[socket] Read the LwIP interface list under the core lock (#20583)

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: J. Nick Koston <nick@home-assistant.io>

esphome/components/socket/socket.cpp

@@ -8,6 +8,7 @@
 #include <string>
 #include "esphome/core/log.h"
 #include "esphome/core/application.h"
+#include "esphome/core/helpers.h"
 #ifdef USE_HOST
 #include "esphome/core/wake.h"
 #endif
@@ -198,24 +199,36 @@ template<typename F> static bool foreach_eligible_ipv6_if(F &&callback) {
   }
   return ctx.found_any;
 #else
-  bool found_any = false;
-  struct netif *netif;
-  NETIF_FOREACH(netif) {
-    if (netif->name[0] == 'l' && netif->name[1] == 'o') {
-      continue;
+  // The netif list needs the core lock, but the callback's setsockopt() takes that lock itself, so the
+  // indices are collected first. One that goes away before the callback runs is rejected by setsockopt().
+  // Devices have only a few interfaces, so the cap drops nothing in practice.
+  static constexpr size_t MAX_IFS = 8;
+  StaticVector<uint8_t, MAX_IFS> indices;
+  {
+    LwIPLock lock;
+    struct netif *netif;
+    NETIF_FOREACH(netif) {
+      if (indices.size() == MAX_IFS) {
+        break;
+      }
+      if (netif->name[0] == 'l' && netif->name[1] == 'o') {
+        continue;
+      }
+      if (!(netif->flags & NETIF_FLAG_UP) || !(netif->flags & NETIF_FLAG_MLD6)) {
+        continue;
+      }
+      indices.push_back(netif_get_index(netif));
     }
-    if (!(netif->flags & NETIF_FLAG_UP) || !(netif->flags & NETIF_FLAG_MLD6)) {
-      continue;
-    }
-    found_any = true;
-    if (callback(netif_get_index(netif))) {
+  }
+  for (uint8_t index : indices) {
+    if (callback(index)) {
       break;
     }
   }
-  if (!found_any) {
+  if (indices.empty()) {
     errno = EADDRNOTAVAIL;
   }
-  return found_any;
+  return !indices.empty();
 #endif
 }
 #endif  // USE_SOCKET_IMPL_BSD_SOCKETS || USE_SOCKET_IMPL_LWIP_SOCKETS

tests/components/socket/test_multicast_host.cpp

@@ -46,10 +46,14 @@ TEST(JoinMulticastGroup, IPv4JoinSetsIfIndexToZero) {
 }
 
 TEST(JoinMulticastGroup, IPv4JoinNullIfIndexOut) {
-  auto sock = esphome::socket::socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
-  ASSERT_NE(sock, nullptr);
-  errno = 0;
-  EXPECT_TRUE(join_multicast_group(sock.get(), "239.0.60.53", nullptr) || errno != 0);
+  // Whether the host accepts the join or not, a null if_index_out must give the same answer
+  auto sock_with = esphome::socket::socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
+  auto sock_without = esphome::socket::socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
+  ASSERT_NE(sock_with, nullptr);
+  ASSERT_NE(sock_without, nullptr);
+  uint32_t if_index = 0;
+  const bool with_out = join_multicast_group(sock_with.get(), "239.0.60.53", &if_index);
+  EXPECT_EQ(join_multicast_group(sock_without.get(), "239.0.60.53", nullptr), with_out);
 }
 
 // =========================================================================
@@ -62,11 +66,13 @@ TEST(JoinMulticastGroup, IPv6JoinSucceedsOrNoInterface) {
   auto sock = esphome::socket::socket(AF_INET6, SOCK_DGRAM, IPPROTO_UDP);
   ASSERT_NE(sock, nullptr);
   uint32_t if_index = 0;
-  bool ok = join_multicast_group(sock.get(), "ff12::cafe", &if_index);
-  if (ok) {
+  errno = 0;
+  if (join_multicast_group(sock.get(), "ff12::cafe", &if_index)) {
     EXPECT_GT(if_index, 0u);
-  } else {
-    EXPECT_EQ(errno, EADDRNOTAVAIL);
+  } else if (errno != EADDRNOTAVAIL) {
+    // EINVAL can only come from our own parsing, since the address literal is valid
+    ASSERT_NE(errno, EINVAL);
+    GTEST_SKIP() << "Host refused the IPv6 multicast join, errno " << errno;
   }
 }
 
@@ -74,7 +80,9 @@ TEST(SetIPv6MulticastIf, ProbeWithZeroIndexSucceedsOrNoInterface) {
   auto sock = esphome::socket::socket(AF_INET6, SOCK_DGRAM, IPPROTO_UDP);
   ASSERT_NE(sock, nullptr);
   errno = 0;
-  EXPECT_TRUE(set_ipv6_multicast_if(sock.get(), 0) || errno == EADDRNOTAVAIL);
+  if (!set_ipv6_multicast_if(sock.get(), 0) && errno != EADDRNOTAVAIL) {
+    GTEST_SKIP() << "Host refused IPV6_MULTICAST_IF, errno " << errno;
+  }
 }
 
 TEST(SetIPv6MulticastIf, ExplicitIndexFromJoinSucceeds) {
@@ -83,10 +91,10 @@ TEST(SetIPv6MulticastIf, ExplicitIndexFromJoinSucceeds) {
   ASSERT_NE(sock_join, nullptr);
   ASSERT_NE(sock_send, nullptr);
   uint32_t if_index = 0;
-  bool joined = join_multicast_group(sock_join.get(), "ff12::cafe", &if_index);
-  if (joined && if_index != 0) {
-    EXPECT_TRUE(set_ipv6_multicast_if(sock_send.get(), if_index));
+  if (!join_multicast_group(sock_join.get(), "ff12::cafe", &if_index) || if_index == 0) {
+    GTEST_SKIP() << "Host has no interface that accepts an IPv6 multicast join";
   }
+  EXPECT_TRUE(set_ipv6_multicast_if(sock_send.get(), if_index));
 }
 
 #endif  // USE_NETWORK_IPV6