remove obsolete containers first on scale down

Signed-off-by: Nicolas De Loof <nicolas.deloof@gmail.com>

pkg/compose/convergence.go

@@ -20,6 +20,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"slices"
 	"sort"
 	"strconv"
 	"strings"
@@ -136,16 +137,18 @@ func (c *convergence) ensureService(ctx context.Context, project *types.Project,
 		ni, erri := strconv.Atoi(containers[i].Labels[api.ContainerNumberLabel])
 		nj, errj := strconv.Atoi(containers[j].Labels[api.ContainerNumberLabel])
 		if erri == nil && errj == nil {
-			return ni < nj
+			return ni > nj
 		}
 
 		// If we don't get a container number (?) just sort by creation date
 		return containers[i].Created < containers[j].Created
 	})
 
+	slices.Reverse(containers)
 	for i, container := range containers {
 		if i >= expected {
 			// Scale Down
+			// As we sorted containers, obsolete ones and/or highest number will be removed
 			container := container
 			traceOpts := append(tracing.ServiceOptions(service), tracing.ContainerOptions(container)...)
 			eg.Go(tracing.SpanWrapFuncForErrGroup(ctx, "service/scale/down", traceOpts, func(ctx context.Context) error {

pkg/e2e/fixtures/scale/Dockerfile

@@ -0,0 +1,17 @@
+#   Copyright 2020 Docker Compose CLI authors
+
+#   Licensed under the Apache License, Version 2.0 (the "License");
+#   you may not use this file except in compliance with the License.
+#   You may obtain a copy of the License at
+
+#       http://www.apache.org/licenses/LICENSE-2.0
+
+#   Unless required by applicable law or agreed to in writing, software
+#   distributed under the License is distributed on an "AS IS" BASIS,
+#   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#   See the License for the specific language governing permissions and
+#   limitations under the License.
+
+FROM nginx:alpine
+ARG FOO
+LABEL FOO=$FOO

pkg/e2e/fixtures/scale/build.yaml

@@ -0,0 +1,3 @@
+services:
+  test:
+    build: .

pkg/e2e/scale_test.go

@@ -184,3 +184,34 @@ func checkServiceContainer(t *testing.T, stdout, containerName, containerState s
 	}
 	testify.Fail(t, errMessage, stdout)
 }
+
+func TestScaleDownNoRecreate(t *testing.T) {
+	const projectName = "scale-down-recreated-test"
+	c := NewCLI(t, WithEnv(
+		"COMPOSE_PROJECT_NAME="+projectName))
+
+	reset := func() {
+		c.RunDockerComposeCmd(t, "down", "--rmi", "all")
+	}
+	t.Cleanup(reset)
+	c.RunDockerComposeCmd(t, "-f", "fixtures/scale/build.yaml", "build", "--build-arg", "FOO=test")
+	c.RunDockerComposeCmd(t, "-f", "fixtures/scale/build.yaml", "up", "-d", "--scale", "test=2")
+
+	c.RunDockerComposeCmd(t, "-f", "fixtures/scale/build.yaml", "build", "--build-arg", "FOO=updated")
+	c.RunDockerComposeCmd(t, "-f", "fixtures/scale/build.yaml", "up", "-d", "--scale", "test=4", "--no-recreate")
+
+	res := c.RunDockerComposeCmd(t, "ps", "--format", "{{.Name}}", "test")
+	res.Assert(t, icmd.Success)
+	assert.Check(t, strings.Contains(res.Stdout(), "scale-down-recreated-test-test-1"))
+	assert.Check(t, strings.Contains(res.Stdout(), "scale-down-recreated-test-test-2"))
+	assert.Check(t, strings.Contains(res.Stdout(), "scale-down-recreated-test-test-3"))
+	assert.Check(t, strings.Contains(res.Stdout(), "scale-down-recreated-test-test-4"))
+
+	t.Log("scale down removes obsolete replica #1 and #2")
+	c.NewDockerComposeCmd(t, "--project-directory", "fixtures/scale", "up", "-d", "--scale", "test=2")
+
+	res = c.RunDockerComposeCmd(t, "ps", "--format", "{{.Name}}", "test")
+	res.Assert(t, icmd.Success)
+	assert.Check(t, strings.Contains(res.Stdout(), "scale-down-recreated-test-test-3"))
+	assert.Check(t, strings.Contains(res.Stdout(), "scale-down-recreated-test-test-4"))
+}