Connect back: a connection one client takes leaves the others waiting

The clients of a -R service all wait for the connection back on the
same listening socket, which was blocking: one connection woke them
all, one took it, and the others were left in accept() until the next,
past their time out and through a reload. The socket is non-blocking,
and an accept() which finds nothing, the connection taken by another
client or gone, is not one of the client's tries. nonblock() makes a
socket non-blocking, acceptagain() tells such an accept().

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/common.c

@@ -674,6 +674,26 @@ int parseconnusername(char *username, struct clientparam *param, int extpasswd,
 }
 
 
+/* What waits for a socket waits in sockpoll(): a call on it never does. */
+void nonblock(SOCKET sock){
+#ifdef _WIN32
+	unsigned long ul = 1;
+	ioctlsocket(sock, FIONBIO, &ul);
+#else
+	fcntl(sock, F_SETFL, O_NONBLOCK | fcntl(sock, F_GETFL));
+#endif
+}
+
+/* accept() with nothing to accept: another took it, or it was gone */
+int acceptagain(void){
+#ifdef _WIN32
+	int err = WSAGetLastError();
+	return err == WSAEWOULDBLOCK || err == WSAECONNRESET || err == WSAEINTR;
+#else
+	return errno == EAGAIN || errno == EWOULDBLOCK || errno == ECONNABORTED || errno == EINTR;
+#endif
+}
+
 int connectwithpoll(struct clientparam *param, SOCKET sock, struct sockaddr *sa, SASIZETYPE size, int to){
 		struct pollfd fds[1];
 #ifdef _WIN32

src/mainclient.c

@@ -54,6 +54,8 @@ static void serveparam(struct clientparam *param){
 		}
 		param->remsock = param->srv->so._accept(param->sostate, param->srv->cbsock, (struct sockaddr*)&param->sinsr, &size);
 		if(param->remsock == INVALID_SOCKET) {
+			/* another client of the service took it: not a try */
+			if(acceptagain()) { i++; continue; }
 			dolog(param, (unsigned char *)"Connect back accept() failed");
 			continue;
 		}

src/maintcp.c

@@ -23,6 +23,8 @@ static int cblisten(struct srvstart *s){
 	return -6;
  }
  setopts(srv->cbsock, &srv->cbssockopts, srv->maxseg);
+ /* the clients of the service all wait for it: one accepts what comes */
+ nonblock(srv->cbsock);
 
  if(srv->so._bind(srv->so.state, srv->cbsock, (struct sockaddr*)&cbsa, SASIZE(&cbsa))==-1) {
 	dolog(defparam, (unsigned char *)"Failed to bind connect back socket");

src/proxy.h

@@ -333,6 +333,8 @@ int nsbroken(void);
    lines among its arguments */
 SOCKET nssocket(struct srvparam *srv, struct sockfuncs *sf, void *state, int side, int domain, int type, int protocol);
 int sockpoll(struct sockfuncs *sf, void *state, struct pollfd *fds, unsigned nfds, int timeout);
+void nonblock(SOCKET sock);
+int acceptagain(void);
 int socksend(struct clientparam *param, SOCKET sock, unsigned char * buf, int bufsize, int to);
 int socksendto(struct clientparam *param, SOCKET sock, struct sockaddr * sin, unsigned char * buf, int bufsize, int to);
 int sockrecvfrom(struct clientparam *param, SOCKET sock, struct sockaddr * sin, unsigned char * buf, int bufsize, int to);