Dedupe samples in the mergeIterator.

Signed-off-by: Tom Wilkie <tom.wilkie@gmail.com>

storage/fanout.go

@@ -432,7 +432,6 @@ func (c *mergeIterator) At() (t int64, v float64) {
 		panic("mergeIterator.At() called after .Next() returned false.")
 	}
 
-	// TODO do I need to dedupe or just merge?
 	return c.h[0].At()
 }
 
@@ -443,6 +442,7 @@ func (c *mergeIterator) Next() bool {
 				heap.Push(&c.h, iter)
 			}
 		}
+
 		return len(c.h) > 0
 	}
 
@@ -450,10 +450,18 @@ func (c *mergeIterator) Next() bool {
 		return false
 	}
 
+	currt, currv := c.At()
+	for len(c.h) > 0 {
+		nextt, nextv := c.h[0].At()
+		if nextt != currt || nextv != currv {
+			break
+		}
+
 		iter := heap.Pop(&c.h).(SeriesIterator)
 		if iter.Next() {
 			heap.Push(&c.h, iter)
 		}
+	}
 
 	return len(c.h) > 0
 }

storage/fanout_test.go

@@ -136,6 +136,14 @@ func TestMergeIterator(t *testing.T) {
 			},
 			expected: []sample{{0, 0}, {1, 1}, {2, 2}, {3, 3}, {4, 4}, {5, 5}},
 		},
+		{
+			input: []SeriesIterator{
+				newListSeriesIterator([]sample{{0, 0}, {1, 1}}),
+				newListSeriesIterator([]sample{{0, 0}, {2, 2}}),
+				newListSeriesIterator([]sample{{2, 2}, {3, 3}}),
+			},
+			expected: []sample{{0, 0}, {1, 1}, {2, 2}, {3, 3}},
+		},
 	} {
 		merged := newMergeIterator(tc.input)
 		actual := drainSamples(merged)