functional: a new test TestScheduleReplace

TestScheduleReplace starts 1 unit, followed by starting another unit
that replaces the 1st unit. Then it verifies that the 2 units are
started on different machines.

functional/fixtures/units/replace.0.service

@@ -0,0 +1,5 @@
+[Unit]
+Description=Test Unit
+
+[Service]
+ExecStart=/bin/bash -c "while true; do echo Hello, World!; sleep 1; done"

functional/fixtures/units/replace.1.service

@@ -0,0 +1,8 @@
+[Unit]
+Description=Test Unit
+
+[Service]
+ExecStart=/bin/bash -c "while true; do echo Hello, World!; sleep 1; done"
+
+[X-Fleet]
+Replaces=replace.0.service

functional/scheduling_test.go

@@ -17,6 +17,7 @@ package functional
 import (
 	"fmt"
 	"os"
+	"path"
 	"path/filepath"
 	"strings"
 	"testing"
@@ -320,6 +321,75 @@ func TestScheduleOneWayConflict(t *testing.T) {
 
 }
 
+// TestScheduleReplace starts 1 unit, followed by starting another unit
+// that replaces the 1st unit. Then it verifies that the 2 units are
+// started on different machines.
+func TestScheduleReplace(t *testing.T) {
+	cluster, err := platform.NewNspawnCluster("smoke")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer cluster.Destroy(t)
+
+	// Start with a simple three-node cluster
+	members, err := platform.CreateNClusterMembers(cluster, 2)
+	if err != nil {
+		t.Fatal(err)
+	}
+	m0 := members[0]
+	if _, err := cluster.WaitForNMachines(m0, 2); err != nil {
+		t.Fatal(err)
+	}
+
+	// Start a unit without Replaces
+	uName0 := "fixtures/units/replace.0.service"
+	if stdout, stderr, err := cluster.Fleetctl(m0, "start", "--no-block", uName0); err != nil {
+		t.Fatalf("Failed starting unit %s: \nstdout: %s\nstderr: %s\nerr: %v", uName0, stdout, stderr, err)
+	}
+
+	active, err := cluster.WaitForNActiveUnits(m0, 1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = util.ActiveToSingleStates(active)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Start a unit that replaces the former one, replace.0.service
+	uName1 := "fixtures/units/replace.1.service"
+	if stdout, stderr, err := cluster.Fleetctl(m0, "start", "--no-block", uName1); err != nil {
+		t.Fatalf("Failed starting unit %s: \nstdout: %s\nstderr: %s\nerr: %v", uName1, stdout, stderr, err)
+	}
+
+	// Both units should show up
+	stdout, _, err := cluster.Fleetctl(m0, "list-unit-files", "--no-legend")
+	if err != nil {
+		t.Fatalf("Failed to run list-unit-files: %v", err)
+	}
+	units := strings.Split(strings.TrimSpace(stdout), "\n")
+	if len(units) != 2 {
+		t.Fatalf("Did not find two units in cluster: \n%s", stdout)
+	}
+	active, err = cluster.WaitForNActiveUnits(m0, 2)
+	if err != nil {
+		t.Fatal(err)
+	}
+	states, err := util.ActiveToSingleStates(active)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// unit replace.1.service must be located on a different machine from 0
+	uNameBase0 := path.Base(uName0)
+	uNameBase1 := path.Base(uName1)
+	mach0 := states[uNameBase0].Machine
+	mach1 := states[uNameBase1].Machine
+	if mach0 == mach1 {
+		t.Fatalf("machine for %s is %s, the same as that of %s.", uNameBase0, mach0, uNameBase1)
+	}
+}
+
 // Ensure units can be scheduled directly to a given machine using the
 // MachineID unit option.
 func TestScheduleConditionMachineID(t *testing.T) {