From a8c3d5e127776d74eb068c95610277feb99adcf0 Mon Sep 17 00:00:00 2001
From: Misaki Shioi <31817032+shioimm@users.noreply.github.com>
Date: Sat, 27 Dec 2025 18:26:56 +0900
Subject: Fix: Do not fast_fallback if local_port is explicitly specified
 (#15732)

`fast fallback` cannot be used with explicitly specified local port,
because concurrent binds to the same `local_host:local_port`
can raise `Errno::EADDRINUSE`.

This issue is more likely to occur on hosts with `IPV6_V6ONLY` disabled,
because IPv6 binds can also occupy IPv4-mapped IPv6 address space.
---
 ext/socket/ipsocket.c    | 24 +++++++++++++++++++++---
 ext/socket/lib/socket.rb |  5 ++---
 2 files changed, 23 insertions(+), 6 deletions(-)

(limited to 'ext')

diff --git a/ext/socket/ipsocket.c b/ext/socket/ipsocket.c
index e952b7871b..758d37293f 100644
--- a/ext/socket/ipsocket.c
+++ b/ext/socket/ipsocket.c
@@ -258,6 +258,22 @@ is_specified_ip_address(const char *hostname)
             inet_pton(AF_INET, hostname, &ipv4addr) == 1);
 }
 
+static int
+is_local_port_fixed(const char *portp)
+{
+    if (!portp) return 0;
+
+    char *endp;
+    errno = 0;
+    long port = strtol(portp, &endp, 10);
+
+    if (endp == portp) return 0;
+    if (errno == ERANGE) return 0;
+    if (port <= 0) return 0;
+
+    return port != 0;
+}
+
 struct fast_fallback_inetsock_arg
 {
     VALUE self;
@@ -1314,13 +1330,15 @@ rsock_init_inetsock(
 
     if (type == INET_CLIENT && FAST_FALLBACK_INIT_INETSOCK_IMPL == 1 && RTEST(fast_fallback)) {
         struct rb_addrinfo *local_res = NULL;
-        char *hostp, *portp;
-        char hbuf[NI_MAXHOST], pbuf[NI_MAXSERV];
+        char *hostp, *portp, *local_portp;
+        char hbuf[NI_MAXHOST], pbuf[NI_MAXSERV], local_pbuf[NI_MAXSERV];
         int additional_flags = 0;
+        int local_flags = 0;
         hostp = raddrinfo_host_str(remote_host, hbuf, sizeof(hbuf), &additional_flags);
         portp = raddrinfo_port_str(remote_serv, pbuf, sizeof(pbuf), &additional_flags);
+        local_portp = raddrinfo_port_str(local_serv, local_pbuf, sizeof(local_pbuf), &local_flags);
 
-        if (!is_specified_ip_address(hostp)) {
+        if (!is_specified_ip_address(hostp) && !is_local_port_fixed(local_portp)) {
             int target_families[2] = { 0, 0 };
             int resolving_family_size = 0;
 
diff --git a/ext/socket/lib/socket.rb b/ext/socket/lib/socket.rb
index e74eaec43a..36fcceaee9 100644
--- a/ext/socket/lib/socket.rb
+++ b/ext/socket/lib/socket.rb
@@ -660,12 +660,11 @@ class Socket < BasicSocket
   #     puts sock.read
   #   }
   def self.tcp(host, port, local_host = nil, local_port = nil, connect_timeout: nil, resolv_timeout: nil, open_timeout: nil, fast_fallback: tcp_fast_fallback, &) # :yield: socket
-
     if open_timeout && (connect_timeout || resolv_timeout)
       raise ArgumentError, "Cannot specify open_timeout along with connect_timeout or resolv_timeout"
     end
 
-    sock = if fast_fallback && !(host && ip_address?(host))
+    sock = if fast_fallback && !(host && ip_address?(host)) && !(local_port && local_port.to_i != 0)
       tcp_with_fast_fallback(host, port, local_host, local_port, connect_timeout:, resolv_timeout:, open_timeout:)
     else
       tcp_without_fast_fallback(host, port, local_host, local_port, connect_timeout:, resolv_timeout:, open_timeout:)
@@ -736,7 +735,7 @@ class Socket < BasicSocket
           if local_addrinfos.any?
             local_addrinfo = local_addrinfos.find { |lai| lai.afamily == addrinfo.afamily }
 
-            if local_addrinfo.nil? # Connecting addrinfoと同じアドレスファミリのLocal addrinfoがない
+            if local_addrinfo.nil?
               if resolution_store.any_addrinfos?
                 # Try other Addrinfo in next "while"
                 next
-- 
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