[uart] Add discard_available to drop stale receive bytes (#21029)

esphome/components/uart/uart.cpp

@@ -33,6 +33,13 @@ void UARTDevice::check_uart_settings(uint32_t baud_rate, uint8_t stop_bits, UART
   }
 }
 
+void UARTDevice::discard_available() {
+  UARTComponent *parent = this->parent_;
+  uint8_t byte;
+  while (parent->available() && parent->read_byte(&byte)) {
+  }
+}
+
 const LogString *parity_to_str(UARTParityOptions parity) {
   return UARTParityStrings::get_log_str(static_cast<uint8_t>(parity), UARTParityStrings::LAST_INDEX);
 }

esphome/components/uart/uart.h

@@ -90,6 +90,9 @@ class UARTDevice {
         count);
   }
 
+  /// Read and drop every byte waiting in the receive buffer.
+  void discard_available();
+
   UARTFlushResult flush() { return this->parent_->flush(); }
 
   // Compat APIs

tests/components/uart/uart_read_available.cpp

@@ -141,4 +141,42 @@ TEST(UARTReadAvailableTest, ChunksWithCountStopEarly) {
   EXPECT_EQ(uart.remaining(), 14u);
 }
 
+TEST(UARTReadAvailableTest, DiscardDropsEveryWaitingByte) {
+  ::testing::NiceMock<FakeRxUART> uart(make_bytes(5));
+  UARTDevice dev(&uart);
+  dev.discard_available();
+  EXPECT_EQ(uart.remaining(), 0u);
+}
+
+TEST(UARTReadAvailableTest, DiscardWithNothingAvailableDoesNotRead) {
+  ::testing::NiceMock<FakeRxUART> uart(std::vector<uint8_t>{});
+  UARTDevice dev(&uart);
+  dev.discard_available();
+  EXPECT_TRUE(uart.read_sizes.empty());
+}
+
+TEST(UARTReadAvailableTest, DiscardDropsBytesThatArriveWhileDraining) {
+  ::testing::NiceMock<FakeRxUART> uart(std::vector<uint8_t>{1, 2});
+  bool appended = false;
+  ON_CALL(uart, available()).WillByDefault(Invoke([&]() -> size_t {
+    if (!appended && uart.read_sizes.size() == 1) {
+      uart.append({9, 9});
+      appended = true;
+    }
+    return uart.remaining();
+  }));
+  UARTDevice dev(&uart);
+  dev.discard_available();
+  EXPECT_TRUE(appended);
+  EXPECT_EQ(uart.remaining(), 0u);
+}
+
+TEST(UARTReadAvailableTest, DiscardStopsOnReadFailure) {
+  ::testing::NiceMock<FakeRxUART> uart(make_bytes(5));
+  uart.fail_after_reads = 2;
+  UARTDevice dev(&uart);
+  dev.discard_available();
+  EXPECT_EQ(uart.read_sizes.size(), 3u);
+}
+
 }  // namespace esphome::uart::testing