[udp] Keep the copy buffer in static memory

Code generation declares the UDP link's buffer for packets split across
buffers as a static array of max_packet_size bytes, so it is not
allocated at run time.

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

esphome/components/udp/packet_interface/__init__.py

@@ -1,9 +1,11 @@
 import esphome.codegen as cg
 from esphome.components import packet_interface
 from esphome.components.packet_interface import (
+    CONF_MAX_PACKET_SIZE,
     new_packet_interface,
     packet_interface_schema,
 )
+from esphome.const import CONF_ID
 from esphome.types import ConfigType
 
 from .. import CONF_UDP_ID, UDP_SCHEMA, udp_ns
@@ -30,4 +32,9 @@ async def to_code(config: ConfigType) -> None:
     cg.add(udp_var.set_should_listen())
     # The udp component sends to its addresses through its broadcast socket
     cg.add(udp_var.set_should_broadcast())
-    await new_packet_interface(config, udp_var)
+    # The copy buffer's size is known here, so it is a static array rather than allocated at run time
+    tx_buffer = f"{config[CONF_ID].id}__tx_buffer"
+    cg.add_global(
+        cg.RawStatement(f"static uint8_t {tx_buffer}[{config[CONF_MAX_PACKET_SIZE]}];")
+    )
+    await new_packet_interface(config, udp_var, cg.RawExpression(tx_buffer))

esphome/components/udp/packet_interface/udp_packet_interface.cpp

@@ -11,7 +11,6 @@ using packet_interface::PacketMetaData;
 ESPHOME_LOG_TAG(TAG, "udp.packet_interface");
 
 void UdpPacketInterface::setup() {
-  this->tx_buffer_ = std::make_unique<uint8_t[]>(this->max_packet_size_);
   this->parent_->add_listener([this](std::span<const uint8_t> data) { this->receive_(data); });
 }
 
@@ -36,8 +35,8 @@ 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_.get(), size);
-  return this->parent_->send_packet(this->tx_buffer_.get(), size);
+  data.copy_to(this->tx_buffer_, size);
+  return this->parent_->send_packet(this->tx_buffer_, size);
 }
 
 }  // namespace esphome::udp

esphome/components/udp/packet_interface/udp_packet_interface.h

@@ -4,7 +4,6 @@
 
 // udp_component.h declares nothing without USE_NETWORK
 #ifdef USE_NETWORK
-#include <memory>
 #include <span>
 
 #include "esphome/components/packet_interface/packet_interface.h"
@@ -20,7 +19,8 @@ namespace esphome::udp {
  */
 class UdpPacketInterface : public packet_interface::PacketInterface {
  public:
-  explicit UdpPacketInterface(UDPComponent *udp_component) : parent_(udp_component) {}
+  /// @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) {}
 
   void setup() override;
   bool send_to_interface(const packet_interface::PacketBuffer &data,
@@ -30,8 +30,7 @@ class UdpPacketInterface : public packet_interface::PacketInterface {
   void receive_(std::span<const uint8_t> data);
 
   UDPComponent *parent_;
-  // For packets split across buffers, allocated in setup()
-  std::unique_ptr<uint8_t[]> tx_buffer_;
+  uint8_t *const tx_buffer_;
 };
 }  // namespace esphome::udp
 #endif

tests/components/udp/udp_packet_interface.cpp

@@ -22,13 +22,15 @@ using Packet = std::vector<uint8_t>;
 
 class TestUdp : public UdpPacketInterface {
  public:
-  explicit TestUdp(UDPComponent *udp) : UdpPacketInterface(udp) {
+  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;
 };