Any real-world object will exceed the pipe buffer in size. Unless the
filter program which we invoke implemented an infinite buffer (and
requiring something like this would miss the point of stream
processing), we can only write to the program as much data as it
accepts for its input buffering (plus the pipe buffer size), before we
need to read its output.

Thus, we need to ensure that write(fds[STDIN_FILENO].fd) does not
block.