// The Construct
//
// Copyright (C) The Construct Developers, Authors & Contributors
// Copyright (C) 2016-2020 Jason Volk <jason@zemos.net>
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice is present in all copies. The
// full license for this software is available in the LICENSE file.

namespace ircd::m::fed::well_known
{
	static std::tuple<ircd::http::code, ircd::string_view, ircd::string_view>
	fetch(const mutable_buffer &,
	      const net::hostport &,
	      const string_view &url);

	extern log::log log;
}

decltype(ircd::m::fed::well_known::log)
ircd::m::fed::well_known::log
{
	"m.well-known"
};

decltype(ircd::m::fed::well_known::cache_default)
ircd::m::fed::well_known::cache_default
{
	{ "name",     "ircd.m.fed.well-known.cache.default" },
	{ "default",  24 * 60 * 60L                         },
};

decltype(ircd::m::fed::well_known::cache_error)
ircd::m::fed::well_known::cache_error
{
	{ "name",     "ircd.m.fed.well-known.cache.error" },
	{ "default",  36 * 60 * 60L                       },
};

///NOTE: not yet used until HTTP cache headers in response are respected.
decltype(ircd::m::fed::well_known::cache_max)
ircd::m::fed::well_known::cache_max
{
	{ "name",     "ircd.m.fed.well-known.cache.max" },
	{ "default",  48 * 60 * 60L                     },
};

decltype(ircd::m::fed::well_known::fetch_timeout)
ircd::m::fed::well_known::fetch_timeout
{
	{ "name",     "ircd.m.fed.well-known.fetch.timeout" },
	{ "default",  15L                                   },
};

decltype(ircd::m::fed::well_known::fetch_redirects)
ircd::m::fed::well_known::fetch_redirects
{
	{ "name",     "ircd.m.fed.well-known.fetch.redirects" },
	{ "default",  2L                                      },
};

ircd::string_view
ircd::m::fed::well_known::get(const mutable_buffer &buf,
                              const string_view &origin)
try
{
	static const string_view type
	{
		"well-known.matrix.server"
	};

	const m::room::id::buf room_id
	{
		"dns", m::my_host()
	};

	const m::room room
	{
		room_id
	};

	const m::event::idx event_idx
	{
		room.get(std::nothrow, type, origin)
	};

	const milliseconds origin_server_ts
	{
		m::get<time_t>(std::nothrow, event_idx, "origin_server_ts", time_t(0))
	};

	const json::object content
	{
		origin_server_ts > 0ms?
			m::get(std::nothrow, event_idx, "content", buf):
			const_buffer{}
	};

	const seconds ttl
	{
		content.get<time_t>("ttl", time_t(86400))
	};

	const string_view cached
	{
		data(buf), move(buf, json::string(content["m.server"]))
	};

	const system_point expires
	{
		origin_server_ts + ttl
	};

	const bool expired
	{
		ircd::now<system_point>() > expires
	};

	const bool valid
	{
		!empty(cached)
	};

	// Crucial value that will provide us with a return string for this
	// function in any case. This is obtained by either using the value
	// found in cache or making a network query for a new value. expired=true
	// when a network query needs to be made, otherwise we can return the
	// cached value. If the network query fails, this value is still defaulted
	// as the origin string to return and we'll also cache that too.
	const string_view delegated
	{
		expired || !valid?
			fetch(buf + size(cached), origin):
			cached
	};

	// Branch on valid cache hit to return result.
	if(valid && !expired)
	{
		char tmbuf[48];
		log::debug
		{
			log, "%s found in cache delegated to %s event_idx:%u expires %s",
			origin,
			cached,
			event_idx,
			timef(tmbuf, expires, localtime),
		};

		return delegated;
	}

	// Conditions for valid expired cache hit w/ failure to reacquire.
	const bool fallback
	{
		valid
		&& expired
		&& cached != delegated
		&& origin == delegated
		&& now<system_point>() < expires + seconds(cache_max)
	};

	if(fallback)
	{
		char tmbuf[48];
		log::debug
		{
			log, "%s found in cache delegated to %s event_idx:%u expired %s",
			origin,
			cached,
			event_idx,
			timef(tmbuf, expires, localtime),
		};

		return cached;
	}

	// Any time the well-known result is the same as the origin (that
	// includes legitimate errors where fetch_well_known() returns the
	// origin to default) we consider that an error and use the error
	// TTL value. Sorry, no exponential backoff implemented yet.
	const auto cache_ttl
	{
		origin == delegated?
			seconds(cache_error).count():
			seconds(cache_default).count()
	};

