From 0c80c9f526dea0e2c8b3262cbad173b7f0e8653c Mon Sep 17 00:00:00 2001
From: Vladimir Dubrovin <3proxy@3proxy.ru>
Date: Sat, 24 Oct 2026 20:56:15 +0300
Subject: [PATCH] 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    | 20 ++++++++++++++++++++
 src/proxy.h     |  2 ++
 src/proxymain.c |  4 ++++
 3 files changed, 26 insertions(+)

diff --git a/src/common.c b/src/common.c
index d6c942b..ee2cece 100644
--- a/src/common.c
+++ b/src/common.c
@@ -634,6 +634,26 @@ 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
diff --git a/src/proxy.h b/src/proxy.h
index 1197254..a7e8407 100644
--- a/src/proxy.h
+++ b/src/proxy.h
@@ -405,6 +405,8 @@ 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 bindwithrange(struct clientparam *param, SOCKET sock, PROXYSOCKADDRTYPE *sa, uint32_t range);
 int bindtodevice(struct sockfuncs *sf, void *state, SOCKET sock, const char *device, int family);
 int extbindtodevice(struct clientparam *param, SOCKET sock);
diff --git a/src/proxymain.c b/src/proxymain.c
index 9b84206..7edcc44 100644
--- a/src/proxymain.c
+++ b/src/proxymain.c
@@ -63,6 +63,8 @@ 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;
 		}
@@ -864,6 +866,8 @@ static int cblisten(struct srvstart *s){
 	return -6;
  }
  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");