issue with sampling during querying with aggregates and descending order

[ramazanyich]

Discovered an issue with result of querying from kairosdb. I need to get last x items from the timeseries DB for a defined timerange and perform aggregation of returned datapoints with sampling of 1 second on this aggregates.
Then I do query using default ascending order response is correctly constructed with correctly sampled datapoints per second.
But it I set order to descending, when sampling is broken: aggregation performed on all data and set to last sampled period.
Below are the example datapoints and queries.
Created 4 datapoints using following REST API call:

curl -v -X POST -d '[ {                                                                                                                                                                               base 10:15:39
  "name" : "my.new.metric2x",     
  "tags" : {     
    "a" : "b"                      
  },                   
  "type" : "long",   
  "value" : 1,      
  "timestamp" : 238433400000
}, {                      
  "name" : "my.new.metric2x",
  "tags" : {                    
    "a" : "b"                 
  },    
  "type" : "long",
  "value" : 3,                 
  "timestamp" : 238433400999                                   
}, {
  "name" : "my.new.metric2x",
  "tags" : {
    "a" : "b"
  },
  "type" : "long",
  "value" : 5,
  "timestamp" : 238433401000
}, {
  "name" : "my.new.metric2x",
  "tags" : {
    "a" : "b"
  },
  "type" : "long",
  "value" : 7,
  "timestamp" : 238433401999
} ]'  http://127.0.0.1:8080/api/v1/datapoints

then perform query using default order:

curl -v -X POST -d '{                                                                                                                                                                         ✘ INT   base 10:17:37
  "start_absolute" : 238433400000,
  "metrics" : [ {
    "name" : "my.new.metric2x",    
    "aggregators" : [ {
      "name" : "sum",
      "sampling" : {
        "value" : "1",
        "unit" : "seconds"
      },
      "align_start_time" : true,
      "align_sampling" : false
    } ] 
  } ],
  "end_absolute" : 238433402000
}'  http://127.0.0.1:8080/api/v1/datapoints/query

and got expected result with correctly sampled aggregation:

{"queries":[{"sample_size":4,"results":[{"name":"my.new.metric2x","group_by":[{"name":"type","type":"number"}],"tags":{"a":["b"]},"values":[[238433400000,4],[238433401000,12]]}]}]}

but if I set order to desc:

curl -v -X POST -d '{                                                                                                                                                                                 base 10:17:43
  "start_absolute" : 238433400000,
  "metrics" : [ {
    "name" : "my.new.metric2x",
    "aggregators" : [ {
      "name" : "sum",
      "sampling" : {
        "value" : "1",
        "unit" : "seconds"
      },
      "align_start_time" : true,
      "align_sampling" : false
    } ],
    "order" : "desc"
  } ],                         
  "end_absolute" : 238433402000                                
}'  http://127.0.0.1:8080/api/v1/datapoints/query

then I got following response:

{"queries":[{"sample_size":4,"results":[{"name":"my.new.metric2x","group_by":[{"name":"type","type":"number"}],"tags":{"a":["b"]},"values":[[238433401000,16]]}]}]}

as you can see the aggregation does not have samping per second and all aggregation happened on all data and put on the last samping period.