What conditions result in an opened, nonblocking named pipe (fifo) being “unavailable” for reads?

Question

Situation:

new_pipe = os.open(pipe_path, os.O_RDONLY | os.O_NONBLOCK) # pipe_path points to a FIFO
data = os.read(new_pipe, 1024)

The read occasionally raises errno -11: Resource temporarily unavailable.

When is this error raised? It seems very rare, as the common cases return data:

• If no writer has the pipe opened, empty str ('') is returned.
• If the writer has the pipe opened, but no data is in the fifo, empty str ('') is also returned
• And of course if the writer puts data in the fifo, that data will be read.

Answer 1

From the POSIX specification of the read system call (emphasis mine):

When attempting to read from an empty pipe or FIFO:

• If no process has the pipe open for writing, read() shall return 0 to indicate end-of-file.

• If some process has the pipe open for writing and O_NONBLOCK is set, read() shall return -1 and set errno to [EAGAIN].

So basically your second assumption is wrong:

If the writer has the pipe opened, but no data is in the fifo, empty str ('') is also returned

This would be against the specification and I can't reproduce that behaviour on my machine (it raises EAGAIN for me). This is not a big problem however, you can just catch the exception and retry:

import errno

def safe_read(fd, size=1024):
   ''' reads data from a pipe and returns `None` on EAGAIN '''
   try:
      return os.read(fd, size)
   except OSError, exc:
      if exc.errno == errno.EAGAIN:
         return None
      raise