// Package symlinkguard is the hyperguild#157 tripwire. The brain directory is
// a Syncthing sendreceive folder and Syncthing syncs symlinks; several brain
// readers (search, vectorstore, context, graph) follow a symlink out of the
// brain and would index or return whatever it points to. Until every reader
// reads through an os.Root, a symlink inside the brain, or a directory the
// scan cannot read (which could hide one), is an incident: the server refuses
// to start on one (main), and Watch reports one that appears while it runs.
package symlinkguard

import (
	"context"
	"fmt"
	"io/fs"
	"log/slog"
	"os"
	"path/filepath"
	"sort"
	"strconv"
	"strings"
	"time"
)

// DefaultInterval is used when Watch is given a non-positive interval.
const DefaultInterval = 5 * time.Minute

// Notifier is the out-of-band alert sink (audit.Notifier satisfies it).
type Notifier interface {
	Notify(ctx context.Context, msg string) error
}

// Result is what a scan found: symlinks, and directories it could not read
// (each of which could hide a symlink). Paths are brain-relative.
type Result struct {
	Symlinks   []string
	Unreadable []string
}

// Clean reports whether the scan found nothing to act on.
func (r Result) Clean() bool { return len(r.Symlinks) == 0 && len(r.Unreadable) == 0 }

// String describes the findings for a log line or alert. Paths are quoted, so
// a name containing a newline cannot blur two findings together.
func (r Result) String() string {
	q := func(xs []string) string {
		out := make([]string, len(xs))
		for i, x := range xs {
			out[i] = strconv.Quote(x)
		}
		return strings.Join(out, ", ")
	}
	var parts []string
	if len(r.Symlinks) > 0 {
		parts = append(parts, fmt.Sprintf("%d symlink(s): %s", len(r.Symlinks), q(r.Symlinks)))
	}
	if len(r.Unreadable) > 0 {
		parts = append(parts, fmt.Sprintf("%d unreadable dir(s): %s", len(r.Unreadable), q(r.Unreadable)))
	}
	return strings.Join(parts, "; ")
}

// Scan walks the brain dir and reports every symlink in it and every
// directory it could not read, continuing past both. Only the ROOT .git is
// skipped (it is brain-sync's, and no reader walks it); a nested .git is
// scanned like any other directory. brainDir itself may be a symlink (a
// volume mount); it is resolved first. The error is non-nil when the root
// cannot be resolved (wrapping os.ErrNotExist when it is missing) or cannot be
// listed.
func Scan(brainDir string) (Result, error) {
	root, err := filepath.EvalSymlinks(brainDir)
	if err != nil {
		return Result{}, fmt.Errorf("resolve brain dir: %w", err)
	}
	var r Result
	rootGit := filepath.Join(root, ".git")
	walkErr := filepath.WalkDir(root, func(p string, d fs.DirEntry, err error) error {
		rel, _ := filepath.Rel(root, p)
		rel = filepath.ToSlash(rel)
		if err != nil {
			if p == root {
				return err
			}
			r.Unreadable = append(r.Unreadable, rel)
			if d != nil && d.IsDir() {
				return filepath.SkipDir
			}
			return nil
		}
		if d.IsDir() && p == rootGit {
			return filepath.SkipDir
		}
		if d.Type()&os.ModeSymlink != 0 {
			r.Symlinks = append(r.Symlinks, rel)
		}
		return nil
	})
	sort.Strings(r.Symlinks)
	sort.Strings(r.Unreadable)
	if walkErr != nil {
		// Only the root's own error reaches here (deeper errors are recorded
		// as Unreadable). A root that cannot be listed cannot be shown clean
		// (PR #159 round 2: discarding this reported clean while search
		// served a planted symlink).
		return r, fmt.Errorf("scan brain dir: %w", walkErr)
	}
	return r, nil
}

// Watch rescans every interval (DefaultInterval if non-positive) until ctx
// ends, and notifies once per distinct non-clean result. A result counts as
// reported only after Notify succeeds, so an alert lost to a notifier outage
// is retried on the next tick.
func Watch(ctx context.Context, brainDir string, interval time.Duration, n Notifier) {
	if interval <= 0 {
		interval = DefaultInterval
	}
	t := time.NewTicker(interval)
	defer t.Stop()
	reported := ""
	for {
		select {
		case <-ctx.Done():
			return
		case <-t.C:
		}
		r, err := Scan(brainDir)
		var key, msg string
		switch {
		case err != nil:
			key = "error: " + err.Error()
			msg = "brain symlink tripwire (hyperguild#157): cannot scan the brain dir: " + err.Error()
		case r.Clean():
			reported = ""
			continue
		default:
			key = r.String()
			msg = "brain symlink tripwire (hyperguild#157): " + key +
				". Readers may follow symlinks out of the brain; remove them."
		}
		if key == reported {
			continue
		}
		slog.Error("brain symlink tripwire", "finding", key)
		if n == nil {
			reported = key
			continue
		}
		if err := n.Notify(ctx, msg); err != nil {
			slog.Warn("symlink tripwire notify failed; will retry", "err", err)
			continue
		}
		reported = key
	}
}