chore: implement VolumesConstraint to only schedule on machines with required volumes

pkg/client/deploy/scheduler/constraint.go

@@ -1,8 +1,10 @@
 package scheduler
 
 import (
+	"slices"
 	"strings"
 
+	"github.com/docker/docker/api/types/volume"
 	"github.com/psviderski/uncloud/pkg/api"
 )
 
@@ -15,6 +17,7 @@ type Constraint interface {
 	Description() string
 }
 
+// constraintsFromSpec derives scheduling constraints from the service specification.
 func constraintsFromSpec(spec api.ServiceSpec) []Constraint {
 	var constraints []Constraint
 
@@ -24,7 +27,18 @@ func constraintsFromSpec(spec api.ServiceSpec) []Constraint {
 		})
 	}
 
-	// TODO: inspect and add VolumeConstraint.
+	// Add a VolumesConstraint for named Docker volumes that are mounted in the container.
+	var volumes []api.VolumeSpec
+	for _, m := range spec.Container.VolumeMounts {
+		if v, ok := spec.Volume(m.VolumeName); ok && v.Type == api.VolumeTypeVolume {
+			volumes = append(volumes, v)
+		}
+	}
+	if len(volumes) > 0 {
+		constraints = append(constraints, &VolumesConstraint{
+			Volumes: volumes,
+		})
+	}
 
 	return constraints
 }
@@ -45,5 +59,51 @@ func (c *PlacementConstraint) Evaluate(machine *Machine) bool {
 }
 
 func (c *PlacementConstraint) Description() string {
+	slices.Sort(c.Machines)
 	return "Placement constraint by machines: " + strings.Join(c.Machines, ", ")
 }
+
+// VolumesConstraint restricts container placement to machines that have the required named Docker volumes.
+type VolumesConstraint struct {
+	// Volumes is a list of named Docker volumes of type api.VolumeTypeVolume that must exist on the machine.
+	Volumes []api.VolumeSpec
+}
+
+// Evaluate determines if a machine has all the required volumes.
+// Returns true if all required volumes exist on the machine or if there are no required volumes.
+func (c *VolumesConstraint) Evaluate(machine *Machine) bool {
+	if len(c.Volumes) == 0 {
+		return true
+	}
+
+	for _, v := range c.Volumes {
+		if v.Type != api.VolumeTypeVolume {
+			continue
+		}
+
+		// TODO: should we check the volume driver to be local or any matched volume by name is ok?
+		if !slices.ContainsFunc(machine.Volumes, func(vol volume.Volume) bool {
+			return vol.Name == v.DockerVolumeName()
+		}) {
+			return false
+		}
+	}
+
+	return true
+}
+
+func (c *VolumesConstraint) Description() string {
+	volumeNames := make([]string, 0, len(c.Volumes))
+	for _, v := range c.Volumes {
+		if v.Type == api.VolumeTypeVolume {
+			volumeNames = append(volumeNames, v.DockerVolumeName())
+		}
+	}
+	slices.Sort(volumeNames)
+
+	if len(volumeNames) == 0 {
+		return "No volumes constraint"
+	}
+
+	return "Volumes: " + strings.Join(volumeNames, ", ")
+}

pkg/client/deploy/scheduler/service.go

@@ -53,5 +53,7 @@ func (s *ServiceScheduler) evaluateConstraints(machine *Machine) bool {
 }
 
 func (s *ServiceScheduler) ScheduleContainer() ([]*pb.MachineInfo, error) {
+	// TODO: organise machines in a heap and supply a sort function from the strategy. Each scheduled container
+	//  should update the machine and reorder it in the heap.
 	return nil, errors.New("not implemented")
 }