RECVFROM addresses with FORK option hung after processing the first packet

CHANGES

@@ -4,6 +4,9 @@ corrections:
 	with the first received packet an error occurred:
 	socket_init(): unknown address family 0
 
+	RECVFROM addresses with FORK option hung after processing the first
+	packet.
+
 ####################### V 1.6.0.1:
 
 new features:

VERSION

@@ -1 +1 @@
-"1.6.0.1+ip4bind"
+"1.6.0.1+ip4bind+recvfromfork"

test.sh

@@ -8195,6 +8195,54 @@ PORT=$((PORT+1))
 N=$((N+1))
 
 
+# there was a bug in *-recvfrom with fork: due to an error in the appropriate
+# signal handler the master process would hang after forking off the first
+# child process.
+NAME=UDP4RECVFROM_FORK
+case "$TESTS" in
+*%functions%*|*%ip4%*|*%udp%*|*%dgram%*|*%$NAME%*)
+TEST="$NAME: test if UDP4-RECVFROM handles more than one packet"
+# idea: run a UDP4-RECVFROM process with fork and -T. Send it one packet;
+# send it a second packet and check if this is processed properly. If yes, the
+# test succeeded.
+tf="$td/test$N.stdout"
+te="$td/test$N.stderr"
+tdiff="$td/test$N.diff"
+tsp=$PORT
+ts="$LOCALHOST:$tsp"
+da=$(date)
+CMD1="$SOCAT $opts -T 2 UDP4-RECVFROM:$tsp,reuseaddr,fork PIPE"
+CMD2="$SOCAT $opts -T 1 - UDP4-SENDTO:$ts"
+printf "test $F_n $TEST... " $N
+$CMD1 >/dev/null 2>"${te}1" &
+pid1=$!
+waitudp4port $tsp 1
+echo "$da" |$CMD2 >/dev/null 2>>"${te}2"	# this should always work
+rc2a=$?
+sleep 1
+echo "$da" |$CMD2 >"$tf" 2>>"${te}3"		# this would fail when bug
+rc2b=$?
+kill $pid1 2>/dev/null; wait
+if [ $rc2b -ne 0 ]; then
+    $PRINTF "$NO_RESULT\n"
+    numCANT=$((numCANT+1))
+elif ! echo "$da" |diff - "$tf" >"$tdiff"; then
+    $PRINTF "$FAILED: $SOCAT:\n"
+    echo "$CMD1 &"
+    echo "$CMD2"
+    cat "${te}1" "${te}2" "${te}3"
+    cat "$tdiff"
+    numFAIL=$((numFAIL+1))
+else
+    $PRINTF "$OK\n"
+    if [ -n "$debug" ]; then cat "${te}1" "${te}2" "${te}3"; fi
+    numOK=$((numOK+1))
+fi ;;
+esac
+PORT=$((PORT+1))
+N=$((N+1))
+
+
 echo "summary: $((N-1)) tests; $numOK ok, $numFAIL failed, $numCANT could not be performed"
 
 if [ "$numFAIL" -gt 0 ]; then

xio-socket.c

@@ -509,8 +509,26 @@ int _xioopen_dgram_sendto(/* them is already in xfd->peersa */
 }
 
 
-static pid_t xio_waitingfor;
+/* when the recvfrom address (with option fork) receives a packet it keeps this
-static bool xio_hashappened;
+   packet in the IP stacks input queue and forks a sub process. The sub process
+   then reads this packet for processing its data.
+   There is a problem because the parent process would find the same packet
+   again if it calls select()/poll() before the client process reads the
+   packet.
+   To solve this problem we implement the following mechanism:
+   The sub process sends a SIGUSR1 when it has read the packet (or a SIGCHLD if
+   it dies before). The parent process waits until it receives that signal and
+   only then continues to listen.
+   To prevent a signal from another process to trigger our loop, we pass the
+   pid of the sub process to the signal handler in xio_waitingfor. The signal
+   handler sets xio_hashappened if the pid matched.
+*/
+static pid_t xio_waitingfor;	/* info from recv loop to signal handler:
+				   indicates the pid that of the child process
+				   that should send us the USR1 signal */
+static bool xio_hashappened;	/* info from signal handler to loop: child
+				   process has read ("consumed") the packet */
+/* this is the signal handler for USR1 and CHLD */
 void xiosigaction_hasread(int signum, siginfo_t *siginfo, void *ucontext) {
    pid_t pid;
    int _errno;
@@ -519,6 +537,7 @@ void xiosigaction_hasread(int signum, siginfo_t *siginfo, void *ucontext) {
    Debug5("xiosigaction_hasread(%d, {%d,%d,%d,"F_pid"}, )",
 	  signum, siginfo->si_signo, siginfo->si_errno, siginfo->si_code,
 	  siginfo->si_pid);
+   if (signum == SIGCHLD) {
       _errno = errno;
       do {
 	 pid = Waitpid(-1, &status, WNOHANG);
@@ -527,12 +546,14 @@ void xiosigaction_hasread(int signum, siginfo_t *siginfo, void *ucontext) {
 		"waitpid(-1, {}, WNOHANG): no child has exited");
 	    Info("childdied() finished");
 	    errno = _errno;
+	    Debug("xiosigaction_hasread() ->");
 	    return;
 	 } else if (pid < 0 && errno == ECHILD) {
 	    Msg1(wassig?E_INFO:E_WARN,
 		 "waitpid(-1, {}, WNOHANG): %s", strerror(errno));
 	    Info("childdied() finished");
 	    errno = _errno;
+	    Debug("xiosigaction_hasread() ->");
 	    return;
 	 }
 	 wassig = true;
@@ -540,9 +561,11 @@ void xiosigaction_hasread(int signum, siginfo_t *siginfo, void *ucontext) {
 	    Warn2("waitpid(-1, {%d}, WNOHANG): %s", status, strerror(errno));
 	    Info("childdied() finished");
 	    errno = _errno;
+	    Debug("xiosigaction_hasread() ->");
 	    return;
 	 }
       } while (1);
+   }
    if (xio_waitingfor == siginfo->si_pid) {
       xio_hashappened = true;
    }