Multiple file transfer on persistent tcp using python - python-3.x

Question

I am trying to transfer multiple images over the persistent TCP connection in python. According to my knowledge, this code should work fine but sometimes it shows the following error ValueError: invalid literal for int() with base 10: '6789:'
For each image, I am first trying to send the size of that image to the client.
Can someone please help me with what I am doing wrong or otherwise guide how to do this in other ways
Server code
import socket
import time
import os
port = 6004
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
host = socket.gethostname() # Get local machine name
s.bind((host, port)) # Bind to the port
s.listen(5) # Now wait for client connection.
print ('Server listening....')
conn, addr = s.accept()
print ('Got connection from', addr)
# data = conn.recv(1024)
# print('Server received', repr(data))
start =time.time()
for i in range(2):
filename="sc" + str(i) + ".jpg"
size = str(os.path.getsize(filename))
try:
conn.send(size.encode())
except:
print('ckckckckckck')
print('filesize', size, len(size))
f = open(filename,'rb')
current=0
size = int(size)
while current <size:
l = f.read(1024)
conn.send(l)
current+=len(l)
print('current',current)
print('l',len(l))
if(current==size):
break
print('close current',current)
f.close()
print('Done sending')
conn.send(b'Thank you for connecting')
#conn.close()
print(time.time()-start)
Client code
import os
import socket
import time # Import socket module
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) # Create a socket object
host = socket.gethostname() # Get local machine name
port = 6004 # Reserve a port for your service.
s.connect((host, port))
s.send(b'Hello server!')
for i in range(2):
data = None
data = s.recv(1024)
data = data.decode()
size = int(data)
print(size)
filename = "sc" + str(i) + ".jpg"
f = open(filename,'wb')
current =0
while current<size:
data = s.recv(1024)
f.write(data)
current+=len(data)
# print('data',len(data))
# print('current',current)
if(current==size):
break
print('close current',current)
f.close()
print('Successfully get the file')
s.close()
print('connection closed')

Answer 1

data = s.recv(5)
You expect that the size is within the first 5 bytes and you expect to get it with one recv. But, the actual size can be less than 5 bytes or more since you don't enforce a specific number of bytes when sending:
conn.send(size.encode())
Thus what could happen here is that the size you read in the client is either too small (if the original size was larger) or that the size used less than 5 bytes and there are already data from the file contents within the 5 bytes you read. And this causes the error you see.
Note that TCP is a byte-stream protocol and not a message protocol, which means that what you send is not necessarily what you recv. Instead the data from multiple send could be read within one recv or a single send could make multiple recv necessary. Thus a better design of your code would make sure that it always writes a fixed length for the length and always makes sure that it got the full length before trying to interpret the data as length.