OSError: [Errno 11] Resource temporarily unavailable. What causes this?

Background

I have two python processes that need to communicate with each other. The comminication is handled by a class named Pipe. I made a seperate class for this because most of the information that needs to be communicated comes in the form of dictionaries so Pipe implements a pretty simple protocol for doing this.

Here is the Pipe constructor:

def __init__(self,sPath):
    """
    create the fifo. if it already exists just associate with it
    """
    self.sPath = sPath
    if not os.path.exists(sPath):
        try:
            os.mkfifo(sPath)
        except:
            raise Exception('cannot mkfifo at path \n {0}'.format(sPath))
    self.iFH = os.open(sPath,os.O_RDWR | os.O_NONBLOCK)
    self.iFHBlocking = os.open(sPath,os.O_RDWR)

So ideally I would just construct a Pipe in each process with the same path and they would be able to talk nice.

I'm going to skip out stuff about the protocol because I think it is largely unnecessary here.

All read and write operations make use of the following 'base' functions:

def base_read_blocking(self,iLen):
    self.lock()
    lBytes = os.read(self.iFHBlocking,iLen)
    self.unlock()
    return lBytes

def base_read(self,iLen):
    print('entering base read')
    self.lock()
    lBytes = os.read(self.iFH,iLen)
    self.unlock()
    print('exiting base read')
    return lBytes

def base_write_blocking(self,lBytes):
    self.lock()
    safe_write(self.iFHBlocking,lBytes)
    self.unlock()

def base_write(self,lBytes):
    print('entering base write')
    self.lock()
    safe_write(self.iFH,lBytes)
    self.unlock()
    print('exiting base write')

safe_write was suggested in another post

def safe_write(*args, **kwargs):
    while True:
        try:
            return os.write(*args, **kwargs)
        except OSError as e:
            if e.errno == 35:
                import time