William Ahern:

Make UDP sockets non-blocking. I've confirmed that at least on Linux 2.4 a
  read event can come back from poll() on a valid SOCK_DGRAM socket but
  recv(2) will still block. This patch doesn't ignore EAGAIN in
  read_udp_packets(), though maybe it should. (This patch was edited by Daniel
  Stenberg and a new configure test was added (imported from curl's configure)
  to properly detect what non-blocking socket approach to use.)

ares/CHANGES

@@ -4,6 +4,13 @@
 
 - William Ahern:
 
+  Make UDP sockets non-blocking. I've confirmed that at least on Linux 2.4 a
+  read event can come back from poll() on a valid SOCK_DGRAM socket but
+  recv(2) will still block. This patch doesn't ignore EAGAIN in
+  read_udp_packets(), though maybe it should. (This patch was edited by Daniel
+  Stenberg and a new configure test was added (imported from curl's configure)
+  to properly detect what non-blocking socket approach to use.)
+
   I'm not quite sure how this was happening, but I've been seeing PTR queries
   which seem to return empty responses. At least, they were empty when calling
   ares_expand_name() on the record. Here's a patch which guarantees to

ares/acinclude.m4

@@ -1,3 +1,126 @@
+dnl Check for how to set a socket to non-blocking state. There seems to exist
+dnl four known different ways, with the one used almost everywhere being POSIX
+dnl and XPG3, while the other different ways for different systems (old BSD,
+dnl Windows and Amiga).
+dnl
+dnl There are two known platforms (AIX 3.x and SunOS 4.1.x) where the
+dnl O_NONBLOCK define is found but does not work. This condition is attempted
+dnl to get caught in this script by using an excessive number of #ifdefs...
+dnl
+AC_DEFUN([CURL_CHECK_NONBLOCKING_SOCKET],
+[
+  AC_MSG_CHECKING([non-blocking sockets style])
+
+  AC_TRY_COMPILE([
+/* headers for O_NONBLOCK test */
+#include <sys/types.h>
+#include <unistd.h>
+#include <fcntl.h>
+],[
+/* try to compile O_NONBLOCK */
+
+#if defined(sun) || defined(__sun__) || defined(__SUNPRO_C) || defined(__SUNPRO_CC)
+# if defined(__SVR4) || defined(__srv4__)
+#  define PLATFORM_SOLARIS
+# else
+#  define PLATFORM_SUNOS4
+# endif
+#endif
+#if (defined(_AIX) || defined(__xlC__)) && !defined(_AIX4)
+# define PLATFORM_AIX_V3
+#endif
+
+#if defined(PLATFORM_SUNOS4) || defined(PLATFORM_AIX_V3) || defined(__BEOS__)
+#error "O_NONBLOCK does not work on this platform"
+#endif
+  int socket;
+  int flags = fcntl(socket, F_SETFL, flags | O_NONBLOCK);
+],[
+dnl the O_NONBLOCK test was fine
+nonblock="O_NONBLOCK"
+AC_DEFINE(HAVE_O_NONBLOCK, 1, [use O_NONBLOCK for non-blocking sockets])
+],[
+dnl the code was bad, try a different program now, test 2
+
+  AC_TRY_COMPILE([
+/* headers for FIONBIO test */
+#include <unistd.h>
+#include <stropts.h>
+],[
+/* FIONBIO source test (old-style unix) */
+ int socket;
+ int flags = ioctl(socket, FIONBIO, &flags);
+],[
+dnl FIONBIO test was good
+nonblock="FIONBIO"
+AC_DEFINE(HAVE_FIONBIO, 1, [use FIONBIO for non-blocking sockets])
+],[
+dnl FIONBIO test was also bad
+dnl the code was bad, try a different program now, test 3
+
+  AC_TRY_COMPILE([
+/* headers for ioctlsocket test (cygwin?) */
+#include <windows.h>
+],[
+/* ioctlsocket source code */
+ int socket;
+ unsigned long flags = ioctlsocket(socket, FIONBIO, &flags);
+],[
+dnl ioctlsocket test was good
+nonblock="ioctlsocket"
+AC_DEFINE(HAVE_IOCTLSOCKET, 1, [use ioctlsocket() for non-blocking sockets])
+],[
+dnl ioctlsocket didnt compile!, go to test 4
+
+  AC_TRY_LINK([
+/* headers for IoctlSocket test (Amiga?) */
+#include <sys/ioctl.h>
+],[
+/* IoctlSocket source code */
+ int socket;
+ int flags = IoctlSocket(socket, FIONBIO, (long)1);
+],[
+dnl ioctlsocket test was good
+nonblock="IoctlSocket"
+AC_DEFINE(HAVE_IOCTLSOCKET_CASE, 1, [use Ioctlsocket() for non-blocking sockets])
+],[
+dnl Ioctlsocket didnt compile, do test 5!
+  AC_TRY_COMPILE([
+/* headers for SO_NONBLOCK test (BeOS) */
+#include <socket.h>
+],[
+/* SO_NONBLOCK source code */
+ long b = 1;
+ int socket;
+ int flags = setsockopt(socket, SOL_SOCKET, SO_NONBLOCK, &b, sizeof(b));
+],[
+dnl the SO_NONBLOCK test was good
+nonblock="SO_NONBLOCK"
+AC_DEFINE(HAVE_SO_NONBLOCK, 1, [use SO_NONBLOCK for non-blocking sockets])
+],[
+dnl test 5 didnt compile!
+nonblock="nada"
+AC_DEFINE(HAVE_DISABLED_NONBLOCKING, 1, [disabled non-blocking sockets])
+])
+dnl end of fifth test
+
+])
+dnl end of forth test
+
+])
+dnl end of third test
+
+])
+dnl end of second test
+
+])
+dnl end of non-blocking try-compile test
+  AC_MSG_RESULT($nonblock)
+
+  if test "$nonblock" = "nada"; then
+    AC_MSG_WARN([non-block sockets disabled])
+  fi
+])
 
