system updater: complete overhaul

Move system update flow to GO, with a CLI (convenient AIO tool) and
server integration. All lifecycle, scheduling, execution occurs on
backend side.

Run some backends via pkexec, some via terminal like paru/yay.

Incorporate flatpak as an option to update.

Add terminal override setting in GUI, in addition to $TERMINAL env
variable.

fixes #2307
fixes #822
fixes #1102
fixes #1812
fixes #1087
fixes #1743

core/internal/server/sysupdate/backend_apt.go

@@ -0,0 +1,75 @@
+package sysupdate
+
+import (
+	"context"
+	"os/exec"
+	"regexp"
+	"strings"
+)
+
+func init() {
+	RegisterSystemBackend(func() Backend { return &aptBackend{} })
+}
+
+var aptUpgradableLine = regexp.MustCompile(`^([^/]+)/\S+\s+(\S+)\s+\S+\s+\[upgradable from:\s+([^\]]+)\]`)
+
+type aptBackend struct{}
+
+func (aptBackend) ID() string           { return "apt" }
+func (aptBackend) DisplayName() string  { return "APT" }
+func (aptBackend) Repo() RepoKind       { return RepoSystem }
+func (aptBackend) NeedsAuth() bool      { return true }
+func (aptBackend) RunsInTerminal() bool { return false }
+func (aptBackend) IsAvailable(_ context.Context) bool {
+	return commandExists("apt") || commandExists("apt-get")
+}
+
+func (aptBackend) CheckUpdates(ctx context.Context) ([]Package, error) {
+	cmd := exec.CommandContext(ctx, "apt", "list", "--upgradable")
+	cmd.Env = append(cmd.Environ(), "LC_ALL=C")
+	out, err := cmd.Output()
+	if err != nil {
+		return nil, err
+	}
+	return parseAptUpgradable(string(out)), nil
+}
+
+func (aptBackend) Upgrade(ctx context.Context, opts UpgradeOptions, onLine func(string)) error {
+	bin := "apt-get"
+	if !commandExists(bin) {
+		bin = "apt"
+	}
+	if opts.DryRun {
+		return Run(ctx, []string{bin, "upgrade", "--dry-run"}, RunOptions{
+			Env:    []string{"DEBIAN_FRONTEND=noninteractive", "LC_ALL=C"},
+			OnLine: onLine,
+		})
+	}
+	argv := []string{"pkexec", "env", "DEBIAN_FRONTEND=noninteractive", "LC_ALL=C", bin, "upgrade", "-y"}
+	return Run(ctx, argv, RunOptions{OnLine: onLine})
+}
+
+func parseAptUpgradable(text string) []Package {
+	if text == "" {
+		return nil
+	}
+	var pkgs []Package
+	for line := range strings.SplitSeq(text, "\n") {
+		line = strings.TrimSpace(line)
+		if line == "" {
+			continue
+		}
+		m := aptUpgradableLine.FindStringSubmatch(line)
+		if m == nil {
+			continue
+		}
+		pkgs = append(pkgs, Package{
+			Name:        m[1],
+			Repo:        RepoSystem,
+			Backend:     "apt",
+			FromVersion: m[3],
+			ToVersion:   m[2],
+		})
+	}
+	return pkgs
+}