A named pipe (also known as FIFO) is a IPC channel, it allows one process to write and another to read. Like all things in Linux, it is implemented as a file like entity using either mkfifo or pipe system calls.

Pipes on Linux have a limited size, typically set to 65536 bytes on modern systems. I recently came across a situation where I needed to know the current remaining capacity of a given pipe without consuming data from it, while one process is writing to it and another is reading from it. I found that the ioctl system call takes in a flag FIONREAD that makes it read the count of unread bytes in a given pipe, identified by the file descriptor of either ends. The count is placed in an int buffer that ioctl takes in.

The relevant section in https://man7.org/linux/man-pages/man7/pipe.7.html reads

The following ioctl(2) operation, which can be applied to a file 
descriptor that refers to  either end of a pipe, places a count 
of the number of unread bytes in the pipe in the int buffer 
pointed to by the final argument of the call:

ioctl(fd, FIONREAD, &nbytes);

The FIONREAD operation is not specified in any standard, but is 
provided on many implementations.

We assume that the name of the pipe is already known, and the reader and writer are active. The resulting python code looks like this:

import fcntl
import os
import termios

fd = os.open(pipe_name, os.O_RDONLY | os.O_NONBLOCK)
buf = array.array('i', [0])
fcntl.ioctl(fd, termios.FIONREAD, buf)
current_size = buf[0]
remaining_capacity = 65536 - current_size

Shout out to my colleague Nicolas Thauvin for pointing me in this direction.