package main

// What wants, requires or triggers one unit (novox/hq issue 332), read-only. Two sources, because each misses
// what the other sees:
//
//   - **the manager**, asked for the unit's reverse dependencies (systemctl show's WantedBy, RequiredBy and
//     the rest, and list-dependencies --reverse for what pulls it in through another unit). It knows what is
//     loaded now, including a dependency a unit file states in its own [Unit] section;
//   - **the configuration directories**, read for an enable link naming the unit in a *.wants, *.requires or
//     *.upholds directory. A link outlives the unit file it pointed at: once the package that shipped the unit
//     is gone, the link dangles, and the manager asks for a unit it cannot find at every boot or login. That
//     is the case a failed record of a unit whose load state is not-found comes from, and the answer says
//     each link's target and whether it dangles.
//
// Nothing here changes anything. Removing a dangling link is unlink.go's; clearing the record is
// `reset-failed`'s.

import (
	"errors"
	"fmt"
	"io/fs"
	"os"
	"path/filepath"
	"sort"
	"strconv"
	"strings"
)

// reverseProperties are the unit's properties that name a unit depending on it.
var reverseProperties = []string{"WantedBy", "RequiredBy", "RequisiteOf", "BoundBy", "UpheldBy", "ConsistsOf", "TriggeredBy", "PartOf"}

// stateProperties say what the manager knows of the unit itself, so a not-found unit is recognised.
var stateProperties = []string{"LoadState", "ActiveState", "SubState", "UnitFileState", "FragmentPath"}

// dependencyDirs are the suffixes of the directories an enable link sits in, and the dependency each makes.
var dependencyDirs = map[string]string{".wants": "Wants", ".requires": "Requires", ".upholds": "Upholds"}

// Entry is one name in a directory as lstat sees it: a link is never followed to say what it is.
type Entry struct {
	Name string
	// Dir is whether the name is itself a directory; a link to one is not.
	Dir bool
	// Link is where a symbolic link points, as written; empty when the name is not a link.
	Link string
	// Dangles is whether a link points at nothing: what it names does not exist.
	Dangles bool
	// Error is why a link's target could not be looked at, when that is anything but its not existing.
	Error string
}

// ConfigDir is one directory a manager reads units and enable links from, and whose it is.
type ConfigDir struct {
	Path  string `json:"path"`
	Whose string `json:"whose"`
	// Removable is whether a dangling link here is a person's or an administrator's to remove: their own
	// configuration, not a package's, a generator's or the runtime's, which put theirs back.
	Removable bool `json:"-"`
}

// EnableLink is one link in a dependency directory that names the unit.
type EnableLink struct {
	Path string `json:"path"`
	// Of is the unit that wants (or requires, or upholds) this one: the directory's name less its suffix.
	Of         string `json:"of"`
	Dependency string `json:"dependency"`
	Target     string `json:"target,omitempty"`
	Dangles    bool   `json:"dangles"`
	// Error is why the target could not be looked at, other than its not existing: neither dangling nor not.
	Error string `json:"error,omitempty"`
	// Link is false for a plain file in a dependency directory, which the manager reads the same way.
	Link  bool   `json:"link"`
	Whose string `json:"whose"`

	removable bool
}

// configDirs are the directories a scope's manager reads unit files and enable links from, in its own order
// of precedence (systemd.unit(5), "Unit File Load Path"). The account's own are left out, and said, when its
// home is not known.
func (m *Manager) configDirs(scope Scope) []ConfigDir {
	if scope == User {
		account := "the operator account's own configuration"
		dirs := []ConfigDir{}
		if m.Home != "" {
			dirs = append(dirs, ConfigDir{filepath.Join(m.Home, ".config", "systemd", "user.control"), account + " (set by a program through the manager)", true})
		}
		dirs = append(dirs, ConfigDir{fmt.Sprintf("/run/user/%d/systemd/user", m.UID), "made at runtime, gone at the next login", false})
		if m.Home != "" {
			dirs = append(dirs, ConfigDir{filepath.Join(m.Home, ".config", "systemd", "user"), account, true})
		}
		dirs = append(dirs,
			ConfigDir{"/etc/systemd/user", "the machine's configuration, for every account", true},
			ConfigDir{"/etc/xdg/systemd/user", "the machine's configuration, for every account", true})
		if m.Home != "" {
			dirs = append(dirs, ConfigDir{filepath.Join(m.Home, ".local", "share", "systemd", "user"), "a program installed in the account's own home", false})
		}
		return append(dirs,
			ConfigDir{"/usr/local/lib/systemd/user", "a program installed by hand, for every account", false},
			ConfigDir{"/usr/lib/systemd/user", "a package's", false})
	}
	return []ConfigDir{
		{"/etc/systemd/system.control", "the machine's configuration (set by a program through the manager)", true},
		{"/run/systemd/transient", "made at runtime, gone at the next boot", false},
		{"/run/systemd/generator.early", "a generator's, made again at every boot and reload", false},
		{"/etc/systemd/system", "the machine's configuration", true},
		{"/run/systemd/system", "made at runtime, gone at the next boot", false},
		{"/run/systemd/generator", "a generator's, made again at every boot and reload", false},
		{"/usr/local/lib/systemd/system", "a program installed by hand", false},
		{"/usr/lib/systemd/system", "a package's", false},
		{"/run/systemd/generator.late", "a generator's, made again at every boot and reload", false},
	}
}

