[packet_interface] Fixes from code review

- Reject an addressing destination or peer that is this device's own
  address, whose packets would be dropped as echoes
- Show 0 fragments, not SIZE_MAX, for a link with no room
- Test the check that applications use a peer's id

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

esphome/components/packet_interface/basic_fragmentation.cpp

@@ -44,7 +44,8 @@ void BasicFragmentation::setup() {
 }
 
 void BasicFragmentation::dump_config() {
-  size_t fragments = this->fragments_for_(this->max_packet_size_);
+  // A link with no room for fragments fails setup, which core reports below
+  size_t fragments = this->fragment_size_() == 0 ? 0 : this->fragments_for_(this->max_packet_size_);
   ESP_LOGCONFIG(TAG,
                 "Basic Fragmentation on '%s':\n"
                 "  Max Packet Size: %zu\n"

esphome/components/packet_interface/layers/basic_addressing.py

@@ -49,6 +49,22 @@ def _unique_destinations(peers: list[ConfigType]) -> list[ConfigType]:
     return peers
 
 
+def _not_own_address(config: ConfigType) -> ConfigType:
+    """Packets from this device's own address are dropped as echoes, so it cannot be a destination."""
+    address = config[CONF_ADDRESS]
+    if config.get(CONF_DESTINATION) == address:
+        raise cv.Invalid(
+            "Must not be this device's own address", path=[CONF_DESTINATION]
+        )
+    for index, peer in enumerate(config.get(CONF_PEERS, [])):
+        if peer[CONF_DESTINATION] == address:
+            raise cv.Invalid(
+                "Must not be this device's own address",
+                path=[CONF_PEERS, index, CONF_DESTINATION],
+            )
+    return config
+
+
 # A function, as each peer holds the other layers, whose schemas exist only once every layer type is registered
 def _basic_addressing_schema() -> cv.Schema:
     peer_schema = cv.All(
@@ -74,6 +90,7 @@ def _basic_addressing_schema() -> cv.Schema:
             }
         ).extend(cv.COMPONENT_SCHEMA),
         cv.has_at_most_one_key(CONF_DESTINATION, CONF_PEERS),
+        _not_own_address,
     )
 
 

tests/unit_tests/test_packet_interface.py

@@ -23,6 +23,8 @@ from esphome.components.packet_interface.layers.hdlc_reliability import (
 from esphome.components.uart import packet_interface as uart_packet_interface
 from esphome.components.udp import packet_interface as udp_packet_interface
 from esphome.const import CONF_ENCRYPTION
+from esphome.core import ID
+import esphome.final_validate as fv
 
 
 @pytest.mark.parametrize(
@@ -292,3 +294,36 @@ def test_auto_load_follows_layers(
 def test_auto_load_without_config() -> None:
     """Tooling that lists dependencies gets everything a layer could load."""
     assert packet_interface.AUTO_LOAD() == ["binary_sensor", "xxtea"]
+
+
+@pytest.mark.parametrize(
+    "addressing",
+    [
+        {"address": 1, "destination": 1},
+        {"address": 1, "peers": [_peer("a", 2), _peer("b", 1)]},
+    ],
+)
+def test_addressing_own_address_rejected(setup_core, addressing: dict) -> None:
+    """A device cannot send to its own address, as its own packets are dropped as echoes."""
+    with pytest.raises(cv.Invalid, match="own address"):
+        uart_packet_interface.CONFIG_SCHEMA({CONF_ADDRESSING: addressing})
+
+
+def test_application_needs_peer_id_with_several_peers(setup_core) -> None:
+    """An application must use one peer's id, not the id of a link with several peers."""
+    validator = packet_interface.final_validate_interface("packet_interface")
+    token = fv.full_config.set(
+        {
+            "packet_interface": [
+                {"id": ID("bus"), CONF_ADDRESSING: {"peers": [_peer("a", 2)]}},
+                {"id": ID("single"), CONF_ADDRESSING: {"destination": 2}},
+            ]
+        }
+    )
+    try:
+        with pytest.raises(cv.Invalid, match="several peers"):
+            validator({"packet_interface": ID("bus")})
+        assert validator({"packet_interface": ID("single")})
+        assert validator({"packet_interface": ID("a")})
+    finally:
+        fv.full_config.reset(token)