mesh-catalog/modules/systemd/cmd/systemd-tools/unlink.go

package main

// Removing a dangling enable link (novox/hq issue 332), on a person's word. `systemctl disable` does not do it
// once the unit's file is gone: it reports the unit file as not existing and leaves the link, and the manager
// goes on asking for the unit at every boot or login.
//
// It removes as little as can be named, and nothing else:
//
//   - only a symbolic link, named for the unit, in a *.wants, *.requires or *.upholds directory under the
//     scope's searched directories (wanted.go), whose target does not exist;
//   - only in the account's or the machine's own configuration — never a package's, a generator's or the
//     runtime's, which put theirs back — and never through a dependency directory that is itself a link;
//   - each link read again just before it goes, and left when it changed since it was found.
//
// A record of each (its path, where it pointed, what wanted the unit) and the person's reason is written to
// the module's state before the link is removed, so nothing is removed that is not recorded, and the answer
// says how to put it back. The account's own link is removed by this process, which is that account; any
// other through sudo -n, as every act on the machine's manager is.

import (
	"encoding/json"
	"fmt"
	"os"
	"path/filepath"
	"strings"
	"time"
)

// UnlinkedRecord is the file in the module's state each removal is recorded in, one JSON object a line.
const UnlinkedRecord = "unlinked-links.jsonl"

// Unlinked is one removal, as recorded.
type Unlinked struct {
	At     string `json:"at"`
	Scope  string `json:"scope"`
	Unit   string `json:"unit"`
	Path   string `json:"path"`
	Target string `json:"target"`
	Of     string `json:"of"`
	Why    string `json:"why"`
}

// UnlinkDangling removes the dangling enable links named for one unit, on the person's word (why).
func (m *Manager) UnlinkDangling(scope Scope, unit, why string) (map[string]any, error) {
	if err := unitArg(unit); err != nil {
		return nil, err
	}
	if why = strings.TrimSpace(why); why == "" {
		return nil, fmt.Errorf("why is required: removing a link is the person's act, and their reason is kept with the record")
	}
	if m.State == "" {
		return nil, fmt.Errorf("this module has no state directory to keep the record in, so nothing is removed")
	}
	links, unread := m.links(scope, unit)
	removed, left := []Unlinked{}, []map[string]string{}
	for _, l := range links {
		reason := ""
		switch {
		case !l.Link:
			reason = "a file, not a link"
		case !l.Dangles:
			reason = "its target exists: `disable` is the verb for a unit that is there"
		case filepath.Base(l.Path) != unit:
			reason = "named for another unit, pointing at this one's file"
		case !l.removable:
			reason = "in " + l.Whose + " directory, which puts it back, or reached through a linked directory"
		}
		if reason == "" {
			now, err := m.Lstat(l.Path)
			if err != nil || now.Link != l.Target || !now.Dangles {
				reason = "it changed since it was read"
			}
		}
		if reason != "" {
			left = append(left, map[string]string{"path": l.Path, "why_left": reason})
			continue
		}
		rec := Unlinked{At: m.now().UTC().Format(time.RFC3339), Scope: string(scope), Unit: unit, Path: l.Path,
			Target: l.Target, Of: l.Of, Why: why}
		if err := m.record(rec); err != nil {
			return nil, fmt.Errorf("the record of %s could not be kept, so it was not removed: %w", l.Path, err)
		}
		if err := m.unlink(l.Path); err != nil {
			return nil, fmt.Errorf("%s was recorded and not removed: %w (removed before it: %d)", l.Path, err, len(removed))
		}
		removed = append(removed, rec)
	}
	answer := map[string]any{"unit": unit, "scope": string(scope), "removed": removed, "left": left,
		"record": filepath.Join(m.State, UnlinkedRecord)}
	if len(unread) > 0 {
		answer["unread"] = unread
	}
	if len(removed) == 0 {
		answer["note"] = "no dangling link named for " + unit + " was in the account's or the machine's own configuration: nothing was removed"
		return answer, nil
	}
	// The manager keeps the dependency it loaded from the link until it reloads.
	if _, err := m.call(scope, "systemctl", "daemon-reload"); err != nil {
		answer["reload"] = "the manager was not reloaded, so it still lists the dependency until it is: " + err.Error()
	} else {
		answer["reload"] = "the manager reloaded"
	}
	answer["next"] = "`reset-failed` clears the unit's failed record; each link can be made again from the record (ln -s <target> <path>) if it was wanted"
	return answer, nil
}

func (m *Manager) now() time.Time {
	if m.Now != nil {
		return m.Now()
	}
	return time.Now()
}

func (m *Manager) record(rec Unlinked) error {
	line, err := json.Marshal(rec)
	if err != nil {
		return err
	}
	if err := os.MkdirAll(m.State, 0o700); err != nil {
		return err
	}
	f, err := os.OpenFile(filepath.Join(m.State, UnlinkedRecord), os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0o600)
	if err != nil {
		return err
	}
	if _, err := f.Write(append(line, '\n')); err != nil {
		f.Close()
		return err
	}
	return f.Close()
}

// unlink removes one link: the account's own by this process, which is the account; another as root,
// through sudo -n. unlink(1) takes one path and removes it without following it.
func (m *Manager) unlink(path string) error {
	own := m.Home != "" && strings.HasPrefix(path, filepath.Clean(m.Home)+string(filepath.Separator))
	if own || m.UID == 0 {
		return m.Remove(path)
	}
	r := m.Run("sudo", []string{"-n", "unlink", path}, nil)
	if r.Status == 0 && r.Error == "" {
		return nil
	}
	return m.failure("unlink", "sudo", System, r)
}