// Package release fetches and installs llmgw's own release binaries.
//
// One implementation for everywhere a binary is replaced — the control plane
// upgrading itself, and an agent told to upgrade by it — so the checks are
// the same on both: the release's own checksum is verified before anything is
// installed, the old binary is kept beside the new one, and the swap is a
// rename, so a machine is never left without a binary that runs.
package release

import (
	"bufio"
	"bytes"
	"context"
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"io"
	"net"
	"net/http"
	"net/url"
	"os"
	"path/filepath"
	"regexp"
	"strings"
	"time"
)

// DefaultBase is where CI publishes releases.
const DefaultBase = "https://gitea.mynodes.xyz/ilker/llmgw/releases/download"

// Tag is a version CI published: v0.10.4, or a release candidate v0.10.4-rc.2.
var Tag = regexp.MustCompile(`^v\d+\.\d+\.\d+(-rc\.\d+)?$`)

// AssetName is the release file for a platform.
func AssetName(version, goos, goarch string) string {
	return "llmgw-" + version + "-" + goos + "-" + goarch
}

// Install fetches version for this platform from base, checks it against the
// release's checksums, and puts it at target, keeping the old one as
// target.bak-pre-<version>. It returns the backup's path.
func Install(ctx context.Context, client *http.Client, base, version, goos, goarch, target string) (string, error) {
	if !Tag.MatchString(version) {
		return "", fmt.Errorf("%q is not a release version", version)
	}
	if err := CheckBase(base); err != nil {
		return "", err
	}
	if client == nil {
		client = &http.Client{Timeout: 10 * time.Minute}
	}
	target, err := filepath.EvalSymlinks(target)
	if err != nil {
		return "", err
	}
	dir := filepath.Dir(target)
	asset := AssetName(version, goos, goarch)
	root := strings.TrimSuffix(base, "/") + "/" + version + "/"

	want, err := checksum(ctx, client, root, asset)
	if err != nil {
		return "", err
	}
	// Beside the target, so the final rename is on one filesystem and atomic.
	tmp, err := os.CreateTemp(dir, ".llmgw-"+version+"-*")
	if err != nil {
		return "", err
	}
	defer os.Remove(tmp.Name())
	got, err := download(ctx, client, root+asset, tmp)
	if cerr := tmp.Close(); err == nil {
		err = cerr
	}
	if err != nil {
		return "", err
	}
	if got != want {
		return "", fmt.Errorf("%s does not match the release's checksum (got %s, expected %s)",
			asset, got[:12], want[:12])
	}
	if err := os.Chmod(tmp.Name(), 0o755); err != nil {
		return "", err
	}
	backup := target + ".bak-pre-" + version
	if err := copyFile(target, backup); err != nil {
		return "", fmt.Errorf("keeping the old binary: %w", err)
	}
	if err := os.Rename(tmp.Name(), target); err != nil {
		return "", err
	}
	return backup, nil
}

// CheckBase refuses a release address a binary should not come from: one
// over plain HTTP, where anyone on the path can swap the bytes and, without a
// signing key, the checksums with them. Loopback is allowed, for a mirror on
// this machine and for tests.
//
// With a signing key built in, plain HTTP is allowed too: the checksums are
// then checked against a key no server holds, so what carries them adds
// nothing, and an offline site's mirror serves releases over the plain HTTP
// it serves weights on.
func CheckBase(base string) error {
	u, err := url.Parse(base)
	if err != nil || u.Host == "" {
		return fmt.Errorf("%q is not a release address", base)
	}
	switch u.Scheme {
	case "https":
		return nil
	case "http":
		if ip := net.ParseIP(u.Hostname()); (ip != nil && ip.IsLoopback()) || u.Hostname() == "localhost" {
			return nil
		}
		if TrustedKey() != "" {
			return nil
		}
	}
	return fmt.Errorf("releases are fetched over https, and %s is not", base)
}

// maxBinary bounds a release binary read into memory.
const maxBinary = 512 << 20

// Fetch downloads a release binary for a platform and checks it exactly as
// Install does — signature when this build trusts a key, then checksum —
// returning its bytes: for a machine that installs it on another, which
// then runs nothing that was not checked here.
func Fetch(ctx context.Context, client *http.Client, base, version, goos, goarch string) ([]byte, error) {
	if !Tag.MatchString(version) {
		return nil, fmt.Errorf("%q is not a release version", version)
	}
	if err := CheckBase(base); err != nil {
		return nil, err
	}
	if client == nil {
		client = &http.Client{Timeout: 10 * time.Minute}
	}
	asset := AssetName(version, goos, goarch)
	root := strings.TrimSuffix(base, "/") + "/" + version + "/"
	want, err := checksum(ctx, client, root, asset)
	if err != nil {
		return nil, err
	}
	var buf bytes.Buffer
	got, err := download(ctx, client, root+asset, &limitWriter{w: &buf, n: maxBinary})
	if err != nil {
		return nil, err
	}
	if got != want {
		return nil, fmt.Errorf("%s does not match the release's checksum (got %s, expected %s)",
			asset, got[:12], want[:12])
	}
	return buf.Bytes(), nil
}

