import os
import select
import socket
import sys
import threading
import time

class config: None

config.port = 7777
config.max_threads = 1024
config.recognised_selectors = ['0', '1', '5', '9', 'g', 'h', 'I', 's']

# error(message)
# Print error message to stderr
def error(message):
	program_name = os.path.basename(sys.argv[0])
	print('%s: Error: %s' % (program_name, message), file = sys.stderr)

# die(message, status = 1) -> (Never returns)
# Print error message to stderr and exit with status code
def die(message, status = 1):
	error(message)
	sys.exit(status)

# bind(port, backlog = 1) → [sockets...]
# Binds to all available (TCP) interfaces on specified port and returns the sockets
# backlog controls how many connections allowed to wait handling before system drops new ones
def bind(port, backlog = 1):
	# Based on code in https://docs.python.org/3/library/socket.html
	sockets = []
	for res in socket.getaddrinfo(None, port, socket.AF_UNSPEC, socket.SOCK_STREAM, 0, socket.AI_PASSIVE):
		af, socktype, proto, canonname, sa = res
		
		try:
			s = socket.socket(af, socktype, proto)
		except OSError:
			continue
		try:
			s.setsockopt(socket.IPPROTO_IPV6, socket.IPV6_V6ONLY, 1)
		except OSError:
			pass
		try:
			s.bind(sa)
			s.listen(backlog)
		except OSError:
			s.close()
			continue

		sockets.append(s)
	
	return sockets


# drop_privileges()
# Drops set[ug]id, die()s if unsuccesful
def drop_privileges():
	try:
		uid = os.getuid()
		gid = os.getgid()
		os.setresgid(gid, gid, gid)
		os.setresuid(uid, uid, uid)
	except:
		die('Unable to drop privileges')

class Protocol:
	gopher, http = range(2)

class RequestEerror(Exception):
	def __init__(self, message):
		self.message = message
	def __str__(self):
		return 'Error with handling request: ' + self.message

# extract_selector_path(selector_path) -> selector, path
# Extract selector and path components from a HTTP path
def extract_selector_path(selector_path):
	if len(selector_path) > 0 and selector_path[0] == '/':
		selector_path = selector_path[1:]
	
	if len(selector_path) == 0: # / is by default of type 1
		selector = '1'
		path = selector_path
	elif selector_path[0] in config.recognised_selectors: # Requested path has a selector we recognise, extract it
		selector = selector_path[0]
		path = selector_path[1:]
	else: # We couldn't recognise any selector, return None for it
		selector = None
		path = selector_path
	
	return selector, path

# get_request(sock) -> path, protocol, rest
# Read request from socket and parse it.
# path is the requested path, protocol is Protocol.gopher or Protocol.http depending on the request protocol
# rest is protocol-dependant information
def get_request(sock):
	request = b''
	while True:
		data = sock.recv(1024)
		if not data: # Other end hung up before sending a header
			raise RequestEerror('Remote end hung up unexpectedly')

		request += data

		if b'\n' in request: # First line has been sent, all we care about for now
			break
	
	request = request.decode('utf-8')
	first_line = request.split('\n')[0]
	if first_line[-1] == '\r':
		first_line = first_line[:-1]
	first_line = first_line.split(' ')
	
	if len(first_line) >= 1 and first_line[0] == 'GET':
		selector_path = first_line[1]
		selector, path = extract_selector_path(selector_path)
		return path, Protocol.http, selector
	else:
		if len(first_line) >= 1:
			path = first_line[0]
		else:
			path = ''
		return path, Protocol.gopher, None

# Global variables to keep track of the amount of running worker threads
threads_amount = 0
threads_lock = threading.Lock()

# Worker thread implementation
class Serve(threading.Thread):
	def __init__(self, sock, address):
		self.sock = sock
		self.address = address
		threading.Thread.__init__(self)
	
	def handle_request(self):
		path, protocol, rest  = get_request(self.sock)
		self.sock.sendall(str((path, protocol, rest)).encode('utf-8'))
	
	def run(self):
		global threads_amount, threads_lock

		try:
			self.handle_request()
		#except BaseException as err: # Catch and log exceptions instead of letting to crash, as we need to update the worker thread count on abnormal exit as well
		#	error('Worker thread died with: %s' % err)
		finally:
			self.sock.close()
			with threads_lock:
				threads_amount -= 1
	
# spawn_thread(sock, address)
# Spawn a new thread to serve a connection if possible, do nothing if not
def spawn_thread(sock, address):
	global threads_amount, threads_lock

	# See if we can spawn a new thread. If not, log an error, close the socket and return. If yes, increment the amount of threads running
	with threads_lock:
		if threads_amount >= config.max_threads:
			error('Could not serve a request from %s, worker thread limit exhausted' % address)
			sock.close()
			return
		else:
			threads_amount += 1
	
	# Spawn a new worker thread
	Serve(sock, address).start()

# listen(port) -> (Never returns)
# Binds itself to all interfaces on designated port and listens on incoming connections
# Spawns worker threads to handle the connections
def listen(port):
	# Get sockets that we listen to
	listening_sockets = bind(port)
	# Drop privileges, we don't need them after this
	drop_privileges()

	# If we got no sockets to listen to, die
	if listening_sockets == []:
		die('Could not bind to port %i' % port)

	# Create a poll object for the listening sockets and a fd->socket map
	listening = select.poll()
	sock_by_fd={}
	for s in listening_sockets:
		listening.register(s, select.POLLIN)
		sock_by_fd[s.fileno()] = s
	del listening_sockets
	
	while True:
		# Wait for listening sockets to get activity
		events = listening.poll()
		for fd,event in events:
			assert(event == select.POLLIN)
			# Get socked from table established previously
			s = sock_by_fd[fd]
			# Accept and handle the connection
			conn,addr = s.accept()

			spawn_thread(conn, addr[0])

listen(config.port)