MINOR: fd: implement an optimised my_closefrom() function

The idea is that poll() can set the POLLNVAL flag for each invalid
FD in a pollfd list. Thus this function makes use of poll() when
compiled in, and builds lists of up to 1024 FDs at once, checks the
output and only closes those which do not have this flag set. Tests
show that this is about twice as fast as blindly calling close() for
each closed fd.

src/fd.c

@@ -151,6 +151,11 @@
 #include <sys/types.h>
 #include <sys/resource.h>
 
+#if defined(ENABLE_POLL)
+#include <poll.h>
+#include <errno.h>
+#endif
+
 #include <common/compat.h>
 #include <common/config.h>
 
@@ -461,6 +466,62 @@ void fd_process_cached_events()
 	fdlist_process_cached_events(&fd_cache);
 }
 
+#if defined(ENABLE_POLL)
+/* This is a portable implementation of closefrom(). It closes all open file
+ * descriptors starting at <start> and above. It relies on the fact that poll()
+ * will return POLLNVAL for each invalid (hence close) file descriptor passed
+ * in argument in order to skip them. It acts with batches of FDs and will
+ * typically perform one poll() call per 1024 FDs so the overhead is low in
+ * case all FDs have to be closed.
+ */
+void my_closefrom(int start)
+{
+	struct pollfd poll_events[1024];
+	struct rlimit limit;
+	int nbfds, fd, ret, idx;
+	int step, next;
+
+	if (getrlimit(RLIMIT_NOFILE, &limit) == 0)
+		step = nbfds = limit.rlim_cur;
+	else
+		step = nbfds = 0;
+
+	if (nbfds <= 0) {
+		/* set safe limit */
+		nbfds = 1024;
+		step = 256;
+	}
+
+	if (step > sizeof(poll_events) / sizeof(poll_events[0]))
+		step = sizeof(poll_events) / sizeof(poll_events[0]);
+
+	while (start < nbfds) {
+		next = (start / step + 1) * step;
+
+		for (fd = start; fd < next && fd < nbfds; fd++) {
+			poll_events[fd - start].fd = fd;
+			poll_events[fd - start].events = 0;
+		}
+
+		do {
+			ret = poll(poll_events, fd - start, 0);
+			if (ret >= 0)
+				break;
+		} while (errno == EAGAIN || errno == EINTR || errno == ENOMEM);
+
+		for (idx = 0; idx < ret; idx++) {
+			if (poll_events[idx].revents & POLLNVAL)
+				continue; /* already closed */
+
+			fd = poll_events[idx].fd;
+			close(fd);
+		}
+		start = next;
+	}
+}
+
+#else // defined(ENABLE_POLL)
+
 /* This is a portable implementation of closefrom(). It closes all open file
  * descriptors starting at <start> and above. This is a naive version for use
  * when the operating system provides no alternative.
@@ -481,6 +542,7 @@ void my_closefrom(int start)
 	while (start < nbfds)
 		close(start++);
 }
+#endif // defined(ENABLE_POLL)
 
 /* disable the specified poller */
 void disable_poller(const char *poller_name)