[udp] Limit a split packet to the copy buffer

The copy buffer is sized by code generation to max_packet_size, but
set_max_packet_size() can raise the limit afterwards, and copying a split
packet would then overflow it. The link now takes the buffer as a span
and refuses a split packet larger than it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

esphome/components/udp/packet_interface/udp_packet_interface.cpp

@@ -35,8 +35,13 @@ bool UdpPacketInterface::send_to_interface(const PacketBuffer &data, const Packe
     auto span = data.as_span();
     return this->parent_->send_packet(span.data(), span.size());
   }
-  data.copy_to(this->tx_buffer_, size);
-  return this->parent_->send_packet(this->tx_buffer_, 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);
 }
 
 }  // namespace esphome::udp

esphome/components/udp/packet_interface/udp_packet_interface.h

@@ -19,8 +19,10 @@ namespace esphome::udp {
  */
 class UdpPacketInterface : public packet_interface::PacketInterface {
  public:
-  /// @param tx_buffer Holds max_packet_size bytes, for packets split across buffers; must outlive the link
-  UdpPacketInterface(UDPComponent *udp_component, uint8_t *tx_buffer) : parent_(udp_component), tx_buffer_(tx_buffer) {}
+  /// @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.
+  UdpPacketInterface(UDPComponent *udp_component, std::span<uint8_t> tx_buffer)
+      : parent_(udp_component), tx_buffer_(tx_buffer) {}
 
   void setup() override;
   bool send_to_interface(const packet_interface::PacketBuffer &data,
@@ -30,7 +32,7 @@ class UdpPacketInterface : public packet_interface::PacketInterface {
   void receive_(std::span<const uint8_t> data);
 
   UDPComponent *parent_;
-  uint8_t *const tx_buffer_;
+  const std::span<uint8_t> tx_buffer_;
 };
 }  // namespace esphome::udp
 #endif

tests/components/udp/udp_packet_interface.cpp

@@ -71,6 +71,16 @@ 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