// Package tenantsync drives the offsite endpoint's per-customer PBS tenancy surface (PBS DR tier
// SLICE 1) — the structural twin of internal/wgsync: SSH with a PINNED host key (exact-match or
// refuse, no fallback) to a forced-command script (`felhom-tenantsync` — its OWN key + sudoers
// line; the peersync surface is untouched). JSON on stdin, JSON on stdout, one op per session.
//
// SECRET HYGIENE (load-bearing divergence from wgsync): the script's stdout carries the one-time
// PBS token secret. It is parsed into Result and handed to the caller for the consume-once store
// write — it is NEVER logged, and error messages NEVER embed stdout bytes (stderr only). Do not
// "improve" the diagnostics by quoting the response.
package tenantsync

import (
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"log"
	"net"
	"regexp"
	"strings"
	"time"

	"golang.org/x/crypto/ssh"
)

// ErrTokenExists is the typed "provision refused: the token already exists" outcome — the hub
// treats it as a state mismatch (a descriptor should exist; re-issue is the explicit recovery).
var ErrTokenExists = errors.New("tenantsync: token already exists on the endpoint (re-issue is the explicit path)")

// customerIDRe mirrors the script's validation — refuse client-side before a wasted SSH round-trip.
var customerIDRe = regexp.MustCompile(`^[A-Za-z0-9][A-Za-z0-9_.-]{0,30}$`)

// Config configures the SSH client. Addr/User/HostKeyLine are typically the SAME values as the
// peersync client (same box, same low-priv user, same pinned host key); PrivateKey is tenantsync's
// OWN key (the authorized_keys line selects the forced command).
type Config struct {
	Addr        string        // "host:22"
	User        string        // "felhom-peersync"
	PrivateKey  []byte        // PEM private key (from the mounted Secret file)
	HostKeyLine string        // single authorized_keys-format line of the endpoint's host pubkey
	Timeout     time.Duration // default 60s (tenancy ops run several PBS commands)
}

// Result is the script's ok-response: the descriptor fields + the ONE-TIME token secret.
// TokenSecret is transient custody — store it consume-once immediately, never log the struct.
type Result struct {
	TokenID     string `json:"token_id"`
	TokenSecret string `json:"token_secret"`
	Fingerprint string `json:"fingerprint"`
	Datastore   string `json:"datastore"`
	Namespace   string `json:"namespace"`
}

// Client is a pinned-host-key SSH per-op executor. Construct with New (parses keys up front).
type Client struct {
	addr    string
	user    string
	signer  ssh.Signer
	hostKey ssh.PublicKey
	timeout time.Duration
	logger  *log.Logger
}

// New builds a Client, failing early on an unparsable private key or host-key line.
func New(cfg Config, logger *log.Logger) (*Client, error) {
	if cfg.Addr == "" || cfg.User == "" {
		return nil, fmt.Errorf("tenantsync: Addr and User are required")
	}
	signer, err := ssh.ParsePrivateKey(cfg.PrivateKey)
	if err != nil {
		return nil, fmt.Errorf("tenantsync: parse private key: %w", err)
	}
	hostKey, _, _, _, err := ssh.ParseAuthorizedKey([]byte(cfg.HostKeyLine))
	if err != nil {
		return nil, fmt.Errorf("tenantsync: parse host key line: %w", err)
	}
	timeout := cfg.Timeout
	if timeout == 0 {
		timeout = 60 * time.Second
	}
	if logger == nil {
		logger = log.Default()
	}
	return &Client{
		addr: cfg.Addr, user: cfg.User, signer: signer,
		hostKey: hostKey, timeout: timeout, logger: logger,
	}, nil
}

// Provision creates the customer's namespace + privilege-separated token on the endpoint.
// An already-existing token is the typed ErrTokenExists (never silently re-keyed).
func (c *Client) Provision(ctx context.Context, customerID string) (*Result, error) {
	return c.tenancyOp(ctx, "provision", customerID)
}

// Reissue explicitly re-keys the customer's token (delete + recreate + re-grant, script-side).
func (c *Client) Reissue(ctx context.Context, customerID string) (*Result, error) {
	return c.tenancyOp(ctx, "reissue", customerID)
}

