Honor value for distinct_hosts constraint

jobspec2/hcl_conversions.go

@@ -235,8 +235,11 @@ func decodeConstraint(body hcl.Body, ctx *hcl.EvalContext, val interface{}) hcl.
 		c.RTarget = constraint
 	}
 
-	if d := v.GetAttr(api.ConstraintDistinctHosts); !d.IsNull() && d.True() {
+	// The shortcut form of the distinct_hosts constraint is a cty.Bool
+	// so it can not use the `attr` func defined earlier
+	if d := v.GetAttr(api.ConstraintDistinctHosts); !d.IsNull() {
 		c.Operand = api.ConstraintDistinctHosts
+		c.RTarget = fmt.Sprint(d.True())
 	}
 
 	if property := attr(api.ConstraintDistinctProperty); property != "" {

scheduler/feasible.go

@@ -572,7 +572,14 @@ func (iter *DistinctHostsIterator) SetJob(job *structs.Job) {
 func (iter *DistinctHostsIterator) hasDistinctHostsConstraint(constraints []*structs.Constraint) bool {
 	for _, con := range constraints {
 		if con.Operand == structs.ConstraintDistinctHosts {
-			return true
+			// Maintain the old behavior around unset RTargets
+			if con.RTarget == "" {
+				return true
+			}
+			enabled, err := strconv.ParseBool(con.RTarget)
+			// If the value is unparsable as a boolean, fall back to the old behavior
+			// of enforcing the constraint when it appears.
+			return err != nil || enabled
 		}
 	}