connect back: a replaced -r service stops waiting at the reload

A -r service waited up to 60 s for the go-ahead on the connection it
made, without looking at a reload. One replaced by a reload went on
waiting, holding a slot and a thread, and took a client if the far
side's go-ahead came in that time; every reload left another such
connection in the far side's queue. The wait is now a second at a time
(60 s in all, as before): at a reload the service takes a go-ahead
already there, its client going on in the service which replaced it,
and otherwise closes the connection and stops.

The far side (-R) sent its go-ahead to any connection it accepted, and
a closed one cost it a client. A connection the -r side closed, at a
reload or after its wait, is readable before any go-ahead, since that
side sends nothing first: once past the ACL it is passed over, without
counting as one of the tries for as many as the listen queue holds
(1 + maxchild/16), so that connections made to be closed or to send
something cannot keep a client's thread in the loop.

Tests: reattach's far side passes over closed connections as -R does.

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

src/mainclient.c

@@ -41,6 +41,8 @@ static void serveparam(struct clientparam *param){
  if(param->srv->cbsock != INVALID_SOCKET){
 	SASIZETYPE size = sizeof(param->sinsr);
 	struct pollfd fds;
+	/* closed connections passed over free: the listen queue's length */
+	int skips = 1 + (param->srv->maxchild>>4);
 	fds.fd = param->srv->cbsock;
 	fds.events = POLLIN;
 	fds.revents = 0;
@@ -73,6 +75,24 @@ static void serveparam(struct clientparam *param){
 			continue;
 		}
 #endif
+		{
+			/* the side connecting back sends nothing before the go-ahead:
+			   readable, the connection was closed there, by a service
+			   which stopped waiting (a reload, its time out); the next
+			   one is taken, and this one does not count as a try, for as
+			   many as the queue holds: past that it does */
+			struct pollfd rf;
+
+			rf.fd = param->remsock;
+			rf.events = POLLIN;
+			rf.revents = 0;
+			if(param->srv->so._poll(param->sostate, &rf, 1, 0) == 1){
+				param->srv->so._closesocket(param->sostate, param->remsock);
+				param->remsock = INVALID_SOCKET;
+				if(skips > 0){ skips--; i++; }
+				continue;
+			}
+		}
 		if(socksend(param, param->remsock, (unsigned char *)"C", 1, CONNBACK_TO) != 1){
 			dolog(param, (unsigned char *)"Connect back sending command failed");
 			param->srv->so._closesocket(param->sostate, param->remsock);

src/maintcp.c

@@ -37,6 +37,9 @@ static int cblisten(struct srvstart *s){
 
 #endif
 
+/* How long, in seconds, a connection made back (-r) waits for its go-ahead */
+#define CBWAIT 60
+
 /* What getting the next client came to. */
 enum { NEXT_CLIENT, NEXT_RETRY, NEXT_STOP };
 
@@ -49,7 +52,7 @@ static int tcpnext(struct srvstart *s, SOCKET *client, int *nlog){
  SOCKET new_sock;
  SASIZETYPE size;
  unsigned char buf[256];
- int error = 0, opt = 1;
+ int error = 0, opt = 1, res, waited;
 #ifndef _WIN32
  struct linger lg;
 #else
@@ -81,7 +84,18 @@ static int tcpnext(struct srvstart *s, SOCKET *client, int *nlog){
 		return NEXT_RETRY;
 	}
 
-	if(sockrecvfrom(NULL, new_sock, NULL, buf, 1, 60*1000) != 1 || *buf!='C') {
+	/* the far side's go-ahead, waited for a second at a time to notice
+	   a reload: a service paused then takes no more connections and
+	   closes this one, unless the go-ahead came already, and its client
+	   goes on in the service which took its place */
+	for(waited = 0; !(res = sockrecvfrom(NULL, new_sock, NULL, buf, 1, 1000)); ){
+		if(srv->config->paused != srv->paused){
+			res = sockrecvfrom(NULL, new_sock, NULL, buf, 1, 0);
+			break;
+		}
+		if(++waited >= CBWAIT) break;
+	}
+	if(res != 1 || *buf!='C') {
 		so._closesocket(so.state, new_sock);
 		usleep(SLEEPTIME);
 		return NEXT_RETRY;

tests/cases/reattach.py

@@ -3,6 +3,7 @@ the rules of the newest service on its listener, taking one of its maxconn
 slots, and is dropped where that cannot be."""
 
 import base64
+import select
 import socket
 import threading
 import time
@@ -128,7 +129,13 @@ def run(t):
         """A request over a connection the -r service made: the peer is its
         client once it has sent C."""
         try:
+            # as the far side of a connect-back service does: a connection
+            # closed by a service which stopped waiting is passed over
+            while True:
                 c, _ = peer.accept()
+                if not select.select([c], [], [], 0)[0]:
+                    break
+                c.close()
             c.settimeout(30)
             c.sendall(b"C")
             auth = base64.b64encode(b"alice:one").decode()