feat(platform): add PackageManager interface and Pacman implementation

internal/platform/pacman.go

@@ -0,0 +1,74 @@
+package platform
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"os/exec"
+	"strings"
+)
+
+// ExecRunner runs commands via os/exec. This is the production Runner.
+type ExecRunner struct{}
+
+func (ExecRunner) Run(ctx context.Context, name string, args ...string) error {
+	cmd := exec.CommandContext(ctx, name, args...)
+	cmd.Stdout = nil
+	cmd.Stderr = nil
+	return cmd.Run()
+}
+
+func (ExecRunner) Output(ctx context.Context, name string, args ...string) ([]byte, error) {
+	cmd := exec.CommandContext(ctx, name, args...)
+	var stderr bytes.Buffer
+	cmd.Stderr = &stderr
+	out, err := cmd.Output()
+	if err != nil {
+		return out, fmt.Errorf("%s %s: %w: %s", name, strings.Join(args, " "), err, stderr.String())
+	}
+	return out, nil
+}
+
+// Pacman implements PackageManager via `pacman` on Arch.
+type Pacman struct {
+	Runner Runner
+}
+
+func NewPacman(r Runner) *Pacman {
+	if r == nil {
+		r = ExecRunner{}
+	}
+	return &Pacman{Runner: r}
+}
+
+func (Pacman) Name() string { return "pacman" }
+
+// IsInstalled returns true when `pacman -Q <pkg>` succeeds.
+func (p Pacman) IsInstalled(ctx context.Context, pkg string) (bool, error) {
+	if pkg == "" {
+		return false, errors.New("empty package name")
+	}
+	_, err := p.Runner.Output(ctx, "pacman", "-Q", pkg)
+	if err == nil {
+		return true, nil
+	}
+	// pacman -Q exits 1 when not installed; we treat any error as "not installed"
+	// because false-positives on transient errors here are harmless (Install
+	// is idempotent via --needed).
+	return false, nil
+}
+
+// Install runs `sudo pacman -S --needed --noconfirm <pkgs...>`.
+func (p Pacman) Install(ctx context.Context, pkgs []string) error {
+	if len(pkgs) == 0 {
+		return nil
+	}
+	args := append([]string{"pacman", "-S", "--needed", "--noconfirm"}, pkgs...)
+	return p.Runner.Run(ctx, "sudo", args...)
+}
+
+// Update runs `sudo pacman -Sy`.
+func (p Pacman) Update(ctx context.Context) error {
+	return p.Runner.Run(ctx, "sudo", "pacman", "-Sy")
+}