// Fingerprint returns the endpoint PBS's API cert fingerprint (the descriptor field).
func (c *Client) Fingerprint(ctx context.Context) (string, error) {
	stdout, stderr, runErr := c.exec(ctx, []byte(`{"op":"fingerprint"}`))
	resp, err := parseResponse(stdout, stderr, runErr)
	if err != nil {
		return "", err
	}
	if resp.Fingerprint == "" {
		return "", fmt.Errorf("tenantsync: fingerprint op returned an empty fingerprint")
	}
	return resp.Fingerprint, nil
}

func (c *Client) tenancyOp(ctx context.Context, op, customerID string) (*Result, error) {
	if !customerIDRe.MatchString(customerID) {
		return nil, fmt.Errorf("tenantsync: invalid customer_id %q", customerID)
	}
	payload, err := json.Marshal(map[string]string{"op": op, "customer_id": customerID})
	if err != nil {
		return nil, err
	}
	stdout, stderr, runErr := c.exec(ctx, payload)
	resp, err := parseResponse(stdout, stderr, runErr)
	if err != nil {
		return nil, err
	}
	if resp.TokenSecret == "" || resp.TokenID == "" || resp.Namespace == "" || resp.Fingerprint == "" || resp.Datastore == "" {
		// Field NAMES only — never values (TokenSecret).
		return nil, fmt.Errorf("tenantsync: %s response is missing required fields", op)
	}
	c.logger.Printf("[INFO] tenantsync: %s ok for %s (ns=%s, token_id=%s; secret withheld from logs)",
		op, customerID, resp.Namespace, resp.TokenID)
	return &resp.Result, nil
}

// response is the script's stdout contract — ok carries the Result fields, error carries code+error.
type response struct {
	Status string `json:"status"`
	Code   string `json:"code"`
	Error  string `json:"error"`
	Result
}

// parseResponse turns (stdout, stderr, runErr) into a typed outcome. The script emits its error
// JSON on stdout and exits 1, so a run error is parsed for the typed code FIRST; only when stdout
// carries no usable JSON does the raw failure (with stderr, NEVER stdout) surface.
func parseResponse(stdout, stderr []byte, runErr error) (*response, error) {
	var resp response
	parseOK := json.Unmarshal(bytes.TrimSpace(stdout), &resp) == nil
	if parseOK && resp.Status == "error" {
		if resp.Code == "token_exists" {
			return nil, ErrTokenExists
		}
		return nil, fmt.Errorf("tenantsync: endpoint refused: %s (code %s)", resp.Error, resp.Code)
	}
	if runErr != nil {
		return nil, fmt.Errorf("tenantsync: remote op failed: %w (stderr: %s)",
			runErr, strings.TrimSpace(string(stderr)))
	}
	if !parseOK || resp.Status != "ok" {
		// stdout may carry the secret — report shape only, never bytes.
		return nil, fmt.Errorf("tenantsync: malformed endpoint response (%d stdout bytes; stderr: %s)",
			len(bytes.TrimSpace(stdout)), strings.TrimSpace(string(stderr)))
	}
	return &resp, nil
}

// exec runs one forced-command session: payload on stdin, returns stdout/stderr. The connection
// discipline (pinned key, constrained HostKeyAlgorithms, deadline both sides of the handshake)
// is wgsync.Push's, verbatim — that shape is live-proven against the real sshd.
func (c *Client) exec(ctx context.Context, payload []byte) (stdout, stderr []byte, err error) {
	sshCfg := &ssh.ClientConfig{
		User:            c.user,
		Auth:            []ssh.AuthMethod{ssh.PublicKeys(c.signer)},
		HostKeyCallback: ssh.FixedHostKey(c.hostKey),
		// Constrain negotiation to the PINNED key's algorithm — a multi-hostkey sshd (stock:
		// ECDSA + ed25519) otherwise presents a different type and FixedHostKey refuses a
		// legitimate server (wgsync S1 live finding).
		HostKeyAlgorithms: []string{c.hostKey.Type()},
		Timeout:           c.timeout,
	}
	dialer := net.Dialer{Timeout: c.timeout}
	conn, err := dialer.DialContext(ctx, "tcp", c.addr)
	if err != nil {
		return nil, nil, fmt.Errorf("tenantsync: dial %s: %w", c.addr, err)
	}
	if dl, ok := ctx.Deadline(); ok {
		conn.SetDeadline(dl)
	} else {
		conn.SetDeadline(time.Now().Add(c.timeout))
	}
	sconn, chans, reqs, err := ssh.NewClientConn(conn, c.addr, sshCfg)
	if err != nil {
		conn.Close()
		return nil, nil, fmt.Errorf("tenantsync: ssh handshake %s: %w", c.addr, err)
	}
	client := ssh.NewClient(sconn, chans, reqs)
	defer client.Close()
	conn.SetDeadline(time.Time{})
	if dl, ok := ctx.Deadline(); ok {
		conn.SetDeadline(dl)
	}

	session, err := client.NewSession()
	if err != nil {
		return nil, nil, fmt.Errorf("tenantsync: session: %w", err)
	}
	defer session.Close()

	var outBuf, errBuf bytes.Buffer
	session.Stdin = bytes.NewReader(payload)
	session.Stdout = &outBuf
	session.Stderr = &errBuf

	// The forced command overrides this string; it documents intent on the wire.
	runErr := session.Run("felhom-tenantsync")
	return outBuf.Bytes(), errBuf.Bytes(), runErr
}