no $sampleRate

Author: FGasper

internal/verifier/partition.go

@@ -103,7 +103,18 @@ func (verifier *Verifier) createPartitionTasks(
 	if sampleRate > 0 && sampleRate < 1 {
 		pipeline = append(
 			pipeline,
-			bson.D{{"$match", bson.D{{"$sampleRate", sampleRate}}}},
+			bson.D{
+				{"$match", bson.D{
+					// This is just $sampleRate, but 4.2 doesn’t recognize
+					// that, so we write it out:
+					{"$expr", bson.D{
+						{"$lt", bson.A{
+							bson.D{{"$rand", bson.D{}}},
+							sampleRate,
+						}},
+					}},
+				}},
+			},
 		)
 	}