From 8214676363cdaddb11cb7898ae6a13cbf7e879e1 Mon Sep 17 00:00:00 2001
From: Nathaniel Cook <nvcook42@gmail.com>
Date: Tue, 15 Dec 2015 11:29:20 -0700
Subject: [PATCH] fix panic when merging empty series

---
 CHANGELOG.md                   |  1 +
 services/httpd/handler.go      | 18 ++++++++++--------
 services/httpd/handler_test.go | 19 +++++++++++++++++++
 3 files changed, 30 insertions(+), 8 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4f577cdc8c..4d308b77fc 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,7 @@ With this release InfluxDB is moving to Go 1.5.
 
 ### Bugfixes
 - [#5042](https://github.com/influxdb/influxdb/issues/5042): Count with fill(none) will drop 0 valued intervals.
+- [#4735](https://github.com/influxdb/influxdb/issues/4735): Fix panic when merging empty results.
 - [#5016](https://github.com/influxdb/influxdb/pull/5016): Don't panic if Meta data directory not writable. Thanks @oiooj
 - [#5059](https://github.com/influxdb/influxdb/pull/5059): Fix unmarshal of database error by client code. Thanks @farshidtz
 - [#4940](https://github.com/influxdb/influxdb/pull/4940): Fix distributed aggregate query query error. Thanks @li-ang
diff --git a/services/httpd/handler.go b/services/httpd/handler.go
index 53b7ef2b5d..a1ab8ecb00 100644
--- a/services/httpd/handler.go
+++ b/services/httpd/handler.go
@@ -328,17 +328,19 @@ func (h *Handler) serveQuery(w http.ResponseWriter, r *http.Request, user *meta.
 			resp.Results = append(resp.Results, r)
 		} else if resp.Results[l-1].StatementID == r.StatementID {
 			cr := resp.Results[l-1]
-			lastSeries := cr.Series[len(cr.Series)-1]
 			rowsMerged := 0
+			if len(cr.Series) > 0 {
+				lastSeries := cr.Series[len(cr.Series)-1]
 
-			for _, row := range r.Series {
-				if !lastSeries.SameSeries(row) {
-					// Next row is for a different series than last.
-					break
+				for _, row := range r.Series {
+					if !lastSeries.SameSeries(row) {
+						// Next row is for a different series than last.
+						break
+					}
+					// Values are for the same series, so append them.
+					lastSeries.Values = append(lastSeries.Values, row.Values...)
+					rowsMerged++
 				}
-				// Values are for the same series, so append them.
-				lastSeries.Values = append(lastSeries.Values, row.Values...)
-				rowsMerged++
 			}
 
 			// Append remaining rows as new rows.
diff --git a/services/httpd/handler_test.go b/services/httpd/handler_test.go
index 9702821b64..48576dfcca 100644
--- a/services/httpd/handler_test.go
+++ b/services/httpd/handler_test.go
@@ -195,6 +195,25 @@ func TestHandler_Query_MergeResults(t *testing.T) {
 	}
 }
 
+// Ensure the handler merges results from the same statement.
+func TestHandler_Query_MergeEmptyResults(t *testing.T) {
+	h := NewHandler(false)
+	h.QueryExecutor.ExecuteQueryFn = func(q *influxql.Query, db string, chunkSize int, closing chan struct{}) (<-chan *influxql.Result, error) {
+		return NewResultChan(
+			&influxql.Result{StatementID: 1, Series: models.Rows{}},
+			&influxql.Result{StatementID: 1, Series: models.Rows([]*models.Row{{Name: "series1"}})},
+		), nil
+	}
+
+	w := httptest.NewRecorder()
+	h.ServeHTTP(w, MustNewJSONRequest("GET", "/query?db=foo&q=SELECT+*+FROM+bar", nil))
+	if w.Code != http.StatusOK {
+		t.Fatalf("unexpected status: %d", w.Code)
+	} else if w.Body.String() != `{"results":[{"series":[{"name":"series1"}]}]}` {
+		t.Fatalf("unexpected body: %s", w.Body.String())
+	}
+}
+
 // Ensure the handler can parse chunked and chunk size query parameters.
 func TestHandler_Query_Chunked(t *testing.T) {
 	h := NewHandler(false)