Add error handling to Tacos model

** Why are these changes being introduced:

I realized that the Tacos model as originally proposed had no error
handling when I refreshed a results page without my local TACOS app
running. The result was a pretty ugly blob shown to the user.

** Relevant ticket(s):

* https://mitlibraries.atlassian.net/browse/use-66

** How does this address that need:

This adds two catch statements to the call to TACOS - one for an HTTP
error response, and one for a JSON parsing error.

It also refactors the Tacos model to allow a separately-defined HTTP
object, which we craft in the test suite to always return those two
error conditions.

** Document any side effects to this change:

This is yet another pattern for error handling, not using VCR cassettes
or webmock, neither of which I could get working this afternoon. I'm
not happy about that, but this does work.

Author: matt-bernhardt

app/models/tacos.rb

@@ -1,12 +1,18 @@
 class Tacos
-  def self.call(term)
-    tacos_http = HTTP.persistent(tacos_url)
-                     .headers(accept: 'application/json',
-                              'Content-Type': 'application/json',
-                              origin: origins)
+  # The tacos_client argument here is unused in production - it is provided for
+  # our test suite so that we can mock various error conditions to ensure that
+  # error handling happens as we intend.
+  def self.call(term, tacos_client = nil)
+    tacos_http = setup(tacos_client)
     query = '{ "query": "{ logSearchEvent(searchTerm: \"' + clean_term(term) + '\", sourceSystem: \"' + tacos_source + '\" ) { phrase source detectors { suggestedResources { title url } } } }" }'
-    raw_response = tacos_http.timeout(http_timeout).post(tacos_url, body: query)
-    JSON.parse(raw_response.to_s)
+    begin
+      raw_response = tacos_http.timeout(http_timeout).post(tacos_url, body: query)
+      JSON.parse(raw_response.to_s)
+    rescue HTTP::Error
+      {"error" => "A connection error has occurred"}
+    rescue JSON::ParserError
+      {"error" => "A parsing error has occurred"}
+    end
   end
 
   private
@@ -23,6 +29,16 @@ def self.origins
     ENV.fetch('ORIGINS', nil)
   end
 
+  # We define the HTTP connection this way so that it can be overridden during
+  # testing, to make sure that the .call method can handle specific error
+  # conditions.
+  def self.setup(tacos_client)
+    tacos_client || HTTP.persistent(tacos_url)
+                        .headers(accept: 'application/json',
+                                 'Content-Type': 'application/json',
+                                 origin: origins)
+  end
+
   def self.tacos_source
     ENV.fetch('TACOS_SOURCE', 'unset')
   end

test/models/tacos_test.rb

@@ -1,5 +1,25 @@
 require 'test_helper'
 
+class TacosConnectionError
+  def timeout(_)
+    self
+  end
+
+  def post(_url, body:)
+    raise HTTP::ConnectionError, "forced connection failure"
+  end
+end
+
+class TacosParsingError
+  def timeout(_)
+    self
+  end
+
+  def post(_url, body:)
+    'This is not valid json'
+  end
+end
+
 class TacosTest < ActiveSupport::TestCase
   test 'TACOS model has a call method that reflects a search term back' do
     VCR.use_cassette('tacos popcorn') do
@@ -21,4 +41,22 @@ class TacosTest < ActiveSupport::TestCase
       end
     end
   end
+
+  test 'TACOS model catches connection errors' do
+    tacos_client = TacosConnectionError.new
+
+    result = Tacos.call('popcorn', tacos_client)
+
+    assert_instance_of Hash, result
+    assert_equal 'A connection error has occurred', result['error']
+  end
+
+  test 'TACOS model catches parsing errors' do
+    tacos_client = TacosParsingError.new
+
+    result = Tacos.call('popcorn', tacos_client)
+
+    assert_instance_of Hash, result
+    assert_equal 'A parsing error has occurred', result['error']
+  end
 end