Call poll(2) from within the resolver but adapt to older kernels without it

if necessary.  This removes the need to malloc large fd_set's for selecting
on high fd's (larger than FD_SETSIZE at libc compile time).

The syscall adaptive stuff only happens on the very first call.  SIGSYS
is masked, and if the call to poll fails with ENOSYS, then we use select
for the life of the program.  If poll does not fail with ENOSYS, then we
always use poll and skip the once-off signal masking gunk.

This may be overkill, but it saved my neck a few times while working on
multiple different sets of kernel sources, some with poll, some without.

1 files changed, 68 insertions, 26 deletions

diff --git a/lib/libc/net/res_send.c b/lib/libc/net/res_send.c
index 9061ec6..f3eee5b 100644
--- a/lib/libc/net/res_send.c
+++ b/lib/libc/net/res_send.c
@@ -56,7 +56,7 @@
 #if defined(LIBC_SCCS) && !defined(lint)
 static char sccsid[] = "@(#)res_send.c	8.1 (Berkeley) 6/4/93";
 static char orig_rcsid[] = "From: Id: res_send.c,v 8.13 1997/06/01 20:34:37 vixie Exp";
-static char rcsid[] = "$Id: res_send.c,v 1.17 1997/06/27 13:00:51 peter Exp $";
+static char rcsid[] = "$Id: res_send.c,v 1.18 1997/06/28 04:19:52 peter Exp $";
 #endif /* LIBC_SCCS and not lint */
 
 /*
@@ -81,6 +81,10 @@ static char rcsid[] = "$Id: res_send.c,v 1.17 1997/06/27 13:00:51 peter Exp $";
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
+#include <poll.h>
+
+static int use_poll = 1;	/* adapt to poll() syscall availability */
+				/* 0 = not present, 1 = try it, 2 = exists */
 
 static int s = -1;	/* socket used for communications */
 static int connected = 0;	/* is the socket connected */
@@ -468,6 +472,8 @@ read_len:
 			/*
 			 * Use datagrams.
 			 */
+			struct pollfd pfd;
+			int msec;
 			struct timeval timeout;
 			fd_set dsmask, *dsmaskp;
 			int dsmasklen;
@@ -569,36 +575,72 @@ read_len:
 			/*
 			 * Wait for reply
 			 */
-			timeout.tv_sec = (_res.retrans << try);
-			if (try > 0)
-				timeout.tv_sec /= _res.nscount;
-			if ((long) timeout.tv_sec <= 0)
-				timeout.tv_sec = 1;
-			timeout.tv_usec = 0;
+    othersyscall:
+			if (use_poll) {
+				msec = (_res.retrans << try) * 1000;
+				if (try > 0)
+					msec /= _res.nscount;
+				if (msec <= 0)
+					msec = 1000;
+			} else {
+			    timeout.tv_sec = (_res.retrans << try);
+				if (try > 0)
+					timeout.tv_sec /= _res.nscount;
+				if ((long) timeout.tv_sec <= 0)
+					timeout.tv_sec = 1;
+			}
     wait:
-			dsmasklen = howmany(s+1, NFDBITS) * sizeof(fd_mask);
-			if (dsmasklen > sizeof(fd_set)) {
-				dsmaskp = (fd_set *)malloc(dsmasklen);
-				if (dsmaskp == NULL) {
+			if (use_poll) {
+				struct sigaction sa, osa;
+				int sigsys_installed = 0;
+
+				pfd.fd = s;
+				pfd.events = POLLIN;
+				if (use_poll == 1) {
+					bzero(&sa, sizeof(sa));
+					sa.sa_handler = SIG_IGN;
+					if (sigaction(SIGSYS, &sa, &osa) >= 0)
+						sigsys_installed = 1;
+				}
+				n = poll(&pfd, 1, msec);
+				if (sigsys_installed == 1) {
+					int oerrno = errno;
+					sigaction(SIGSYS, &osa, NULL);
+					errno = oerrno;
+				}
+				if (n < 0) {
+					if (errno == ENOSYS) {
+						use_poll = 0;
+						goto othersyscall;
+					}
+					if (errno == EINTR)
+						goto wait;
+					Perror(stderr, "poll", errno);
 					res_close();
 					goto next_ns;
 				}
-			} else
-				dsmaskp = &dsmask;
-			/* only zero what we need */
-			bzero((char *)dsmaskp, dsmasklen);
-			FD_SET(s, dsmaskp);
-			n = select(s+1, dsmaskp, (fd_set *)NULL,
-				   (fd_set *)NULL, &timeout);
-			if (dsmaskp != &dsmask)
-				free(dsmaskp);
-			if (n < 0) {
-				if (errno == EINTR)
-					goto wait;
-				Perror(stderr, "select", errno);
-				res_close();
-				goto next_ns;
+				if (use_poll == 1)
+					use_poll = 2;
+			} else {
+				dsmasklen = howmany(s + 1, NFDBITS) *
+					    sizeof(fd_mask);
+				if (dsmasklen > sizeof(fd_set)) {
+					dsmaskp = (fd_set *)malloc(dsmasklen);
+					if (dsmaskp == NULL) {
+						res_close();
+						goto next_ns;
+					}
+				} else
+					dsmaskp = &dsmask;
+				/* only zero what we need */
+				bzero((char *)dsmaskp, dsmasklen);
+				FD_SET(s, dsmaskp);
+				n = select(s + 1, dsmaskp, (fd_set *)NULL,
+					   (fd_set *)NULL, &timeout);
+				if (dsmaskp != &dsmask)
+					free(dsmaskp);
 			}
+
 			if (n == 0) {
 				/*
 				 * timeout