 dnl We create a function for detecting which compiler we use and then set as
 dnl pendantic compiler options as possible for that particular compiler. The

ares/ares_process.c

@@ -466,13 +466,76 @@ void ares__send_query(ares_channel channel, struct query *query, time_t now)
     }
 }
 
+/*
+ * nonblock() set the given socket to either blocking or non-blocking mode
+ * based on the 'nonblock' boolean argument. This function is highly portable.
+ */
+static int nonblock(ares_socket_t sockfd,    /* operate on this */
+                    int nonblock   /* TRUE or FALSE */)
+{
+#undef SETBLOCK
+#define SETBLOCK 0
+#ifdef HAVE_O_NONBLOCK
+  /* most recent unix versions */
+  int flags;
+
+  flags = fcntl(sockfd, F_GETFL, 0);
+  if (TRUE == nonblock)
+    return fcntl(sockfd, F_SETFL, flags | O_NONBLOCK);
+  else
+    return fcntl(sockfd, F_SETFL, flags & (~O_NONBLOCK));
+#undef SETBLOCK
+#define SETBLOCK 1
+#endif
+
+#if defined(HAVE_FIONBIO) && (SETBLOCK == 0)
+  /* older unix versions */
+  int flags;
+
+  flags = nonblock;
+  return ioctl(sockfd, FIONBIO, &flags);
+#undef SETBLOCK
+#define SETBLOCK 2
+#endif
+
+#if defined(HAVE_IOCTLSOCKET) && (SETBLOCK == 0)
+  /* Windows? */
+  unsigned long flags;
+  flags = nonblock;
+
+  return ioctlsocket(sockfd, FIONBIO, &flags);
+#undef SETBLOCK
+#define SETBLOCK 3
+#endif
+
+#if defined(HAVE_IOCTLSOCKET_CASE) && (SETBLOCK == 0)
+  /* presumably for Amiga */
+  return IoctlSocket(sockfd, FIONBIO, (long)nonblock);
+#undef SETBLOCK
+#define SETBLOCK 4
+#endif
+
+#if defined(HAVE_SO_NONBLOCK) && (SETBLOCK == 0)
+  /* BeOS */
+  long b = nonblock ? 1 : 0;
+  return setsockopt(sockfd, SOL_SOCKET, SO_NONBLOCK, &b, sizeof(b));
+#undef SETBLOCK
+#define SETBLOCK 5
+#endif
+
+#ifdef HAVE_DISABLED_NONBLOCKING
+  return 0; /* returns success */
+#undef SETBLOCK
+#define SETBLOCK 6
+#endif
+
+#if (SETBLOCK == 0)
+#error "no non-blocking method was found/used/set"
+#endif
+}
+
 static int open_tcp_socket(ares_channel channel, struct server_state *server)
 {
-#if defined(WIN32)
-  u_long flags;
-#else
-  int flags;
-#endif
   ares_socket_t s;
   struct sockaddr_in sockin;
 
@@ -482,25 +545,7 @@ static int open_tcp_socket(ares_channel channel, struct server_state *server)
     return -1;
 
   /* Set the socket non-blocking. */
-
+  nonblock(s, TRUE);
-#if defined(WIN32) || defined(WATT32)
-  flags = 1;
-  ioctlsocket(s, FIONBIO, &flags);
-#else
-  flags = fcntl(s, F_GETFL, 0);
-
-  if (flags == -1)
-    {
-      closesocket(s);
-      return -1;
-    }
-  flags |= O_NONBLOCK;
-  if (fcntl(s, F_SETFL, flags) == -1)
-    {
-      closesocket(s);
-      return -1;
-    }
-#endif
 
   /* Connect to the server. */
   memset(&sockin, 0, sizeof(sockin));
@@ -531,6 +576,9 @@ static int open_udp_socket(ares_channel channel, struct server_state *server)
   if (s == ARES_SOCKET_BAD)
     return -1;
 
+  /* Set the socket non-blocking. */
+  nonblock(s, TRUE);
+
   /* Connect to the server. */
   memset(&sockin, 0, sizeof(sockin));
   sockin.sin_family = AF_INET;

ares/configure.ac

@@ -328,5 +328,6 @@ AC_CHECK_SIZEOF(struct in_addr, ,
 
 AC_CHECK_FUNCS([bitncmp if_indextoname])
 
+CURL_CHECK_NONBLOCKING_SOCKET
 
 AC_OUTPUT(Makefile)