ejabberdctl: support concurrent connections with bound conn names

If flock is available, ejabberdctl will use it to grab one
of a bound number of connection names. This allows concurrent
connections while using a bound number of atoms.

Using PID, timestamps or random strings for transient connection IDs
(which would avoid the need for flock) uses an unbound number of atoms.
This can effectively DoS servers, as these connection names are
not garbage collected.

src/Makefile.in

@@ -110,6 +110,9 @@ MSGSDIR = $(PRIVDIR)/msgs
 # /var/lib/ejabberd/
 SPOOLDIR = $(DESTDIR)@localstatedir@/lib/ejabberd
 
+# /var/lock/ejabberdctl
+CTLLOCKDIR = $(DESTDIR)@localstatedir@/lock/ejabberdctl
+
 # /var/lib/ejabberd/.erlang.cookie
 COOKIEFILE = $(SPOOLDIR)/.erlang.cookie
 
@@ -230,6 +233,12 @@ install: all
 	install -d -m 750 $(O_USER) $(SPOOLDIR)
 	$(CHOWN_COMMAND) -R @INSTALLUSER@ $(SPOOLDIR) >$(CHOWN_OUTPUT)
 	chmod -R 750 $(SPOOLDIR)
+
+	# ejabberdctl lock directory
+	install -d -m 750 $(O_USER) $(CTLLOCKDIR)
+	$(CHOWN_COMMAND) -R @INSTALLUSER@ $(CTLLOCKDIR) >$(CHOWN_OUTPUT)
+	chmod -R 750 $(CTLLOCKDIR)
+
 	[ ! -f $(COOKIEFILE) ] || { $(CHOWN_COMMAND) @INSTALLUSER@ $(COOKIEFILE) >$(CHOWN_OUTPUT) ; chmod 400 $(COOKIEFILE) ; }
 	#
 	# Log directory
@@ -265,6 +274,7 @@ uninstall-all: uninstall-binary
 	rm -rf $(ETCDIR)
 	rm -rf $(EJABBERDDIR)
 	rm -rf $(SPOOLDIR)
+	rm -rf $(CTLLOCKDIR)
 	rm -rf $(LOGDIR)
 
 clean: clean-recursive clean-local

src/ejabberdctl.template

@@ -14,6 +14,17 @@ ERLANG_NODE=$NODE@$HOST
 ERL=@erl@
 INSTALLUSER=@installuser@
 
+# Control number of connections identifiers
+# using flock if available. Expects a linux-style
+# flock that can lock a file descriptor.
+MAXCONNID=100
+CONNLOCKDIR=@LOCALSTATEDIR@/lock/ejabberdctl
+FLOCK='/usr/bin/flock'
+if [ ! -x "$FLOCK" ];then
+    FLOCK=""
+fi
+ 
+
 # parse command line parameters
 ARGS=
 while [ $# -ne 0 ] ; do
@@ -228,13 +239,54 @@ help ()
 ctl ()
 {
     COMMAND=$@
-    $EXEC_CMD "$ERL \
-      $NAME ctl-${ERLANG_NODE} \
-      -noinput \
-      -hidden \
-      -pa $EJABBERD_EBIN_PATH \
-      -s ejabberd_ctl -extra $ERLANG_NODE $COMMAND"
-    result=$?
+
+    if [ ! "$FLOCK" ];then
+	# no flock, simply invoke ctlexec()
+	CTL_CONN="ctl-${EJABBERD_NODE}"
+	ctlexec $CTL_CONN $COMMAND
+	result=$?
+    else
+	# we have flock so we get a lock
+	# on one of a limited number of
+	# conn names -- this allows
+	# concurrent invokations using a bound
+	# number of atoms
+	for N in $(seq 1 $MAXCONNID); do
+	    CTL_CONN="ejabberdctl-$N"
+	    CTL_LOCKFILE="$CONNLOCKDIR/$CTL_CONN"
+	    (
+		exec 8>"$CTL_LOCKFILE"
+		if flock --nb 8; then
+		    ctlexec $CTL_CONN $COMMAND
+                    ssresult=$?
+                    # segregate from possible flock exit(1)
+		    ssresult=$(expr $ssresult \* 10)
+		    exit $ssresult
+		else
+		    exit 1
+		fi
+            )
+	    result=$?
+	    if [ $result -eq 1 ]; then
+                # means we errored out in flock
+                # rather than in the exec - stay in the loop
+                # trying other conn names...
+		badlock=1
+	    else
+		badlock=""
+		break;
+	    fi
+	done
+	result=$(expr $result / 10)
+    fi
+
+    if [ "$badlock" ];then
+        echo "Ran out of connections to try. Your ejabberd processes" >&2
+        echo "may be stuck or this is a very busy server. For very"   >&2
+       echo "busy servers, consider raising MAXCONNIDS"          >&2
+       exit 1;
+    fi
+
     case $result in
     0) :;;
     1) :;;
@@ -244,6 +296,18 @@ ctl ()
     return $result
 }
 
+ctlexec ()
+{
+    CONN_NAME=$1; shift
+    COMMAND=$@
+    $EXEC_CMD "$ERL \
+      $NAME ${CONN_NAME} \
+      -noinput \
+      -hidden \
+      -pa $EJABBERD_EBIN_PATH \
+      -s ejabberd_ctl -extra $ERLANG_NODE $COMMAND"
+}
+
 # display ctl usage
 usage ()
 {