[udp] Take the packet size from the copy buffer

The link derives max_packet_size from the span it is given, so a packet
larger than the copy buffer cannot be copied into it.

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

esphome/components/udp/packet_interface/udp_packet_interface.cpp

@@ -35,11 +35,6 @@ bool UdpPacketInterface::send_to_interface(const PacketBuffer &data, const Packe
     auto span = data.as_span();
     return this->parent_->send_packet(span.data(), span.size());
   }
-  if (size > this->tx_buffer_.size()) {
-    ESP_LOGW(TAG, "%s: Packet of %zu bytes is larger than the %zu byte buffer", this->name_, size,
-             this->tx_buffer_.size());
-    return false;
-  }
   data.copy_to(this->tx_buffer_.data(), size);
   return this->parent_->send_packet(this->tx_buffer_.data(), size);
 }

esphome/components/udp/packet_interface/udp_packet_interface.h

@@ -19,10 +19,9 @@ namespace esphome::udp {
  */
 class UdpPacketInterface : public packet_interface::PacketInterface {
  public:
-  /// @param tx_buffer Joins packets split across buffers, sized by code generation to max_packet_size; must outlive
-  /// the link. A larger max_packet_size set later is limited to it for those packets.
+  /// @param tx_buffer Joins packets split across buffers; its size is the largest packet, and it must outlive the link
   UdpPacketInterface(UDPComponent *udp_component, std::span<uint8_t> tx_buffer)
-      : parent_(udp_component), tx_buffer_(tx_buffer) {}
+      : packet_interface::PacketInterface(tx_buffer.size()), parent_(udp_component), tx_buffer_(tx_buffer) {}
 
   void setup() override;
   bool send_to_interface(const packet_interface::PacketBuffer &data,

tests/components/udp/udp_packet_interface.cpp

@@ -20,17 +20,19 @@ using packet_interface::PacketBuffer;
 using packet_interface::PacketMetaData;
 using Packet = std::vector<uint8_t>;
 
-class TestUdp : public UdpPacketInterface {
+// The copy buffer is a base, so it is set up before the link's constructor sees it
+struct UdpBufferStorage {
+  uint8_t tx_buffer_storage[8]{};
+};
+
+class TestUdp : public UdpBufferStorage, public UdpPacketInterface {
  public:
   explicit TestUdp(UDPComponent *udp) : UdpPacketInterface(udp, this->tx_buffer_storage) {
-    this->set_max_packet_size(8);
     this->add_packet_interface_listener(
         [this](const PacketBuffer &data, const PacketMetaData &) { this->received.push_back(to_vector(data)); });
   }
   using UdpPacketInterface::receive_;
 
-  // The base class only keeps the pointer, so the storage can be a member set up after it
-  uint8_t tx_buffer_storage[8]{};
   std::vector<Packet> received;
 };
 
@@ -71,16 +73,6 @@ TEST_F(UdpPacketInterfaceTest, OversizedPacketNotSent) {
   EXPECT_FALSE(this->link_.send_to_interface(PacketBuffer(&header, 1, &body, nullptr, 0), {}));
 }
 
-TEST_F(UdpPacketInterfaceTest, SplitPacketLimitedToBuffer) {
-  this->link_.setup();
-  // Raised above the copy buffer after code generation sized it
-  this->link_.set_max_packet_size(100);
-  uint8_t header = 0;
-  Packet body_data{1, 2, 3, 4, 5, 6, 7, 8};
-  PacketBuffer body(body_data);
-  EXPECT_FALSE(this->link_.send_to_interface(PacketBuffer(&header, 1, &body, nullptr, 0), {}));
-}
-
 }  // namespace esphome::udp::testing
 
 #endif