Fix scalar-vector comparisons (#5454)

* Fix scalar-vector comparisons

Fixes https://github.com/prometheus/prometheus/issues/5452

Signed-off-by: Julius Volz <julius.volz@gmail.com>

CHANGELOG.md

@@ -24,6 +24,7 @@
 * [BUGFIX] Set TLSHandshakeTimeout in HTTP transport. common#179
 * [BUGFIX] Use fsync to be more resilient to machine crashes. tsdb#573 tsdb#578
 * [BUGFIX] Keep series that are still in WAL in checkpoints. tsdb#577
+* [BUGFIX] Fix output sample values for scalar-to-vector comparison operations. #5454
 
 ## 2.8.1 / 2019-03-28
 
@@ -195,8 +196,8 @@ This release includes multiple bugfixes and features. Further, the WAL implement
 * [FEATURE] Persist alert 'for' state across restarts #4061
 * [FEATURE] Add API providing per target metric metadata #4183
 * [FEATURE] Add API providing recording and alerting rules #4318 #4501
-* [ENHANCEMENT] Brand new WAL implementation for TSDB. Forwards incompatible with previous WAL. 
-* [ENHANCEMENT] Show rule evaluation errors in UI #4457 
+* [ENHANCEMENT] Brand new WAL implementation for TSDB. Forwards incompatible with previous WAL.
+* [ENHANCEMENT] Show rule evaluation errors in UI #4457
 * [ENHANCEMENT] Throttle resends of alerts to Alertmanager #4538
 * [ENHANCEMENT] Send EndsAt along with the alert to Alertmanager #4550
 * [ENHANCEMENT] Limit the samples returned by remote read endpoint #4532

promql/engine.go

@@ -1581,6 +1581,11 @@ func (ev *evaluator) VectorscalarBinop(op ItemType, lhs Vector, rhs Scalar, swap
 			lv, rv = rv, lv
 		}
 		value, keep := vectorElemBinop(op, lv, rv)
+		// Catch cases where the scalar is the LHS in a scalar-vector comparison operation.
+		// We want to always keep the vector element value as the output value, even if it's on the RHS.
+		if op.isComparisonOperator() && swap {
+			value = rv
+		}
 		if returnBool {
 			if keep {
 				value = 1.0

promql/testdata/operators.test

@@ -220,7 +220,7 @@ eval instant at 50m SUM(http_requests) BY (job) > 1000
 	{job="app-server"} 2600
 
 eval instant at 50m 1000 < SUM(http_requests) BY (job)
-	{job="app-server"} 1000
+	{job="app-server"} 2600
 
 eval instant at 50m SUM(http_requests) BY (job) <= 1000
 	{job="api-server"} 1000
@@ -388,7 +388,7 @@ eval instant at 5m node_cpu % 2
   {instance="def",job="node",mode="idle"} 0
   {instance="def",job="node",mode="user"} 0
 
-	
+
 clear
 
 load 5m
@@ -414,4 +414,4 @@ load 5m
   testmetric1{src="a",dst="b"} 0
   testmetric2{src="a",dst="b"} 1
 
-eval_fail instant at 0m -{__name__=~'testmetric1|testmetric2'}
+eval_fail instant at 0m -{__name__=~'testmetric1|testmetric2'}