package apply

import (
	"context"
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"fmt"
	"os"
	"regexp"
	"strings"
	"sync"
	"time"

	"github.com/novox/mesh-host/internal/declaration"
	"github.com/novox/mesh-host/internal/store"
	"github.com/novox/mesh-host/internal/system"
)

// An access is an operator's path a module mounts and the mesh does not own (novox/hq ADR 0051): mounted when
// present, refused when absent, nothing changed on it. ADR 0325 kept a shared folder through ACL entries; ADR 0328
// supersedes it with libraries (library.go), and an access is again exactly what it was before: an access's `users`
// and `taken` are accepted and not acted on.

// applyAccess is one access, applied as ADR 0051 has it; what ADR 0312's grant put an account in is taken back
// first, so no account is in a group for data but a library's (ADR 0328 §11).
func applyAccess(ctx context.Context, sys system.System, r *declaration.Access, run Runner, previous store.Applied,
	in inputs) (Outcome, error) {
	gave, left, said := takeBackGranted(ctx, sys, previous, run, in.groups, in.handed)
	out, err := legacyAccess(r)
	if len(left) > 0 {
		out.user, out.groups = previous.User, left
	}
	if said != "" {
		out.Detail = joinDetail(out.Detail, said)
		if gave && out.Action == "unchanged" {
			// Updated, so what names the access under restart-on — the tool runner — holds the groups it has now.
			out.Action = "updated"
		}
	}
	return out, err
}

// takeBackGranted takes an account out of every group ADR 0312's grant put it in for an access, as removeUser takes
// back a user's (ADR 0252 §3): unless another declared resource still asks for it, which is then handed the group.
// Whether any was taken back, the groups still to take back — one whose leaving failed stays recorded, so the next
// apply tries again — and what was said; nothing for an access that granted nothing.
func takeBackGranted(ctx context.Context, sys system.System, previous store.Applied, run Runner, wanted Wanted,
	handed Handed) (bool, []string, string) {
	if previous.User == "" || len(previous.Groups) == 0 {
		return false, nil, ""
	}
	account := previous.User
	in, err := system.GroupsOf(ctx, system.Runner(run), account)
	if err != nil {
		return false, previous.Groups, fmt.Sprintf("the groups ADR 0312's grant put %s in (%s) were not taken "+
			"back, and are tried again: %v", account, strings.Join(previous.Groups, ", "), err)
	}
	already := setOf(in)
	gave := false
	var left, said []string
	for _, g := range previous.Groups {
		switch {
		case !already[g]:
			continue
		case len(wanted.othersAsk(account, g, previous.ID)) > 0:
			said = append(said, fmt.Sprintf("left in %s, which %s still asks for", g,
				strings.Join(wanted.othersAsk(account, g, previous.ID), ", ")))
			handed.hand(account, g)
			continue
		}
		gone, why := leaveGroup(ctx, sys, account, g, run)
		if !gone {
			left = append(left, g)
			why += "; tried again on the next apply"
		}
		gave = gave || gone
		said = append(said, why)
	}
	return gave, left, "the group grant ADR 0312 made is taken back: " + account + " " + strings.Join(said, "; ")
}

// legacyAccess is an access from a controller older than ADR 0325, applied exactly as before it (ADR 0051 §7).
//
// **Absent is refused, not created.** A bind mount whose source does not exist is made for you by the container
// runtime, as root, with whatever mode it picks — which is exactly the silent wrong-ownership 04-ISSUES/026
// records. So the host checks first and says plainly that the operator must provide the path, rather than
// conjuring a directory it does not own and cannot give the right owner. Nothing is written, so this never reports
// a change: the machine did not move, the host merely confirmed a fact about it.
func legacyAccess(r *declaration.Access) (Outcome, error) {
	out := begin(r)
	info, err := os.Stat(r.Path)
	if errors.Is(err, os.ErrNotExist) {
		return out, fmt.Errorf(
			"%s is not there, and the mesh does not own it — the operator must provide it. It is "+
				"shared, pre-existing data (novox/hq ADR 0051): the host mounts it and creates "+
				"nothing, so a missing one is said here rather than made as root by the container "+
				"runtime", r.Path)
	}
	if err != nil {
		return out, err
	}
	if !info.IsDir() {
		return out, fmt.Errorf(
			"%s is not a directory, and an access is a shared directory the operator provides",
			r.Path)
	}
	mode := declaration.AccessRead
	if r.Mode != "" {
		mode = r.Mode
	}
	out.Action = "unchanged"
	out.Detail = "operator-owned; present, " + mode + ", nothing managed"
	return out, nil
}