// within is whether a path lies in one of the directories.
func within(path string, dirs []ConfigDir) bool {
	path = filepath.Clean(path)
	for _, d := range dirs {
		if path == d.Path || strings.HasPrefix(path, d.Path+string(filepath.Separator)) {
			return true
		}
	}
	return false
}

// links finds every enable link naming the unit under the scope's configuration directories: by its own
// name, or by the name of the file it points at. A directory that cannot be read is said, never skipped as
// empty; one that does not exist is the ordinary case and is not. **A dependency directory that is itself a
// link is followed only when it leads into the searched directories**: elsewhere it is said, and not read,
// so a link cannot point the search, or a removal after it, at any directory on the machine.
func (m *Manager) links(scope Scope, unit string) ([]EnableLink, []string, []string) {
	found, unread, same := []EnableLink{}, []string{}, []string{}
	dirs := m.configDirs(scope)
	seen := map[string]string{}
	for _, dir := range dirs {
		// One directory reached by two names is read once, under the first: the same link is not listed
		// twice, nor removed through the second name.
		if m.Resolve != nil {
			if real, err := m.Resolve(dir.Path); err == nil {
				if first, twice := seen[real]; twice {
					same = append(same, dir.Path+" is "+first+", read under that name")
					continue
				}
				seen[real] = dir.Path
			}
		}
		entries, err := m.ReadDir(dir.Path)
		if err != nil {
			if !errors.Is(err, fs.ErrNotExist) {
				unread = append(unread, dir.Path+": "+err.Error())
			}
			continue
		}
		for _, e := range entries {
			dependency, of := "", ""
			for suffix, dep := range dependencyDirs {
				if strings.HasSuffix(e.Name, suffix) {
					dependency, of = dep, strings.TrimSuffix(e.Name, suffix)
				}
			}
			if dependency == "" || (!e.Dir && e.Link == "") {
				continue
			}
			sub := filepath.Join(dir.Path, e.Name)
			read := sub
			if e.Link != "" {
				target := e.Link
				if !filepath.IsAbs(target) {
					target = filepath.Join(dir.Path, target)
				}
				if !within(target, dirs) {
					unread = append(unread, sub+": a link to "+e.Link+", outside the directories searched, not followed")
					continue
				}
				read = filepath.Clean(target)
			}
			inside, err := m.ReadDir(read)
			if err != nil {
				if !errors.Is(err, fs.ErrNotExist) {
					unread = append(unread, sub+": "+err.Error())
				}
				continue
			}
			for _, l := range inside {
				if l.Name != unit && (l.Link == "" || filepath.Base(l.Link) != unit) {
					continue
				}
				found = append(found, EnableLink{Path: filepath.Join(sub, l.Name), Of: of, Dependency: dependency,
					Target: l.Link, Dangles: l.Link != "" && l.Dangles, Error: l.Error, Link: l.Link != "",
					Whose: dir.Whose, removable: dir.Removable && e.Link == ""})
			}
		}
	}
	sort.Slice(found, func(i, j int) bool { return found[i].Path < found[j].Path })
	return found, unread, same
}

// WantedBy is what wants, requires or triggers one unit: the manager's reverse dependencies and the enable
// links naming it, with a finding in words. A manager that does not answer is said beside the links, which
// are read from disk and do not need it.
func (m *Manager) WantedBy(scope Scope, unit string) (map[string]any, error) {
	if err := unitArg(unit); err != nil {
		return nil, err
	}
	answer := map[string]any{"unit": unit, "scope": string(scope)}

	manager := map[string]any{}
	args := []string{"show", "--no-pager"}
	for _, p := range append(append([]string{}, stateProperties...), reverseProperties...) {
		args = append(args, "--property="+p)
	}
	loadState := ""
	dependents := 0
	if out, err := m.call(scope, "systemctl", append(args, "--", unit)...); err != nil {
		manager["error"] = err.Error()
	} else {
		for _, line := range strings.Split(out, "\n") {
			k, v, ok := strings.Cut(line, "=")
			if !ok || k == "" {
				continue
			}
			if isReverse(k) {
				if names := strings.Fields(v); len(names) > 0 {
					manager[k] = names
					dependents += len(names)
				}
				continue
			}
			manager[k] = v
		}
		loadState, _ = manager["LoadState"].(string)
	}
	// Through other units: list-dependencies --reverse follows the same edges from the units that pull in
	// those that pull this one in. Its first line is the unit itself.
	if out, err := m.call(scope, "systemctl", "list-dependencies", "--reverse", "--plain", "--no-legend", "--no-pager", "--", unit); err != nil {
		manager["reverse_error"] = err.Error()
	} else {
		chain := []string{}
		for i, line := range strings.Split(out, "\n") {
			name := strings.TrimSpace(strings.TrimLeft(line, " │├└─●○×*"))
			if i == 0 || name == "" || name == unit {
				continue
			}
			chain = append(chain, name)
		}
		if len(chain) > 0 {
			manager["reverse"] = chain
		}
	}
	answer["manager"] = manager

