//! `koh update` — put a newer published koh in place of this one (§12).
//!
//! The source is a release document: Kepr's `/koh/releases/latest.json` by
//! default, or whatever `KOH_UPDATE_SOURCE` names. Koh installs only a
//! **newer** version than the one running, never merely a different one, so a
//! stale mirror cannot walk the binary backwards. Nothing is ever built here.

module update

import crypto.blake3
import json2
import net.http
import os
import src.remote

pub const default_source = 'https://kepr.uk/koh/releases/latest.json'
pub const source_env = 'KOH_UPDATE_SOURCE'
pub const repo = 'koh'
const file_scheme = 'file://'

// One platform's file, in the shape Kepr answers with (§22.4).
struct PlatformFile {
	url    string
	blake3 string
	sha256 string
}

// The release document: `notes` is omitted by Kepr when it is empty, so it is
// optional rather than an empty string.
struct Document {
	version   string
	notes     ?string
	platforms map[string]PlatformFile
}

pub struct Available {
pub:
	version  string
	notes    string
	url      string
	blake3   string
	platform string
}

// source is the document to read: the environment wins, so a self-hosted Kepr
// can serve its own koh.
pub fn source() string {
	named := os.getenv(source_env)
	if named != '' {
		return named
	}
	return default_source
}

// Checked is what the source had to say: whether there is anything newer, and
// what it is when there is.
pub struct Checked {
pub:
	update    bool
	available Available
}

// check asks the source what the newest koh is. Errors are errors; "nothing to
// do" is not one of them, so it is a field, not a failed call.
pub fn check(now string, platform string) !Checked {
	url := source()
	response := http.fetch(
		url:          url
		validate:     true
		verify:       remote.tls_roots(url)!
		enable_http2: false
	) or { return error('could not reach ${url}: ${err.msg()}') }
	if response.status_code != 200 {
		return error('${url} answered ${response.status_code}')
	}
	document := json2.decode[Document](response.body) or {
		// The parser's own message, carets and colour included, means nothing here.
		return error('${url} did not answer with a release document — try again later')
	}
	if document.version == '' {
		// A document with no version is not a release document, so say that rather
		// than pretending there is simply nothing to do.
		return error('${url} has no version — it is not a release document')
	}
	if compare_versions(document.version, now) <= 0 {
		return Checked{}
	}
	file := document.platforms[platform] or {
		return error('koh ${document.version} has no ${platform} file — run it on another machine')
	}
	return Checked{
		update:    true
		available: Available{
			version:  document.version
			notes:    document.notes or { '' }
			url:      file.url
			blake3:   file.blake3
			platform: platform
		}
	}
}

// compare_versions orders two versions the way the spec asks: dot-separated
// components compared as numbers, so 2.10.0 is newer than 2.9.9. A component
// that is not a number is compared as text, so a strange version can never win
// by accident. Returns 1, 0 or -1.
pub fn compare_versions(a string, b string) int {
	left := a.split('.')
	right := b.split('.')
	for index in 0 .. if left.len > right.len { left.len } else { right.len } {
		one := if index < left.len { left[index] } else { '0' }
		two := if index < right.len { right[index] } else { '0' }
		order := compare_component(one, two)
		if order != 0 {
			return order
		}
	}
	return 0
}

fn compare_component(a string, b string) int {
	// Leading zeros are not part of a number: 01 and 1 are the same version.
	left := a.trim_left('0')
	right := b.trim_left('0')
	if left == right {
		return 0
	}
	if left.len != right.len {
		return if left.len > right.len { 1 } else { -1 }
	}
	return if left > right { 1 } else { -1 }
}

// install downloads the new binary, checks it, and swaps it in for the one
// running. The rename is the only step that changes anything, and it is the
// last one.
pub fn install(available Available) !string {
	bytes := download(available.url)!
	verify(bytes, available.blake3)!
	target := os.executable()
	fresh := write_beside(os.dir(target), bytes)!
	if os.user_os() == 'macos' {
		// An unsigned binary cannot run on Apple silicon at all, so a refused
		// signature is a refusal to install, not a warning.
		signed := os.execute('codesign -f -s - ${sh_quote(fresh)}')
		if signed.exit_code != 0 {
			os.rm(fresh) or {}
			return error('codesign refused the downloaded binary: ${signed.output.trim_space()}')
		}
	}
	// rename, not move: both paths are in one folder, so this is the atomic step
	// and a failure is a failure, never a half-written binary.
	os.rename(fresh, target)!
	return target
}

fn download(url string) ![]u8 {
	if url.starts_with(file_scheme) {
		// A Kepr with a folder bucket hands out `file://` download URLs (dev and
		// tests only), so updating from one is the same code path with the file
		// read instead of fetched.
		path := url[file_scheme.len..]
		size := os.file_size(path)
		bytes := os.read_file(path) or {
			return error('could not read ${url}: ${err.msg()}')
		}.bytes()
		// The download is verified against the size it was said to be, and then
		// against its Blake3, before anything is signed or moved.
		if size > 0 && bytes.len != size {
			return error('${url} is ${size} bytes but ${bytes.len} arrived — refusing a partial download')
		}
		return bytes
	}
	response := http.fetch(
		url:          url
		validate:     true
		verify:       remote.tls_roots(url)!
		enable_http2: false
	) or { return error('could not download ${url}: ${err.msg()}') }
	if response.status_code != 200 {
		return error('${url} answered ${response.status_code}')
	}
	bytes := response.body.bytes()
	declared := response.header.get(.content_length) or { '' }
	if declared != '' && i64(declared.trim_space().int()) != bytes.len {
		return error('${url} said ${declared} bytes and sent ${bytes.len} — refusing a partial download')
	}
	return bytes
}

// verify is the check that makes installing anything safe: the bytes must be
// what the document said, or they are thrown away.
fn verify(bytes []u8, expected string) ! {
	if bytes.len == 0 {
		return error('the download was empty')
	}
	if expected == '' {
		return error('the release document carries no Blake3 checksum — refusing to install it')
	}
	got := blake3.sum256(bytes).hex()
	if got != expected {
		return error('the download is not the published file (Blake3 ${got} instead of ${expected}) — refusing to install it')
	}
}

// write_beside writes the new binary beside the one it will replace, so the rename is
// within one filesystem and therefore atomic. A folder that cannot be written
// is refused here, with the write's own error.
fn write_beside(dir string, bytes []u8) !string {
	fresh := os.join_path(dir, '.koh-new-${os.getpid()}')
	os.write_file_array(fresh, bytes) or {
		return error('cannot write beside ${dir} — put koh where this user can replace it (${err.msg()})')
	}
	os.chmod(fresh, 0o755)!
	return fresh
}

// sh_quote keeps a path with a space in it one argument, and cannot be broken
// out of by a quote inside the path.
fn sh_quote(text string) string {
	return "'" + text.replace("'", "'\\''") + "'"
}