socket-proxyd: fix off-by-one in connection limit

The limit is checked before the new connection is added to the set, so
with --connections-max=N and N connections open "set_size() > N" is
false and an additional connection is admitted. Use ">=" to refuse new
connections once the limit is reached.

(cherry picked from commit 9b51efd69fade39b0315251d02d1c9eab929891c)

src/socket-proxy/socket-proxyd.c

@@ -409,7 +409,7 @@ static int context_add_connection(Context *context, int fd) {
                 log_debug("New connection from %s", strna(peer));
         }
 
-        if (set_size(context->connections) > arg_connections_max)
+        if (set_size(context->connections) >= arg_connections_max)
                 return log_warning_errno(SYNTHETIC_ERRNO(EBUSY), "Hit connection limit, refusing connection.");
 
         r = sd_event_source_set_enabled(context->idle_time, SD_EVENT_OFF);

test/integration-tests/TEST-74-AUX-UTILS/TEST-74-AUX-UTILS.units/proxy-echo.py

@@ -7,12 +7,17 @@ import sys
 data = sys.stdin.buffer.read()
 s = socket.create_connection(('localhost', 12345), timeout=15)
 s.settimeout(15)
-s.sendall(data)
 received = b''
-while len(received) < len(data):
-    chunk = s.recv(65536)
-    if not chunk:
-        break
-    received += chunk
+try:
+    s.sendall(data)
+    while len(received) < len(data):
+        chunk = s.recv(65536)
+        if not chunk:
+            break
+        received += chunk
+except (ConnectionResetError, BrokenPipeError):
+    # A proxy refusing the connection closes it, which may arrive as a reset rather than EOF
+    if received:
+        raise
 sys.stdout.buffer.write(received)
 s.close()

test/units/TEST-74-AUX-UTILS.socket-proxyd.sh

@@ -65,3 +65,26 @@ PROXY_PID="$(systemctl show -P MainPID test-proxyd.service)"
 read -r _ _ _ SOFT HARD _ < <(grep '^Max open files' "/proc/$PROXY_PID/limits")
 assert_eq "$SOFT" 4096
 assert_eq "$HARD" 4096
+
+# --connections-max= is a hard cap: while one connection is held open, the next one is closed unserved
+systemctl stop test-proxyd.socket test-proxyd.service
+cat >/run/systemd/system/test-proxyd.service <<EOF
+[Service]
+ExecStart=/usr/lib/systemd/systemd-socket-proxyd --connections-max=1 $BACKEND_SOCK
+EOF
+systemctl daemon-reload
+systemctl start test-proxyd.socket
+
+exec {HELD}<>/dev/tcp/127.0.0.1/12345
+echo -n held >&"$HELD"
+read -r -N 4 -t 15 REPLY <&"$HELD"
+assert_eq "$REPLY" "held"
+# Assigned rather than passed to assert_eq, so that set -e fails the test when the client errors out
+REFUSED="$(echo -n refused | proxy_echo)"
+assert_eq "$REFUSED" ""
+journalctl --sync
+journalctl -b -u test-proxyd.service --grep "Hit connection limit" >/dev/null
+echo -n still >&"$HELD"
+read -r -N 5 -t 15 REPLY <&"$HELD"
+assert_eq "$REPLY" "still"
+exec {HELD}>&-