Explain example pipeline from Python subprocess module

Question

Section 17.1.4.2: Replacing shell pipeline of the python subprocess module says to replace

output=`dmesg | grep hda`

with

p1 = Popen(["dmesg"], stdout=PIPE)
p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
p1.stdout.close()  # Allow p1 to receive a SIGPIPE if p2 exits.
output = p2.communicate()[0]

The comment to the third line explains why the close function is being called, but not why it makes sense. It doesn't, to me. Will not closing p1.stdout before the communicate method is called prevent any output from being sent through the pipe? (Obviously it won't, I've tried to run the code and it runs fine). Why is it necessary to call close to make p1 receive SIGPIPE? What kind of close is it that doesn't close? What, exactly, is it closing?

Please consider this an academic question, I'm not trying to accomplish anything except understanding these things better.

Answer 1

You are closing p1.stdout in the parent process, thus leaving dmesg as the only process with that file descriptor open. If you didn't do this, even when dmesg closed its stdout, you would still have it open, and a SIGPIPE would not be generated. (The OS basically keeps a reference count, and generates SIGPIPE when it hits zero. If you don't close the file, you prevent it from ever reaching zero.)