ctdb-common: Set immediate mode for pcap capture

Fix a problem where ctdb_killtcp (almost always) fails to capture
packets with --enable-pcap and libpcap ≥ 1.9.1.  The problem is due to
a gradual change in libpcap semantics when using
pcap_get_selectable_fd(3PCAP) to get a file descriptor and then using
that file descriptor in non-blocking mode.

pcap_set_immediate_mode(3PCAP) says:

  pcap_set_immediate_mode() sets whether immediate mode should be set
  on a capture handle when the handle is activated.  In immediate
  mode, packets are always delivered as soon as they arrive, with no
  buffering.

and

  On Linux, with previous releases of libpcap, capture devices are
  always in immediate mode; however, in 1.5.0 and later, they are, by
  default, not in immediate mode, so if pcap_set_immediate_mode() is
  available, it should be used.

However, it wasn't until libpcap commit
2ade7676101366983bd4f86bc039ffd25da8c126 (before libpcap 1.9.1) that
it became a requirement to use pcap_set_immediate_mode(), even with a
timeout of 0.

More explanation in this libpcap issue comment:

  the-tcpdump-group/libpcap#860 (comment)

Do a configure check for pcap_set_immediate_mode() even though it has
existed for 10 years.  It is easy enough.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15451

Signed-off-by: Martin Schwenke <mschwenke@ddn.com>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>

Autobuild-User(master): Amitay Isaacs <amitay@samba.org>
Autobuild-Date(master): Tue Aug 15 10:53:52 UTC 2023 on atb-devel-224

(cherry picked from commit dc7b48c)

Autobuild-User(v4-19-test): Jule Anger <janger@samba.org>
Autobuild-Date(v4-19-test): Tue Aug 29 09:34:35 UTC 2023 on atb-devel-224

ctdb/common/system_socket.c

@@ -1007,6 +1007,13 @@ int ctdb_sys_open_capture_socket(const char *iface, void **private_data)
 		DBG_ERR("Failed to set timeout for pcap capture\n");
 		goto fail;
 	}
+#ifdef HAVE_PCAP_SET_IMMEDIATE_MODE
+	ret = pcap_set_immediate_mode(pt, 1);
+	if (ret < 0) {
+		DBG_ERR("Failed to set immediate mode for pcap capture\n");
+		goto fail;
+	}
+#endif
 	ret = pcap_activate(pt);
 	if (ret < 0) {
 		DBG_ERR("Failed to activate pcap capture\n");

ctdb/wscript

@@ -221,6 +221,7 @@ def configure(conf):
         if not conf.CHECK_FUNCS_IN('pcap_open_live', 'pcap', headers='pcap.h'):
             Logs.error('Need libpcap')
             sys.exit(1)
+        conf.CHECK_FUNCS_IN('pcap_set_immediate_mode', 'pcap', headers='pcap.h')
 
     if not conf.CHECK_FUNCS_IN('backtrace backtrace_symbols', 'execinfo',
                                checklibc=True, headers='execinfo.h'):