return
+    end
+
     # get all the nodes
     nodes = Node.find(:all, :conditions => ['latitude > ? AND longitude > ? AND latitude < ? AND longitude < ? AND visible = 1', min_lat, min_lon, max_lat, max_lon])
 
     node_ids = nodes.collect {|node| node.id }
+
+    # (in the future, we may wish to abort here if we found too many nodes)
+
+    # grab the segments
     segments = Array.new
     if node_ids.length > 0
       node_ids_sql = "(#{node_ids.join(',')})"
       # get the referenced segments
       segments = Segment.find_by_sql "select * from current_segments where visible = 1 and (node_a in #{node_ids_sql} or node_b in #{node_ids_sql})"
     end
-    # see if we have nay missing nodes
+    # see if we have any missing nodes
     segments_nodes = segments.collect {|segment| segment.node_a }
     segments_nodes += segments.collect {|segment| segment.node_b }
 

app/controllers/application.rb

@@ -38,6 +38,17 @@ class ApplicationController < ActionController::Base
     end 
   end 
 
+  # Report and error to the user
+  # (If anyone ever fixes Rails so it can set a http status "reason phrase",
+  #  rather than only a status code and having the web engine make up a 
+  #  phrase from that, we can also put the error message into the status
+  #  message. For now, rails won't let us)
+  def report_error(message)
+    render :nothing => true, :status => 400
+    # Todo: some sort of escaping of problem characters in the message
+    response.headers['Error'] = message
+  end
+
   # extract authorisation credentials from headers, returns user = nil if none
   private 
   def get_auth_data