Add experimental I2C failure support for SHT4x's

This seems useful to extend more generally as well. It wouldn't remove a
device-specific implementation since some devices get unhappy and start
returning errors when you don't treat them right. That might be
interesting. The goal of this is to get some experience before doing
anything more substantial.

Author: fhunleth

lib/circuits_sim/device/sht4x.ex

@@ -15,6 +15,7 @@ defmodule CircuitsSim.Device.SHT4X do
             humidity_rh: 30.0,
             temperature_c: 22.2,
             crc_injection_count: 0,
+            broken: nil,
             acc: <<>>
 
   @type t() :: %__MODULE__{
@@ -23,6 +24,7 @@ defmodule CircuitsSim.Device.SHT4X do
           humidity_rh: float(),
           temperature_c: float(),
           crc_injection_count: non_neg_integer(),
+          broken: nil | {:error, any()},
           acc: binary()
         }
 
@@ -69,12 +71,27 @@ defmodule CircuitsSim.Device.SHT4X do
     I2CServer.send_message(bus_name, address, {:inject_crc_errors, count})
   end
 
+  @doc """
+  Experimental API to return I2C errors on read/read_writes
+
+  Set the 3rd argument to an error tuple to be returned or `nil` to work
+  normally.
+  """
+  @spec set_broken(String.t(), Circuits.I2C.address(), nil | {:error, any()}) :: :ok
+  def set_broken(bus_name, address, broken) do
+    I2CServer.send_message(bus_name, address, {:set_broken, broken})
+  end
+
   ## protocol implementation
 
   defimpl I2CDevice do
     @crc_alg :cerlc.init(:crc8_sensirion)
 
     @impl I2CDevice
+    def read(%{broken: result} = state, _count) when is_tuple(result) do
+      {result, state}
+    end
+
     def read(%{current: :serial_number} = state, count) do
       state |> binary_for_serial_number() |> flush_read_to_result(count)
     end
@@ -104,6 +121,10 @@ defmodule CircuitsSim.Device.SHT4X do
     def write(state, _), do: state
 
     @impl I2CDevice
+    def write_read(%{broken: result} = state, _to_write, _read_count) when is_tuple(result) do
+      {result, state}
+    end
+
     def write_read(state, _to_write, read_count) do
       {{:ok, :binary.copy(<<0>>, read_count)}, %{state | current: nil}}
     end
@@ -131,6 +152,10 @@ defmodule CircuitsSim.Device.SHT4X do
       {:ok, %{state | crc_injection_count: count}}
     end
 
+    def handle_message(state, {:set_broken, value}) do
+      {:ok, %{state | broken: value}}
+    end
+
     defp binary_for_serial_number(state) do
       state |> add_crcs(<<state.serial_number::32>>)
     end