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_dnf.go

@@ -0,0 +1,108 @@
+package sysupdate
+
+import (
+	"context"
+	"errors"
+	"os/exec"
+	"strings"
+)
+
+func init() {
+	RegisterSystemBackend(func() Backend { return &dnfBackend{bin: "dnf5"} })
+	RegisterSystemBackend(func() Backend { return &dnfBackend{bin: "dnf"} })
+}
+
+type dnfBackend struct {
+	bin string
+}
+
+func (b dnfBackend) ID() string           { return b.bin }
+func (b dnfBackend) DisplayName() string  { return strings.ToUpper(b.bin) }
+func (b dnfBackend) Repo() RepoKind       { return RepoSystem }
+func (b dnfBackend) NeedsAuth() bool      { return true }
+func (b dnfBackend) RunsInTerminal() bool { return false }
+
+func (b dnfBackend) IsAvailable(ctx context.Context) bool {
+	if !commandExists(b.bin) {
+		return false
+	}
+	if commandExists("rpm-ostree") && ostreeBooted(ctx) {
+		return false
+	}
+	return true
+}
+
+func (b dnfBackend) CheckUpdates(ctx context.Context) ([]Package, error) {
+	out, err := dnfListUpgrades(ctx, b.bin)
+	if err != nil {
+		return nil, err
+	}
+	installed := rpmInstalledVersions(ctx)
+	return parseDnfList(out, b.bin, installed), nil
+}
+
+func (b dnfBackend) Upgrade(ctx context.Context, opts UpgradeOptions, onLine func(string)) error {
+	if opts.DryRun {
+		return Run(ctx, []string{b.bin, "upgrade", "--assumeno"}, RunOptions{OnLine: onLine})
+	}
+	return Run(ctx, []string{"pkexec", b.bin, "upgrade", "-y"}, RunOptions{OnLine: onLine})
+}
+
+func dnfListUpgrades(ctx context.Context, bin string) (string, error) {
+	cmd := exec.CommandContext(ctx, bin, "list", "--upgrades", "--quiet")
+	out, err := cmd.Output()
+	if err == nil {
+		return string(out), nil
+	}
+	if exitErr, ok := errors.AsType[*exec.ExitError](err); ok && exitErr.ExitCode() == 1 {
+		return "", nil
+	}
+	return "", err
+}
+
+func rpmInstalledVersions(ctx context.Context) map[string]string {
+	out, err := exec.CommandContext(ctx, "rpm", "-qa", "--qf", `%{NAME}\t%{VERSION}-%{RELEASE}\n`).Output()
+	if err != nil {
+		return nil
+	}
+	m := make(map[string]string)
+	for line := range strings.SplitSeq(string(out), "\n") {
+		name, ver, ok := strings.Cut(line, "\t")
+		if !ok {
+			continue
+		}
+		m[name] = ver
+	}
+	return m
+}
+
+func parseDnfList(text, backendID string, installed map[string]string) []Package {
+	if text == "" {
+		return nil
+	}
+	var pkgs []Package
+	for line := range strings.SplitSeq(text, "\n") {
+		fields := strings.Fields(line)
+		if len(fields) < 2 {
+			continue
+		}
+		nameArch := fields[0]
+		version := fields[1]
+		switch nameArch {
+		case "Available", "Upgrades":
+			continue
+		}
+		name := nameArch
+		if dot := strings.LastIndex(nameArch, "."); dot > 0 {
+			name = nameArch[:dot]
+		}
+		pkgs = append(pkgs, Package{
+			Name:        nameArch,
+			Repo:        RepoSystem,
+			Backend:     backendID,
+			FromVersion: installed[name],
+			ToVersion:   version,
+		})
+	}
+	return pkgs
+}