// SPDX-FileCopyrightText: 2026 Echolot contributors
// SPDX-License-Identifier: GPL-3.0-or-later

// Package selfupdate replaces the running binary with the newest release
// asset from a Gitea repo. Native mode only and strictly opt-in (twice: the
// API base must be configured AND --self-update passed / timer enabled).
// Containers update by pulling a new image tag instead.
package selfupdate

import (
	"crypto/sha256"
	"echo-lot.app/server/internal/relsign"
	"echo-lot.app/server/internal/system"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"io"
	"net/http"
	"os"
	"path/filepath"
	"runtime"
	"strings"
	"time"
)

// DefaultPublicKeyB64 is the reference deployment's release-signing key (ed25519, base64). The
// matching private key lives only in the CI secret store (RELEASE_SIGNING_KEY) — not in this
// repo, not on the Gitea host, not on any server. Operators running their own release pipeline
// override it with ECHOLOT_SELF_UPDATE_PUBKEY (mint a pair with `release-sign -gen`).
const DefaultPublicKeyB64 = "KcytZd4zNIwqfhTyamtdSrXg8ZqYHGAkVxgn5zR7ZQI="

type release struct {
	TagName string  `json:"tag_name"`
	Assets  []asset `json:"assets"`
}
type asset struct {
	Name string `json:"name"`
	URL  string `json:"browser_download_url"`
}

// Run checks <api>/releases/latest for an asset named
// echolot-server_<GOOS>_<GOARCH> newer than currentVersion and atomically
// replaces the current executable. The caller (or systemd Restart=) handles
// the restart; we never exec ourselves.
//
// pubKeyB64 is the release-signing public key; empty means [DefaultPublicKeyB64].
func Run(api, pubKeyB64, currentVersion string) error {
	if api == "" {
		return fmt.Errorf("self-update disabled: no --self-update-api / ECHOLOT_SELF_UPDATE_API configured")
	}
	if pubKeyB64 == "" {
		pubKeyB64 = DefaultPublicKeyB64
	}
	client := &http.Client{Timeout: 30 * time.Second}
	resp, err := client.Get(strings.TrimRight(api, "/") + "/releases/latest")
	if err != nil {
		return err
	}
	defer resp.Body.Close()
	if resp.StatusCode != http.StatusOK {
		return fmt.Errorf("release API: %s", resp.Status)
	}
	var rel release
	if err := json.NewDecoder(resp.Body).Decode(&rel); err != nil {
		return err
	}
	// Tags are namespaced (server-v1.2.3) but binaries are stamped with the
	// bare version (v1.2.3) — compare the normalized forms or the updater
	// would re-download the same version forever.
	latest := strings.TrimPrefix(rel.TagName, "server-")
	if rel.TagName == "" || latest == currentVersion {
		fmt.Printf("already current (%s)\n", currentVersion)
		return nil
	}
	want := fmt.Sprintf("echolot-server_%s_%s", runtime.GOOS, runtime.GOARCH)
	var url string
	for _, a := range rel.Assets {
		if a.Name == want {
			url = a.URL
			break
		}
	}
	if url == "" {
		return fmt.Errorf("release %s has no asset %q", rel.TagName, want)
	}

	// The release must carry SHA256SUMS *and* its detached signature. The checksums alone only
	// protect download integrity (truncation, proxy mangling) — they come from the same place as
	// the binaries, so whoever can alter one can alter both. The signature is the defense against
	// a compromised release host: its private key exists only in the CI secret store, so a valid
	// SHA256SUMS.sig means the project's pipeline published exactly these checksums, and the
	// checksum then extends that trust to the binary.
	fetch := func(name string) ([]byte, error) {
		for _, a := range rel.Assets {
			if a.Name != name {
				continue
			}
			resp, err := client.Get(a.URL)
			if err != nil {
				return nil, err
			}
			defer resp.Body.Close()
			return io.ReadAll(io.LimitReader(resp.Body, 1<<20))
		}
		return nil, fmt.Errorf("release %s has no asset %q", rel.TagName, name)
	}
	sums, err := fetch("SHA256SUMS")
	if err != nil {
		return fmt.Errorf("fetching SHA256SUMS: %w", err)
	}
	sig, err := fetch("SHA256SUMS.sig")
	if err != nil {
		return fmt.Errorf("release %s is unsigned — refusing to update (%v)", rel.TagName, err)
	}
	if err := relsign.Verify(pubKeyB64, sums, string(sig)); err != nil {
		return fmt.Errorf("release %s: SHA256SUMS signature rejected — refusing to update: %w", rel.TagName, err)
	}
	wantSum := ""
	for _, line := range strings.Split(string(sums), "\n") {
		if fields := strings.Fields(line); len(fields) == 2 && fields[1] == want {
			wantSum = fields[0]
		}
	}
	if wantSum == "" {
		return fmt.Errorf("release %s has no SHA256SUMS entry for %q — refusing to update", rel.TagName, want)
	}

	self, err := os.Executable()
	if err != nil {
		return err
	}
	self, _ = filepath.EvalSymlinks(self)
	tmp := self + ".update"
	f, err := os.OpenFile(tmp, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0o755)
	if err != nil {
		return err
	}
	dl, err := client.Get(url)
	if err != nil {
		f.Close()
		os.Remove(tmp)
		return err
	}
	h := sha256.New()
	_, err = io.Copy(io.MultiWriter(f, h), dl.Body)
	dl.Body.Close()
	f.Close()
	if err != nil {
		os.Remove(tmp)
		return err
	}
	if got := hex.EncodeToString(h.Sum(nil)); got != wantSum {
		os.Remove(tmp)
		return fmt.Errorf("checksum mismatch for %s: got %s want %s", want, got, wantSum)
	}
	if err := os.Rename(tmp, self); err != nil {
		os.Remove(tmp)
		return fmt.Errorf("atomic replace failed (filesystem boundaries?): %w", err)
	}
	// Serving became an explicit verb, and a unit written before that change starts this binary
	// with no arguments - which now prints usage and exits non-zero. The unit is not part of what
	// an update replaces, so it is repaired here rather than left to fail at the next restart,
	// which might be a reboot months from now.
	if repaired, err := system.RepairExecStart(); err != nil {
		fmt.Println("WARNING: could not update the systemd unit for --serve:", err)
	} else if repaired {
		fmt.Println("updated the systemd unit to pass --serve (serving is now an explicit verb)")
	}
	fmt.Printf("updated %s -> %s (%s); restart to run it\n", currentVersion, rel.TagName, self)
	return nil
}