Prevent error loop on RECVFROM addresses with fork

CHANGES

@@ -43,6 +43,13 @@ Corrections:
 	is now increased to 2047 bytes.
 	Change suggested by Mario Camou.
 
+	Addresses of type RECVFROM with option fork looped with an error
+	message in case that the second address failed before consuming the
+	packet. The fix makes RECVFROM drop the packet when the second address
+	failed before reading it. Use retry or forever option with the second
+	address if you want to avoid data loss.
+	Thanks to Chunmei Xu for reporting this issue and proving the patch.
+
 Porting:
 	In gcc version 10 the default changed from -fcommon to -fno-common.
 	Consequently, linking filan and procan failed with error

doc/socat.yo

@@ -419,6 +419,7 @@ label(ADDRESS_IP_RECVFROM)dit(bf(tt(IP-RECVFROM:<protocol>)))
    This address works well with IP-SENDTO address peers (see above).
    Protocol 255 uses the raw socket with the IP header being part of the
    data.nl()
+   See the link(note about RECVFROM addresses)(NOTE_RECVFROM).nl()
    Option groups: link(FD)(GROUP_FD),link(SOCKET)(GROUP_SOCKET),link(IP4)(GROUP_IP4),link(IP6)(GROUP_IP6),link(CHILD)(GROUP_CHILD),link(RANGE)(GROUP_RANGE) nl()
    Useful options:
    link(pf)(OPTION_PROTOCOL_FAMILY),
@@ -790,6 +791,7 @@ label(ADDRESS_SOCKET_RECVFROM)dit(bf(tt(SOCKET-RECVFROM:<domain>:<type>:<protoco
    to find the appropriate values. The local-address must be the
    link(data)(TYPE_DATA) representation of a sockaddr structure without
    sa_family and (BSD) sa_len components.nl()
+   See the link(note about RECVFROM addresses)(NOTE_RECVFROM).nl()
    Option groups: link(FD)(GROUP_FD),link(SOCKET)(GROUP_SOCKET),link(CHILD)(GROUP_CHILD),link(RANGE)(GROUP_RANGE)nl()
    Useful options:
    link(fork)(OPTION_FORK),
@@ -1113,11 +1115,15 @@ label(ADDRESS_UDP_RECVFROM)dit(bf(tt(UDP-RECVFROM:<port>)))
    UDP/IP version 4 or 6 
    depending on option link(pf)(OPTION_PROTOCOL_FAMILY).
    It receives one packet from an unspecified peer and may send one or more
-   answer packets to that peer. This mode is particularly useful with fork
+   answer packets to that peer. This mode is particularly useful with
-   option 
+   link(fork)(OPTION_FORK) option 
    where each arriving packet - from arbitrary peers - is handled by its own sub
    process. This allows a behaviour similar to typical UDP based servers like ntpd
    or named. This address works well with socat UDP-SENDTO address peers.nl()
+   label(NOTE_RECVFROM)Note: When the second address fails before entering the transfer loop the
+   packet is dropped. Use option link(retry)(OPTION_RETRY) or
+   link(forever)(OPTION_FOREVER) on the second address to avoid data loss.
+   nl()
    Option groups: link(FD)(GROUP_FD),link(SOCKET)(GROUP_SOCKET),link(IP4)(GROUP_IP4),link(IP6)(GROUP_IP6),link(CHILD)(GROUP_CHILD),link(RANGE)(GROUP_RANGE) nl()
    Useful options:
    link(fork)(OPTION_FORK),
@@ -1233,6 +1239,7 @@ label(ADDRESS_UNIX_RECVFROM)dit(bf(tt(UNIX-RECVFROM:<filename>)))
    This mode is particularly useful with fork option where each arriving packet - from arbitrary peers - is handled by its own sub process.
    This address works well with socat UNIX-SENDTO address peers.nl()
    Option groups: link(FD)(GROUP_FD),link(SOCKET)(GROUP_SOCKET),link(NAMED)(GROUP_NAMED),link(CHILD)(GROUP_CHILD),link(UNIX)(GROUP_SOCK_UNIX) nl()
+   See the link(note about RECVFROM addresses)(NOTE_RECVFROM).nl()
    Useful options:
    link(fork)(OPTION_FORK)nl()
    See also:
@@ -2310,7 +2317,8 @@ label(OPTION_FORK)dit(bf(tt(fork)))
    child:
    OPENSSL-LISTEN forks em(before) the SSL handshake, while OPENSSL-CONNECT
    forks em(afterwards).
-   RETRY and FOREVER options are not inherited by the child process.nl()
+   link(retry)(OPTION_RETRY) and link(forever)(OPTION_FOREVER) options are not
+   inherited by the child process.nl()
    On some operating systems (e.g. FreeBSD) this option does not work for
    UDP-LISTEN addresses.nl()
 enddit()

xio-socket.c

@@ -1128,6 +1128,8 @@ static pid_t xio_waitingfor;	/* info from recv loop to signal handler:
 				   that should send us the USR1 signal */
 static bool xio_hashappened;	/* info from signal handler to loop: child
 				   process has read ("consumed") the packet */
+static int xio_childstatus;
+
 /* this is the signal handler for USR1 and CHLD */
 void xiosigaction_hasread(int signum
 #if HAVE_STRUCT_SIGACTION_SA_SIGACTION && defined(SA_SIGINFO)
@@ -1179,6 +1181,7 @@ void xiosigaction_hasread(int signum
 	 }
 	 if (pid == xio_waitingfor) {
 	    xio_hashappened = true;
+            xio_childstatus = WEXITSTATUS(status);
 	    Debug("xiosigaction_hasread() ->");
 	    diag_in_handler = 0;
 	    errno = _errno;
@@ -1480,6 +1483,12 @@ int _xioopen_dgram_recvfrom(struct single *xfd, int xioflags,
 	 xio_waitingfor = 0;	/* so this child will not set hashappened again */
 	 xio_hashappened = false;
 
+         if (xio_childstatus != 0) {
+             char buff[512];
+             Recv(xfd->fd, buff, sizeof(buff), 0);
+             xio_childstatus = 0;
+             Info("drop data because of child exit failed");
+         }
 	 Info("continue listening");
       } else {
 	break;