Gracefully degrade application failure error when deserializing

lib/temporal/workflow/errors.rb

@@ -10,12 +10,35 @@ class Errors
       def self.generate_error(failure, default_exception_class = StandardError)
         case failure.failure_info
         when :application_failure_info
-          exception_class = safe_constantize(failure.application_failure_info.type)
-          exception_class ||= default_exception_class
           message = from_details_payloads(failure.application_failure_info.details)
-          backtrace = failure.stack_trace.split("\n")
 
-          exception_class.new(message).tap do |exception|
+          exception_class = safe_constantize(failure.application_failure_info.type)
+          if exception_class.nil?
+            Temporal.logger.error(
+              "Could not find original error class. Defaulting to StandardError.", 
+              {original_error: failure.application_failure_info.type},
+            )
+            message = "#{failure.application_failure_info.type}: #{message}"
+            exception_class = default_exception_class
+          end
+
+
+          begin
+            exception = exception_class.new(message)
+          rescue ArgumentError => deserialization_error
+            # We don't currently support serializing/deserializing exceptions with more than one argument.
+            message = "#{exception_class}: #{message}"
+            exception = default_exception_class.new(message)
+            Temporal.logger.error(
+              "Could not instantiate original error. Defaulting to StandardError.", 
+              {
+                original_error: failure.application_failure_info.type,
+                instantiation_error_message: deserialization_error.message,
+              },
+            ) 
+          end
+          exception.tap do |exception|
+            backtrace = failure.stack_trace.split("\n")
             exception.set_backtrace(backtrace) if !backtrace.empty?
           end
         when :timeout_failure_info

spec/fabricators/application_failure_fabricator.rb

@@ -0,0 +1,12 @@
+# Simulates Temporal::Connection::Serializer::Failure
+Fabricator(:application_failure, from: Temporal::Api::Failure::V1::Failure) do
+  transient :error_class, :backtrace
+  message { |attrs| attrs[:message] }
+  stack_trace { |attrs| attrs[:backtrace].join("\n") }
+  application_failure_info do |attrs|
+    Temporal::Api::Failure::V1::ApplicationFailureInfo.new(
+      type: attrs[:error_class],
+      details: TestDeserializer.new.to_details_payloads(attrs[:message]),
+    )
+  end
+end

spec/unit/lib/temporal/workflow/errors_spec.rb

@@ -0,0 +1,86 @@
+require 'temporal/workflow/errors'
+
+class TestDeserializer
+  include Temporal::Concerns::Payloads
+end
+
+class ErrorWithTwoArgs < StandardError
+  def initialize(message, another_argument); end
+end
+
+class SomeError < StandardError; end
+
+describe Temporal::Workflow::Errors do
+  describe '.generate_error' do
+    it "instantiates properly when the client has the error" do
+      message = "An error message"
+      stack_trace = ["a fake backtrace"]
+      failure = Fabricate(
+        :application_failure,
+        message: message,
+        backtrace: stack_trace,
+        error_class: SomeError.to_s
+      )
+
+      e = Temporal::Workflow::Errors.generate_error(failure)
+      expect(e).to be_a(SomeError)
+      expect(e.message).to eq(message)
+      expect(e.backtrace).to eq(stack_trace)
+
+    end
+
+    it "falls back to StandardError when the client doesn't have the error class" do 
+      allow(Temporal.logger).to receive(:error)
+
+      message = "An error message"
+      stack_trace = ["a fake backtrace"]
+      failure = Fabricate(
+        :application_failure,
+        message: message,
+        backtrace: stack_trace,
+        error_class: 'NonexistentError',
+      )
+
+      e = Temporal::Workflow::Errors.generate_error(failure)
+      expect(e).to be_a(StandardError)
+      expect(e.message).to eq("NonexistentError: An error message")
+      expect(e.backtrace).to eq(stack_trace)
+      expect(Temporal.logger)
+        .to have_received(:error)
+        .with(
+          'Could not find original error class. Defaulting to StandardError.',
+          {:original_error=>"NonexistentError"},
+        )
+
+    end
+
+
+    it "falls back to StandardError when the client can't initialize the error class" do 
+      allow(Temporal.logger).to receive(:error)
+
+      message = "An error message"
+      stack_trace = ["a fake backtrace"]
+      failure = Fabricate(
+        :application_failure,
+        message: message,
+        backtrace: stack_trace,
+        error_class: ErrorWithTwoArgs.to_s,
+      )
+
+      e = Temporal::Workflow::Errors.generate_error(failure)
+      expect(e).to be_a(StandardError)
+      expect(e.message).to eq("ErrorWithTwoArgs: An error message")
+      expect(e.backtrace).to eq(stack_trace)
+      expect(Temporal.logger)
+        .to have_received(:error)
+        .with(
+          'Could not instantiate original error. Defaulting to StandardError.',
+          {
+            :original_error=>"ErrorWithTwoArgs",
+            :instantiation_error_message=> "wrong number of arguments (given 1, expected 2)",
+          },
+        )
+    end
+
+  end
+end