make decode error messages more descriptive

Author: nrednav

lib/msgpack/decode_error.ex

@@ -2,6 +2,30 @@ defmodule Msgpack.DecodeError do
   defexception [:message, :reason]
 
   @impl true
+  def message(%{reason: :unexpected_eof}) do
+    "Unexpected end of file. The MessagePack binary is incomplete."
+  end
+
+  def message(%{reason: {:unknown_prefix, prefix}}) do
+    "Unknown type prefix: #{prefix}. The byte `0x#{Integer.to_string(prefix, 16)}` is not a valid MessagePack type marker."
+  end
+
+  def message(%{reason: {:trailing_bytes, rest}}) do
+    "Trailing bytes remaining after decoding. A valid MessagePack binary must have exactly one root object. Remaining data: #{inspect(rest)}"
+  end
+
+  def message(%{reason: {:max_depth_reached, max_depth}}) do
+    "Maximum nesting depth of #{max_depth} reached. This limit can be configured with the `:max_depth` option."
+  end
+
+  def message(%{reason: {:max_byte_size_exceeded, max_size}}) do
+    "A declared object size exceeds the maximum of #{max_size} bytes. This limit can be configured with the `:max_byte_size` option."
+  end
+
+  def message(%{reason: :invalid_timestamp}) do
+    "Invalid timestamp format. The nanosecond field must be less than 1,000,000,000."
+  end
+
   def message(exception) do
     "Failed to decode MessagePack binary. Reason = #{inspect(exception.reason)}"
   end

test/msgpack_test.exs

@@ -162,6 +162,20 @@ defmodule MsgpackTest do
         Msgpack.decode!(input)
       end
     end
+
+    test "raises an error with a descriptive message" do
+      assert_raise Msgpack.DecodeError, "Unexpected end of file. The MessagePack binary is incomplete.", fn ->
+        Msgpack.decode!(<<0x81, 0xA3, "f">>)
+      end
+
+      assert_raise Msgpack.DecodeError, "Unknown type prefix: 193. The byte `0xC1` is not a valid MessagePack type marker.", fn ->
+        Msgpack.decode!(<<0xC1>>)
+      end
+
+      assert_raise Msgpack.DecodeError, "Maximum nesting depth of 2 reached. This limit can be configured with the `:max_depth` option.", fn ->
+        Msgpack.decode!(<<0x91, 0x91, 0x91, 1>>, max_depth: 2)
+      end
+    end
   end
 
   describe "Property Tests" do