Fix possible infinite loop on pipe/sock_drain()

If the returned value of read/recv is 0 (meaning EOF), we'll end up in an
infinite loop (active wait) until something is written on the pipe which is
not really what we want here especially because those functions are called
from the main thread.

Signed-off-by: David Goulet <dgoulet@ev0ke.net>

changes/bug14554

@@ -0,0 +1,4 @@
+  o Major bugfix
+    - Possibility of an infinite loop if the returned value of the read/recv
+      was 0. A returned value of 0 means that we've reached the EOF thus the
+      pipe/sock is drained so return success not an error.

src/common/compat_threads.c

@@ -171,10 +171,12 @@ pipe_drain(int fd)
 {
   char buf[32];
   ssize_t r;
-  while ((r = read_ni(fd, buf, sizeof(buf))) >= 0)
-    ;
-  if (r == 0 || errno != EAGAIN)
+  do {
+    r = read_ni(fd, buf, sizeof(buf));
+  } while (r > 0);
+  if (errno != EAGAIN)
     return -1;
+  /* A value of r = 0 means EOF on the fd so successfully drained. */
   return 0;
 }
 #endif
@@ -193,10 +195,12 @@ sock_drain(tor_socket_t fd)
 {
   char buf[32];
   ssize_t r;
-  while ((r = recv_ni(fd, buf, sizeof(buf), 0)) >= 0)
-    ;
-  if (r == 0 || !ERRNO_IS_EAGAIN(tor_socket_errno(fd)))
+  do {
+    r = recv_ni(fd, buf, sizeof(buf), 0);
+  } while (r > 0);
+  if (!ERRNO_IS_EAGAIN(tor_socket_errno(fd)))
     return -1;
+  /* A value of r = 0 means EOF on the fd so successfully drained. */
   return 0;
 }