// limitWriter refuses to grow past n bytes.
type limitWriter struct {
	w io.Writer
	n int64
}

func (l *limitWriter) Write(p []byte) (int, error) {
	if int64(len(p)) > l.n {
		return 0, fmt.Errorf("the release binary is larger than %d bytes", maxBinary)
	}
	l.n -= int64(len(p))
	return l.w.Write(p)
}

// checksum reads the release's checksum for asset — after checking the
// checksums' signature, when this build trusts a signing key.
func checksum(ctx context.Context, client *http.Client, root, asset string) (string, error) {
	c, err := fetchChecksums(ctx, client, root)
	if err != nil {
		return "", err
	}
	return c.For(asset)
}

// Checksums are a release's checksums.txt and its signature, as published.
type Checksums struct {
	Text []byte
	// Signature is checksums.txt.sig; empty for a release read by a build
	// that trusts no key.
	Signature string
}

// FetchChecksums reads a release's checksums and their signature, checked
// as Install checks them: for a machine that keeps them — in an offline
// bundle — to be checked again where they are used.
func FetchChecksums(ctx context.Context, client *http.Client, base, version string) (Checksums, error) {
	if !Tag.MatchString(version) {
		return Checksums{}, fmt.Errorf("%q is not a release version", version)
	}
	if err := CheckBase(base); err != nil {
		return Checksums{}, err
	}
	if client == nil {
		client = &http.Client{Timeout: 10 * time.Minute}
	}
	return fetchChecksums(ctx, client, strings.TrimSuffix(base, "/")+"/"+version+"/")
}

// fetchChecksums reads checksums.txt under root, and its signature when this
// build trusts a signing key. A release with no signature is then refused
// rather than taken on its checksums: a missing signature is exactly what
// replacing a release would look like.
func fetchChecksums(ctx context.Context, client *http.Client, root string) (Checksums, error) {
	body, err := get(ctx, client, root+"checksums.txt")
	if err != nil {
		return Checksums{}, err
	}
	defer body.Close()
	sums, err := io.ReadAll(io.LimitReader(body, 1<<20))
	if err != nil {
		return Checksums{}, err
	}
	c := Checksums{Text: sums}
	if TrustedKey() == "" {
		return c, nil
	}
	sigBody, err := get(ctx, client, root+SignatureName)
	if err != nil {
		return Checksums{}, fmt.Errorf("this build only installs signed releases, and %s could not be read: %w", SignatureName, err)
	}
	sig, err := io.ReadAll(io.LimitReader(sigBody, 4096))
	sigBody.Close()
	if err != nil {
		return Checksums{}, err
	}
	c.Signature = string(sig)
	return c, c.Verify()
}

// Verify checks the checksums against the key this build trusts. With none
// they are taken as they are, as every release before signing was.
func (c Checksums) Verify() error {
	key := TrustedKey()
	if key == "" {
		return nil
	}
	if strings.TrimSpace(c.Signature) == "" {
		return fmt.Errorf("this build only installs signed releases, and these checksums carry no %s", SignatureName)
	}
	return Verify(key, c.Text, c.Signature)
}

// For is the checksum the release gives for one asset.
func (c Checksums) For(asset string) (string, error) {
	sc := bufio.NewScanner(bytes.NewReader(c.Text))
	for sc.Scan() {
		f := strings.Fields(sc.Text())
		if len(f) == 2 && f[1] == asset && len(f[0]) == 64 {
			return f[0], nil
		}
	}
	return "", fmt.Errorf("the release's checksums list no %s", asset)
}

func download(ctx context.Context, client *http.Client, url string, dst io.Writer) (string, error) {
	body, err := get(ctx, client, url)
	if err != nil {
		return "", err
	}
	defer body.Close()
	h := sha256.New()
	if _, err := io.Copy(io.MultiWriter(dst, h), body); err != nil {
		return "", fmt.Errorf("downloading %s: %w", url, err)
	}
	return hex.EncodeToString(h.Sum(nil)), nil
}

func get(ctx context.Context, client *http.Client, url string) (io.ReadCloser, error) {
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
	if err != nil {
		return nil, err
	}
	resp, err := client.Do(req)
	if err != nil {
		return nil, fmt.Errorf("fetching %s: %w", url, err)
	}
	if resp.StatusCode != http.StatusOK {
		resp.Body.Close()
		return nil, fmt.Errorf("fetching %s: %s", url, resp.Status)
	}
	return resp.Body, nil
}

func copyFile(src, dst string) error {
	in, err := os.Open(src) //nolint:gosec // our own binary
	if err != nil {
		return err
	}
	defer in.Close()
	out, err := os.OpenFile(dst, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0o755) //nolint:gosec // beside it
	if err != nil {
		return err
	}
	if _, err := io.Copy(out, in); err != nil {
		out.Close()
		return err
	}
	return out.Close()
}