Fix POLLHUP in pipes

The code in pipe_buffer.cc had the meaning of POLLHUP and POLLRDHUP
reversed, and as a result POLLHUP wasn't correctly returned when polling
a read-end of the pipe whose write-end is closed.

This fixes the mixup, and as a result the test added to tst-pipe.cc in
the previous commit, and also another one added in this patch, all pass.

We are still incompatible with Linux with what we do regarding POLLRDHUP.
This will be a separate patch.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>

libc/pipe_buffer.cc

@@ -18,7 +18,7 @@ void pipe_buffer::detach_sender()
     if (sender) {
         sender = nullptr;
         if (receiver)
-            poll_wake(receiver, POLLRDHUP);
+            poll_wake(receiver, POLLHUP);
         may_read.wake_all();
     }
 }
@@ -29,7 +29,7 @@ void pipe_buffer::detach_receiver()
     if (receiver) {
         receiver = nullptr;
         if (sender)
-            poll_wake(sender, POLLHUP);
+            poll_wake(sender, POLLRDHUP);
         may_write.wake_all();
     }
 }
@@ -50,14 +50,14 @@ int pipe_buffer::read_events_unlocked()
 {
     int ret = 0;
     ret |= !q.empty() ? POLLIN : 0;
-    ret |= !sender ? POLLRDHUP : 0;
+    ret |= !sender ? POLLHUP : 0;
     return ret;
 }
 
 int pipe_buffer::write_events_unlocked()
 {
     if (!receiver) {
-        return POLLHUP;
+        return POLLRDHUP;
     }
     int ret = 0;
     ret |= q.size() < max_buf ? POLLOUT : 0;

tests/tst-pipe.cc

@@ -307,6 +307,19 @@ int main(int ac, char** av)
     r = close(s[0]);
     report(r == 0, "close also read side");
 
+    // Test poll() on read side, when write side is already closed (POLLHUP expected)
+    r = pipe(s);
+    report(r == 0, "pipe call");
+    r = close(s[1]);
+    report(r == 0, "close write side");
+    poller = { s[0], POLLIN, 0 };
+    r = poll(&poller, 1, 0);
+    report(r==1, "pipe is readable");
+    report(poller.revents & POLLHUP, "POLLHUP signaled");
+    t6.join();
+    r = close(s[0]);
+    report(r == 0, "close also read side");
+
 
     debug("SUMMARY: %d tests, %d failures\n", tests, fails);
 }