	links, unread, same := m.links(scope, unit)
	answer["links"] = links
	if len(same) > 0 {
		answer["same_directory"] = same
	}
	answer["searched"] = m.configDirs(scope)
	if len(unread) > 0 {
		answer["unread"] = unread
	}
	answer["findings"] = findings(scope, unit, loadState, dependents, links, unread, manager["error"] != nil, m.Home == "")
	return answer, nil
}

func isReverse(property string) bool {
	for _, p := range reverseProperties {
		if p == property {
			return true
		}
	}
	return false
}

// findings say what the answer means, so a caller does not have to put the parts together.
func findings(scope Scope, unit, loadState string, dependents int, links []EnableLink, unread []string, managerFailed, noHome bool) []string {
	out := []string{}
	dangling, removable, unknown := 0, 0, 0
	for _, l := range links {
		switch {
		case l.Dangles:
			dangling++
			if l.removable {
				removable++
			}
			out = append(out, l.Path+" makes "+l.Of+" "+strings.TrimSuffix(strings.ToLower(l.Dependency), "s")+" "+unit+" and points at "+l.Target+", which does not exist ("+l.Whose+")")
		case l.Error != "":
			unknown++
			out = append(out, l.Path+" points at "+l.Target+", which could not be looked at: "+l.Error)
		}
	}
	if loadState == "not-found" && len(links) > 0 && dangling == 0 && unknown == 0 {
		out = append(out, "the unit's file is not found, and "+plural(len(links), "enable link")+" still name it")
	}
	if removable > 0 {
		out = append(out, "`disable` does not remove a link whose unit file is gone; `unlink-dangling` in this scope removes "+
			"the dangling links named for the unit in the account's or the machine's own configuration, keeping a record of each; "+
			"then `reset-failed` clears the unit's failed record")
	} else if dangling > 0 {
		out = append(out, "the dangling links are in a package's, a generator's or the runtime's directory, which puts them back: they are not the person's to remove")
	}
	incomplete := managerFailed || len(unread) > 0 || unknown > 0 || (scope == User && noHome)
	if len(links) == 0 && dependents == 0 {
		where := "in the directories searched"
		if incomplete {
			where += ", which are not all of them (below)"
		}
		if loadState == "not-found" {
			out = append(out, "nothing names "+unit+" "+where+": a failed record of it is stale, and `reset-failed` clears it")
		} else if !managerFailed {
			out = append(out, "no unit and no enable link pulls "+unit+" in "+where+": it starts only when asked by name, by a timer, socket or path unit the manager does not list, or by a program")
		}
	}
	if managerFailed {
		out = append(out, "the manager did not answer, so only the links on disk are known")
	}
	if scope == User && noHome {
		out = append(out, "the account's home is not known to this process, so its own directories were not searched")
	}
	if len(unread) > 0 {
		out = append(out, "some directories could not be read, so a link may be missing from this answer")
	}
	return out
}

func plural(n int, what string) string {
	if n == 1 {
		return "1 " + what
	}
	return strconv.Itoa(n) + " " + what + "s"
}

// readDir lists a directory as enable links need it, by lstat: each name, whether it is itself a directory,
// where a link points, and whether what it points at does not exist — any other failure to look is said as
// the error it was, never as dangling.
func readDir(dir string) ([]Entry, error) {
	entries, err := os.ReadDir(dir)
	if err != nil {
		return nil, err
	}
	out := make([]Entry, 0, len(entries))
	for _, e := range entries {
		out = append(out, entryOf(filepath.Join(dir, e.Name()), e.Name(), e.Type()))
	}
	return out, nil
}

// lstatEntry is one path as readDir would list it, read again just before it is acted on.
func lstatEntry(path string) (Entry, error) {
	info, err := os.Lstat(path)
	if err != nil {
		return Entry{}, err
	}
	return entryOf(path, filepath.Base(path), info.Mode().Type()), nil
}

func entryOf(full, name string, mode fs.FileMode) Entry {
	entry := Entry{Name: name, Dir: mode.IsDir()}
	if mode&fs.ModeSymlink == 0 {
		return entry
	}
	target, err := os.Readlink(full)
	if err != nil {
		target, entry.Error = "(unreadable)", err.Error()
	}
	entry.Link = target
	if _, err := os.Stat(full); errors.Is(err, fs.ErrNotExist) {
		entry.Dangles = true
	} else if err != nil {
		entry.Error = err.Error()
	}
	return entry
}