For many-to-one matches, always copy label from one side.

This is a breaking change for everyone using the machine roles
labeling approach.

promql/engine.go

@@ -942,6 +942,8 @@ func resultMetric(lhs, rhs metric.Metric, op itemType, matching *VectorMatching)
 		// Included labels from the `group_x` modifier are taken from the "one"-side .
 		if v, ok := rhs.Metric[ln]; ok {
 			m[ln] = v
+		} else {
+			delete(m, ln)
 		}
 	}
 	return metric.Metric{Metric: m, Copied: false}

promql/parse.go

@@ -479,8 +479,6 @@ func (p *parser) expr() Expr {
 				}
 				if p.peek().typ == itemLeftParen {
 					vecMatching.Include = p.labels()
-				} else if !vecMatching.Ignoring {
-					p.errorf("must specify labels in INCLUDE clause when using ON")
 				}
 			}
 		}

promql/parse_test.go

@@ -250,10 +250,6 @@ var testExpr = []struct {
 		input:  "1 offset 1d",
 		fail:   true,
 		errMsg: "offset modifier must be preceded by an instant or range selector",
-	}, {
-		input:  "a - on(b) group_left d",
-		fail:   true,
-		errMsg: "must specify labels in INCLUDE clause when using ON",
 	}, {
 		input:  "a - on(b) ignoring(c) d",
 		fail:   true,
@@ -504,6 +500,27 @@ var testExpr = []struct {
 				On:   model.LabelNames{"test", "blub"},
 			},
 		},
+	}, {
+		input: "foo * on(test,blub) group_left bar",
+		expected: &BinaryExpr{
+			Op: itemMUL,
+			LHS: &VectorSelector{
+				Name: "foo",
+				LabelMatchers: metric.LabelMatchers{
+					{Type: metric.Equal, Name: model.MetricNameLabel, Value: "foo"},
+				},
+			},
+			RHS: &VectorSelector{
+				Name: "bar",
+				LabelMatchers: metric.LabelMatchers{
+					{Type: metric.Equal, Name: model.MetricNameLabel, Value: "bar"},
+				},
+			},
+			VectorMatching: &VectorMatching{
+				Card: CardManyToOne,
+				On:   model.LabelNames{"test", "blub"},
+			},
+		},
 	}, {
 		input: "foo and on(test,blub) bar",
 		expected: &BinaryExpr{
@@ -737,10 +754,6 @@ var testExpr = []struct {
 		input:  "foo unless on(bar) group_right(baz) bar",
 		fail:   true,
 		errMsg: "no grouping allowed for \"unless\" operation",
-	}, {
-		input:  `http_requests{group="production"} / on(instance) group_left cpu_count{type="smp"}`,
-		fail:   true,
-		errMsg: "parse error at char 61: must specify labels in INCLUDE clause when using ON",
 	}, {
 		input:  `http_requests{group="production"} + on(instance) group_left(job,instance) cpu_count{type="smp"}`,
 		fail:   true,

promql/testdata/operators.test

@@ -227,10 +227,10 @@ load 5m
   random{foo="bar"} 1
 
 # Copy machine role to node variable.
-eval instant at 5m node_role * on (instance,job) group_left (role) node_var
+eval instant at 5m node_role * on (instance) group_right (role) node_var
   {instance="abc",job="node",role="prometheus"} 2
 
-eval instant at 5m node_var * on (instance,job) group_right (role) node_role
+eval instant at 5m node_var * on (instance) group_left (role) node_role
   {instance="abc",job="node",role="prometheus"} 2
 
 eval instant at 5m node_var * ignoring (role) group_left (role) node_role
@@ -244,19 +244,23 @@ eval instant at 5m node_cpu * ignoring (role, mode) group_left (role) node_role
   {instance="abc",job="node",mode="idle",role="prometheus"} 3
   {instance="abc",job="node",mode="user",role="prometheus"} 1
 
+eval instant at 5m node_cpu * on (instance) group_left (role) node_role
+  {instance="abc",job="node",mode="idle",role="prometheus"} 3
+  {instance="abc",job="node",mode="user",role="prometheus"} 1
+
 
 # Ratio of total.
-eval instant at 5m node_cpu / on (instance,job) group_left (mode) sum by (instance,job)(node_cpu)
+eval instant at 5m node_cpu / on (instance) group_left sum by (instance,job)(node_cpu)
   {instance="abc",job="node",mode="idle"} .75
   {instance="abc",job="node",mode="user"} .25
   {instance="def",job="node",mode="idle"} .80
   {instance="def",job="node",mode="user"} .20
 
-eval instant at 5m sum by (mode, job)(node_cpu) / on (job) group_left (mode) sum by (job)(node_cpu)
+eval instant at 5m sum by (mode, job)(node_cpu) / on (job) group_left sum by (job)(node_cpu)
   {job="node",mode="idle"} 0.7857142857142857
   {job="node",mode="user"} 0.21428571428571427
 
-eval instant at 5m sum(sum by (mode, job)(node_cpu) / on (job) group_left (mode) sum by (job)(node_cpu))
+eval instant at 5m sum(sum by (mode, job)(node_cpu) / on (job) group_left sum by (job)(node_cpu))
   {} 1.0