Handle argument errors when http request raises argument error for headers having CR/LF characters

app/lib/link_checker/uri_checker/http_checker.rb

@@ -89,7 +89,7 @@ def initialize(options = {})
     end
   end
 
-  class FaradayError < Error
+  class HttpCommunicationError < Error
     def initialize(options = {})
       super(suggested_fix: :determine_if_temporary, **options)
     end
@@ -238,7 +238,7 @@ def make_request(method, check_ssl: true)
       response
     rescue Faraday::ConnectionFailed
       add_problem(
-        FaradayError.new(
+        HttpCommunicationError.new(
           summary: :website_unavailable,
           message: :website_host_offline,
           from_redirect: from_redirect?,
@@ -247,7 +247,7 @@ def make_request(method, check_ssl: true)
       nil
     rescue Faraday::TimeoutError
       add_problem(
-        FaradayError.new(
+        HttpCommunicationError.new(
           summary: :website_unavailable,
           message: :page_is_not_responding,
           from_redirect: from_redirect?,
@@ -269,9 +269,26 @@ def make_request(method, check_ssl: true)
       make_request(method, check_ssl: false)
     rescue Faraday::Error
       add_problem(
-        FaradayError.new(
+        HttpCommunicationError.new(
           summary: :page_unavailable,
-          message: :page_failing_to_load,
+          message: :technical_error_on_page,
+          from_redirect: from_redirect?,
+        ),
+      )
+      nil
+    # Ruby net-http cannot handle responses with headers with CR/LF characters in, and such responses raise
+    # an argument error. We want to catch these and add to the report for now, rather than continuing to raise these
+    # as errors, which can trigger alerting. We are planning on raising this to be fixed at the net-http level which may
+    # mean we do not have to handle these here.
+    # Doing some fuzzy matching on the error message to try and ensure a bit of resilience, allowing for changes to the
+    # exact error message in the exception.
+    rescue ArgumentError => e
+      raise e unless e.message =~ /(.*)header(.*) CR\/LF/
+
+      add_problem(
+        HttpCommunicationError.new(
+          summary: :page_unavailable,
+          message: :technical_error_on_page,
           from_redirect: from_redirect?,
         ),
       )

app/lib/link_checker/uri_checker/problem.rb

@@ -45,7 +45,7 @@ def get_string(symbol)
       UnusualUrl
       NotAvailableOnline
       NoHost
-      FaradayError
+      HttpCommunicationError
       PageNotFound
       PageRequiresLogin
       PageIsUnavailable

config/locales/en.yml

@@ -103,9 +103,9 @@ en:
     singular: This page has a security problem that users may be alerted to.
     redirect: This redirects to a page with a security problem.
 
-  page_failing_to_load:
-    singular: This page is failing to load.
-    redirect: This redirects to a page that isn't loading.
+  technical_error_on_page:
+    singular: This page has a technical error.
+    redirect: This redirects to a page that has a technical error.
 
   determine_if_temporary: Determine if this is a temporary issue or the resource is no longer available.
 

spec/lib/link_checker_spec.rb

@@ -142,6 +142,26 @@
       include_examples "has warnings"
     end
 
+    context "header with CR/LF character" do
+      let(:uri) { "http://www.not-gov.uk/header_with_CRLF_character" }
+      before do
+        stub_request(:get, uri)
+          .to_return(headers: { "Invalid" => "A header containing a carriage return \r character" })
+      end
+
+      include_examples "has errors"
+      include_examples "has a problem summary", "Page unavailable"
+    end
+
+    it "does not recue from other argument error" do
+      uri = "http://www.not-gov.uk/raises_argument_error"
+      error = ArgumentError.new("something that's nothing to do with headers and carriage return line feed chars")
+
+      stub_request(:get, uri).to_raise(error)
+
+      expect { described_class.new(uri).call }.to raise_error(error)
+    end
+
     context "slow response" do
       let(:uri) { "http://www.not-gov.uk/slow_response" }