[BugFix] avoid infinite loop with VLLM_PORT and get_open_ports_list (#36191)

Signed-off-by: walterbm <walter.beller.morales@gmail.com>

vllm/utils/network_utils.py

@@ -167,16 +167,34 @@ def get_open_port() -> int:
 
 
 def get_open_ports_list(count: int = 5) -> list[int]:
-    """Get a list of open ports."""
-    ports = set[int]()
-    while len(ports) < count:
-        ports.add(get_open_port())
-    return list(ports)
+    """Get a list of unique open ports.
+
+    When VLLM_PORT is set, scans upward from that port, advancing
+    the start position after each find so every port is unique.
+    """
+    ports_set = set[int]()
+    if envs.VLLM_PORT is not None:
+        next_port = envs.VLLM_PORT
+        for _ in range(count):
+            port = _get_open_port(start_port=next_port, max_attempts=1000)
+            ports_set.add(port)
+            next_port = port + 1
+        return list(ports_set)
+    else:
+        while len(ports_set) < count:
+            ports_set.add(get_open_port())
+
+    return list(ports_set)
 
 
-def _get_open_port() -> int:
-    port = envs.VLLM_PORT
+def _get_open_port(
+    start_port: int | None = None,
+    max_attempts: int | None = None,
+) -> int:
+    start_port = start_port if start_port is not None else envs.VLLM_PORT
+    port = start_port
     if port is not None:
+        attempts = 0
         while True:
             try:
                 with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
@@ -185,6 +203,12 @@ def _get_open_port() -> int:
             except OSError:
                 port += 1  # Increment port number if already in use
                 logger.info("Port %d is already in use, trying port %d", port - 1, port)
+            attempts += 1
+            if max_attempts is not None and attempts >= max_attempts:
+                raise RuntimeError(
+                    f"Could not find open port after {max_attempts} "
+                    f"attempts starting from port {start_port}"
+                )
     # try ipv4
     try:
         with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s: