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().
(cherry picked from commit 126c7d0)

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

src/common.c

@@ -661,6 +661,26 @@ int parseconnusername(char *username, struct clientparam *param, int extpasswd,
 }
 
 
+/* What waits for a socket waits in poll(): 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/proxy.h

@@ -369,6 +369,8 @@ unsigned char * dologname (unsigned char *buf, unsigned char *name, const unsign
 int readconfig(FILE * fp);
 void initcommands(void);
 int connectwithpoll(struct clientparam *param, SOCKET sock, struct sockaddr *sa, SASIZETYPE size, int to);
+void nonblock(SOCKET sock);
+int acceptagain(void);
 int bindtodevice(struct sockfuncs *sf, void *state, SOCKET sock, const char *device, int family);
 int extbindtodevice(struct clientparam *param, SOCKET sock);
 int intbindtodevice(struct clientparam *param, SOCKET sock);

src/proxymain.c

@@ -88,6 +88,8 @@ void * threadfunc (void *p) {
 		}
 		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;
 		}
@@ -984,6 +986,8 @@ int MODULEMAINFUNC (int argc, char** argv){
 #endif
 
 	setopts(srv.cbsock, srv.cbssockopts);
+	/* 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");