consul/connect: use additional constraints in scheduling connect tasks

This PR adds two additional constraints on Connect sidecar and gateway tasks,
making sure Nomad schedules them only onto nodes where Connect is actually
enabled on the Consul agent.

Consul requires `connect.enabled = true` and `ports.grpc = <number>` to be
explicitly set on agent configuration before Connect APIs will work. Until
now, Nomad would only validate a minimum version of Consul, which would cause
confusion for users who try to run Connect tasks on nodes where Consul is not
yet sufficiently configured. These contstraints prevent job scheduling on nodes
where Connect is not actually use-able.

Closes #10700

nomad/job_endpoint_hook_connect.go

@@ -89,6 +89,22 @@ func connectGatewayVersionConstraint() *structs.Constraint {
 	}
 }
 
+func connectEnabledConstraint() *structs.Constraint {
+	return &structs.Constraint{
+		LTarget: "${attr.consul.connect}",
+		RTarget: "true",
+		Operand: "=",
+	}
+}
+
+func connectListenerConstraint() *structs.Constraint {
+	return &structs.Constraint{
+		LTarget: "${attr.consul.grpc}",
+		RTarget: "0",
+		Operand: ">",
+	}
+}
+
 // jobConnectHook implements a job Mutating and Validating admission controller
 type jobConnectHook struct{}
 
@@ -414,6 +430,8 @@ func newConnectGatewayTask(prefix, service string, netHost bool) *structs.Task {
 		Resources: connectSidecarResources(),
 		Constraints: structs.Constraints{
 			connectGatewayVersionConstraint(),
+			connectEnabledConstraint(),
+			connectListenerConstraint(),
 		},
 	}
 }
@@ -437,6 +455,8 @@ func newConnectSidecarTask(service string) *structs.Task {
 		},
 		Constraints: structs.Constraints{
 			connectSidecarVersionConstraint(),
+			connectEnabledConstraint(),
+			connectListenerConstraint(),
 		},
 	}
 }

nomad/structs/structs.go

@@ -8234,15 +8234,15 @@ const (
 	ConstraintAttributeIsNotSet = "is_not_set"
 )
 
-// Constraints are used to restrict placement options.
+// A Constraint is used to restrict placement options.
 type Constraint struct {
 	LTarget string // Left-hand target
 	RTarget string // Right-hand target
 	Operand string // Constraint operand (<=, <, =, !=, >, >=), contains, near
 	str     string // Memoized string
 }
 
-// Equal checks if two constraints are equal
+// Equals checks if two constraints are equal
 func (c *Constraint) Equals(o *Constraint) bool {
 	return c == o ||
 		c.LTarget == o.LTarget &&