// PlacedID names the filesystem at mount for a wait, with no path in it: the controller asks again when the
// folder's filesystem is not the one a warrant was given for. Empty when the filesystem cannot be named, and a folder
// on it is then never kept (it fails closed, and waits).
func PlacedID(mount string) string { return placedFrom(mount, filesystemKey(mount)) }

// placedFrom is PlacedID from a filesystem key already read.
func placedFrom(mount, key string) string {
	if mount == "" || key == "" {
		return ""
	}
	sum := sha256.Sum256([]byte(mount + "\x00" + key))
	return "fs-" + hex.EncodeToString(sum[:6])
}

// filesystemKey is what names the filesystem at mount across reboots, in this order:
//
//   - a filesystem on a block device: its UUID, found by matching the mount's device against the devices in
//     /dev/disk/by-uuid (never blkid). xfs's statfs id follows the device number, so the UUID comes first;
//   - else the kernel's filesystem id (statfs f_fsid) when it is not zero: ZFS's from its dataset, btrfs's from its
//     UUID;
//   - else mountinfo's source with the filesystem's type: an NFS export's server:/export, a FUSE filesystem's own
//     source string (both report f_fsid 0).
//
// Never the mount point and type alone, and never a device's name, which a reboot may renumber. Empty when none of
// these answers within statBound: then nothing is kept there. Read once per mount at a time.
func filesystemKey(mount string) string {
	if mount == "" {
		return ""
	}
	v, ok := boundedOnce("fs "+mount, func() any { return readFilesystemKey(mount) })
	if !ok {
		return ""
	}
	return v.(string)
}

func readFilesystemKey(mount string) string {
	if u := MountUUID(mount); u != "" {
		return "uuid:" + u
	}
	if id := MountFsid(mount); strings.Trim(id, "0") != "" {
		return "fsid:" + id
	}
	if source, fstype := mountSourceOf(mount); source != "" && source != "none" {
		return "source:" + fstype + ":" + source
	}
	return ""
}

// statBound is how long a read of a folder may take before it is said not to answer: a hard network mount whose
// server is down blocks a stat for ever, and one access must never hold the apply.
var statBound = 5 * time.Second

// errNoAnswer is a folder whose read did not return within statBound.
var errNoAnswer = errors.New("it does not answer: a read of it did not return within its bound (a mount that is down)")

// lstatWithin is os.Lstat within statBound, and statWithin os.Stat. A read that hangs is left to return on its own.

func lstatWithin(path string) (os.FileInfo, error) { return within("lstat", os.Lstat, path) }
func statWithin(path string) (os.FileInfo, error)  { return within("stat", os.Stat, path) }

// flight is one read in flight, shared by everyone who asks while it is: a read that hangs on a dead mount holds
// one goroutine per path, never one per apply or look.
type flight struct {
	done chan struct{}
	v    any
}

var inFlight = struct {
	sync.Mutex
	reads map[string]*flight
}{reads: map[string]*flight{}}

// boundedOnce is read's answer within statBound, sharing a read of key already in flight; false when it did not
// answer in time.
func boundedOnce(key string, read func() any) (any, bool) {
	inFlight.Lock()
	f, running := inFlight.reads[key]
	if !running {
		f = &flight{done: make(chan struct{})}
		inFlight.reads[key] = f
		go func() {
			f.v = read()
			inFlight.Lock()
			delete(inFlight.reads, key)
			inFlight.Unlock()
			close(f.done)
		}()
	}
	inFlight.Unlock()
	select {
	case <-f.done:
		return f.v, true
	case <-time.After(statBound):
		return nil, false
	}
}

func within(op string, stat func(string) (os.FileInfo, error), path string) (os.FileInfo, error) {
	type read struct {
		info os.FileInfo
		err  error
	}
	v, ok := boundedOnce(op+" "+path, func() any {
		info, err := stat(path)
		return read{info, err}
	})
	if !ok {
		return nil, errNoAnswer
	}
	r := v.(read)
	return r.info, r.err
}

// absPath is an absolute path in words: what pathless takes out.
var absPath = regexp.MustCompile(`(^|[\s"'(=])/[^\s"',;:)]*`)

// Pathless is an error in words with no path (ADR 0234 §6): a path error's operation and cause, and any other
// path said as "a path".
func Pathless(err error) string { return pathless(err) }

func pathless(err error) string {
	var pe *os.PathError
	if errors.As(err, &pe) {
		return pe.Op + ": " + pe.Err.Error()
	}
	return absPath.ReplaceAllString(err.Error(), "${1}a path")
}