fix: address review findings for self-update (#15)

internal/agent/updater/compose.go

@@ -6,6 +6,7 @@ import (
 	"io"
 	"os"
 	"os/exec"
+	"slices"
 	"strings"
 	"time"
 
@@ -98,10 +99,6 @@ func NewComposeExecutor(cli ComposeDocker, resolve *Resolver, selfID string, run
 // When the service is the agent's own container, the recreate runs in a
 // helper container instead (see selfUpdate), because it stops this process.
 func (e *ComposeExecutor) Execute(ctx context.Context, job Job, out io.Writer) (string, error) {
-	// What the job has printed so far goes into the helper's labels.
-	soFar := &tailBuffer{max: selfUpdateOutputMax}
-	out = io.MultiWriter(out, tailWriter{soFar})
-
 	project, service := job.ComposeProject, job.ComposeService
 	cts, err := e.cli.ContainerList(ctx, container.ListOptions{
 		All: true,
@@ -116,14 +113,23 @@ func (e *ComposeExecutor) Execute(ctx context.Context, job Job, out io.Writer) (
 	if len(cts) == 0 {
 		return "", fmt.Errorf("no container for compose service %s/%s on this machine", project, service)
 	}
-	workingDir := cts[0].Labels[labelWorkingDir]
-	configFiles := cts[0].Labels[labelConfigFiles]
-	envFileLabel := cts[0].Labels[labelEnvFile]
-
 	self, err := inspectSelf(ctx, e.cli, e.selfID)
 	if err != nil {
 		return "", err
 	}
+	isSelf := self.ContainerJSONBase != nil && self.ID != "" &&
+		slices.ContainsFunc(cts, func(ct container.Summary) bool { return ct.ID == self.ID })
+	// What an update of the agent's own service has printed so far goes into
+	// the helper's labels.
+	var soFar *tailBuffer
+	if isSelf {
+		soFar = &tailBuffer{max: selfUpdateOutputMax}
+		out = io.MultiWriter(out, tailWriter{soFar})
+	}
+	workingDir := cts[0].Labels[labelWorkingDir]
+	configFiles := cts[0].Labels[labelConfigFiles]
+	envFileLabel := cts[0].Labels[labelEnvFile]
+
 	roots := rootsOf(self)
 	files, dir, err := composeFiles(roots, configFiles)
 	if err != nil {
@@ -170,22 +176,29 @@ func (e *ComposeExecutor) Execute(ctx context.Context, job Job, out io.Writer) (
 	}
 
 	base := composeBase(project, workingDir, files, envs)
-	isSelf := false
-	if self.ContainerJSONBase != nil && self.ID != "" {
-		for _, ct := range cts {
-			isSelf = isSelf || ct.ID == self.ID
-		}
-	}
-	for _, step := range [][]string{{"pull", service}, {"up", "-d", service}} {
-		args := append(append([]string(nil), base...), step...)
-		if isSelf && step[0] == "up" {
-			return e.selfUpdate(ctx, job, self, workingDir, args, pinned, out, soFar)
-		}
+	run := func(args []string) error {
 		fmt.Fprintf(out, "$ docker %s\n", strings.Join(args, " "))
 		tail := &tailBuffer{max: errorOutputTail}
 		if err := e.run(ctx, workingDir, io.MultiWriter(out, tailWriter{tail}), args...); err != nil {
-			return "", fmt.Errorf("docker compose %s: %w (output: %s)", step[0], err, strings.TrimSpace(tail.String()))
+			return composeError(args[len(base)], err, tail)
 		}
+		return nil
+	}
+	if err := run(append(append([]string(nil), base...), "pull", service)); err != nil {
+		return "", err
+	}
+	up := append(append([]string(nil), base...), "up", "-d", service)
+	if isSelf {
+		return e.selfUpdate(ctx, job, self, workingDir, up, pinned, out, soFar)
+	}
+	if err := run(up); err != nil {
+		return "", err
 	}
 	return pinned, nil
 }
+
+// composeError describes a failed `docker compose <step>` call with the tail
+// of its output.
+func composeError(step string, err error, tail *tailBuffer) error {
+	return fmt.Errorf("docker compose %s: %w (output: %s)", step, err, strings.TrimSpace(tail.String()))
+}