	// Write our record to the cache room; note that this doesn't really
	// match the format of other DNS records in this room since it's a bit
	// simpler, but we don't share the ircd.dns.rr type prefix anyway.
	const auto cache_id
	{
		m::send(room, m::me(), type, origin, json::members
		{
			{ "ttl",       cache_ttl  },
			{ "m.server",  delegated  },
		})
	};

	log::debug
	{
		log, "%s caching delegation to %s to cache in %s",
		origin,
		delegated,
		string_view{cache_id},
	};

	return delegated;
}
catch(const ctx::interrupted &)
{
	throw;
}
catch(const std::exception &e)
{
	log::error
	{
		log, "%s :%s",
		origin,
		e.what(),
	};

	return string_view
	{
		data(buf), move(buf, origin)
	};
}

ircd::string_view
ircd::m::fed::well_known::fetch(const mutable_buffer &user_buf,
                                const string_view &origin)
try
{
	static const string_view path
	{
		"/.well-known/matrix/server"
	};

	// If the supplied buffer isn't large enough to make the full
	// HTTP request, allocate one that is.
	const unique_mutable_buffer req_buf
	{
		size(user_buf) < 8_KiB? 8_KiB: 0UL
	};

	const mutable_buffer buf
	{
		req_buf? req_buf: user_buf
	};

	rfc3986::uri uri;
	uri.remote = origin;
	uri.path = path;
	json::object response;
	unique_mutable_buffer carry;
	for(size_t i(0); i < fetch_redirects; ++i)
	{
		const auto &[code, location, content]
		{
			fetch(buf, uri.remote, uri.path)
		};

		// Successful error; bail
		if(code >= 400)
			return string_view
			{
				data(user_buf), move(user_buf, origin)
			};

		// Successful result; response content handled after loop.
		if(code < 300)
		{
			response = content;
			break;
		}

		// Indirection code, but no location response header
		if(!location)
			return string_view
			{
				data(user_buf), move(user_buf, origin)
			};

		// Redirection; carry over the new target by copying it because it's
		// in the buffer which we'll be overwriting for the new request.
		carry = unique_mutable_buffer{location};
		uri = string_view{carry};

		// Indirection code, bad location header.
		if(!uri.path || !uri.remote)
			return string_view
			{
				data(user_buf), move(user_buf, origin)
			};
	}

	const json::string &m_server
	{
		response["m.server"]
	};

	if(!m_server)
		return string_view
		{
			data(user_buf), move(user_buf, origin)
		};

	// This construction validates we didn't get a junk string
	volatile const net::hostport ret
	{
		m_server
	};

	log::debug
	{
		log, "%s query to %s found delegation to %s",
		origin,
		uri.remote,
		m_server,
	};

	// Move the returned string to the front of the buffer; this overwrites
	// any other incoming content to focus on just the unquoted string.
	return string_view
	{
		data(user_buf), move(user_buf, m_server)
	};
}
catch(const ctx::interrupted &)
{
	throw;
}
catch(const std::exception &e)
{
	log::derror
	{
		log, "%s in network query :%s",
		origin,
		e.what(),
	};

	return string_view
	{
		data(user_buf), move(user_buf, origin)
	};
}

/// Return a tuple of the HTTP code, any Location header, and the response
/// content. These values are unconditional if this function doesn't throw,
/// but if there's no Location header and/or content then those will be empty
/// string_view's. This function is intended to be run in a loop by the caller
/// to chase redirection. No HTTP codes will throw from here; server and
/// network errors (and others) will.
std::tuple<ircd::http::code, ircd::string_view, ircd::string_view>
ircd::m::fed::well_known::fetch(const mutable_buffer &buf,
	                            const net::hostport &remote,
	                            const string_view &url)
{
	// Hard target https service; do not inherit any matrix service from remote.
	const net::hostport target
	{
		host(remote), "https", port(remote)
	};

	window_buffer wb
	{
		buf
	};

	const http::header headers[]
	{
		{ "User-Agent", info::user_agent },
	};

	http::request
	{
		wb, host(target), "GET", url, 0, {}, headers
	};

	server::out out;
	out.head = wb.completed();

	// Remaining space in buffer is used for received head; note that below
	// we specify this same buffer for in.content, but that's a trick
	// recognized by ircd::server to place received content directly after
	// head in this buffer without any additional dynamic allocation.
	server::in in;
	in.head = buf + size(out.head);
	in.content = in.head;

	server::request::opts opts;
	opts.http_exceptions = false; // 3xx/4xx/5xx response won't throw.
	server::request request
	{
		target, std::move(out), std::move(in), &opts
	};

	const auto code
	{
		request.get(seconds(fetch_timeout))
	};

	thread_local char rembuf[rfc3986::DOMAIN_BUFSIZE * 2];
	log::debug
	{
		log, "fetch from %s %s :%u %s",
		string(rembuf, target),
		url,
		uint(code),
		http::status(code)
	};

	const http::response::head head
	{
		request.in.gethead(request)
	};

	return
	{
		code,
		head.location,
		request.in.content,
	};
}