Prometheus: Fix creation of invalid dataframes with exemplars (#75187)

for #73654

pkg/util/converter/prom.go

@@ -387,6 +387,7 @@ l1Fields:
 			frame.Meta = &data.FrameMeta{
 				Custom: resultTypeToCustomMeta("exemplar"),
 			}
+			exCount := 0
 			for more, err := iter.ReadArray(); more; more, err = iter.ReadArray() {
 				if err != nil {
 					return nil, nil, err
@@ -417,7 +418,6 @@ l1Fields:
 						timeField.Append(ts)
 
 					case "labels":
-						max := 0
 						pairs, err := readLabelsAsPairs(iter)
 						if err != nil {
 							return nil, nil, err
@@ -427,20 +427,17 @@ l1Fields:
 							v := pair[1]
 							f, ok := lookup[k]
 							if !ok {
-								f = data.NewFieldFromFieldType(data.FieldTypeString, 0)
+								f = data.NewFieldFromFieldType(data.FieldTypeString, exCount)
 								f.Name = k
 								lookup[k] = f
 								frame.Fields = append(frame.Fields, f)
 							}
 							f.Append(v)
-							if f.Len() > max {
-								max = f.Len()
-							}
 						}
 
 						// Make sure all fields have equal length
 						for _, f := range lookup {
-							diff := max - f.Len()
+							diff := exCount + 1 - f.Len()
 							if diff > 0 {
 								f.Extend(diff)
 							}
@@ -457,6 +454,7 @@ l1Fields:
 						})
 					}
 				}
+				exCount++
 			}
 		case "":
 			if err != nil {

pkg/util/converter/prom_test.go

@@ -32,6 +32,7 @@ var files = []string{
 	"prom-error",
 	"prom-exemplars-a",
 	"prom-exemplars-b",
+	"prom-exemplars-diff-labels",
 	"loki-streams-a",
 	"loki-streams-b",
 	"loki-streams-c",

pkg/util/converter/testdata/prom-exemplars-diff-labels-frame.jsonc

@@ -0,0 +1,93 @@
+//  🌟 This was machine generated.  Do not edit. 🌟
+//  
+//  Frame[0] {
+//      "typeVersion": [
+//          0,
+//          0
+//      ],
+//      "custom": {
+//          "resultType": "exemplar"
+//      }
+//  }
+//  Name: 
+//  Dimensions: 4 Fields by 2 Rows
+//  +-----------------------------------+-----------------+------------------+------------------+
+//  | Name: Time                        | Name: Value     | Name: traceID    | Name: ztraceID   |
+//  | Labels:                           | Labels:         | Labels:          | Labels:          |
+//  | Type: []time.Time                 | Type: []float64 | Type: []string   | Type: []string   |
+//  +-----------------------------------+-----------------+------------------+------------------+
+//  | 2020-09-14 15:22:35.479 +0000 UTC | 19              | Olp9XHlq763ccsfa |                  |
+//  | 2020-09-14 15:22:45.489 +0000 UTC | 20              |                  | hCtjygkIHwAN9vs4 |
+//  +-----------------------------------+-----------------+------------------+------------------+
+//  
+//  
+//  🌟 This was machine generated.  Do not edit. 🌟
+{
+  "status": 200,
+  "frames": [
+    {
+      "schema": {
+        "meta": {
+          "typeVersion": [
+            0,
+            0
+          ],
+          "custom": {
+            "resultType": "exemplar"
+          }
+        },
+        "fields": [
+          {
+            "name": "Time",
+            "type": "time",
+            "typeInfo": {
+              "frame": "time.Time"
+            }
+          },
+          {
+            "name": "Value",
+            "type": "number",
+            "typeInfo": {
+              "frame": "float64"
+            },
+            "labels": {}
+          },
+          {
+            "name": "traceID",
+            "type": "string",
+            "typeInfo": {
+              "frame": "string"
+            }
+          },
+          {
+            "name": "ztraceID",
+            "type": "string",
+            "typeInfo": {
+              "frame": "string"
+            }
+          }
+        ]
+      },
+      "data": {
+        "values": [
+          [
+            1600096955479,
+            1600096965489
+          ],
+          [
+            19,
+            20
+          ],
+          [
+            "Olp9XHlq763ccsfa",
+            ""
+          ],
+          [
+            "",
+            "hCtjygkIHwAN9vs4"
+          ]
+        ]
+      }
+    }
+  ]
+}

pkg/util/converter/testdata/prom-exemplars-diff-labels.json

@@ -0,0 +1,23 @@
+{
+    "status": "success",
+    "data": [
+        {
+            "exemplars": [
+                {
+                    "labels": {
+                        "traceID": "Olp9XHlq763ccsfa"
+                    },
+                    "value": "19",
+                    "timestamp": 1600096955.479
+                },
+                {
+                    "labels": {
+                        "ztraceID": "hCtjygkIHwAN9vs4"
+                    },
+                    "value": "20",
+                    "timestamp": 1600096965.489
+                }
+            ]
+        }
+    ]
+}