ignore default username when no password is set

fixes a regression introduced with the api_key support for xpack monitoring and management in elastic#11864 which disabled the possibility to not use any authentication by relying on the default options and only enabling monitoring for example. It now ignores the default username option when no password is explicitly set.

x-pack/lib/helpers/elasticsearch_options.rb

@@ -67,9 +67,12 @@ def es_options_from_settings(feature, settings)
         opts['ssl'] = true
       end
 
-      # the username setting has a default value and should not be included when using another authentication
+      # the username setting has a default value and should not be included when using another authentication such as cloud_auth or api_key.
+      # it should also not be included when no password is set.
       # it is safe to silently remove here since all authentication verifications have been validated at this point.
-      if settings.set?("#{prefix}#{feature}.elasticsearch.cloud_auth") || settings.set?("#{prefix}#{feature}.elasticsearch.api_key")
+      if settings.set?("#{prefix}#{feature}.elasticsearch.cloud_auth") ||
+         settings.set?("#{prefix}#{feature}.elasticsearch.api_key") ||
+         (!settings.set?("#{prefix}#{feature}.elasticsearch.password") && !settings.set?("#{prefix}#{feature}.elasticsearch.username"))
         opts.delete('user')
       end
 
@@ -201,15 +204,6 @@ def validate_authentication!(feature, settings, prefix)
       authentication_count += 1 if provided_password
       authentication_count += 1 if provided_api_key
 
-      if authentication_count == 0
-        # when no explicit authentication is set it is relying on default username
-        # but without and explicit password set
-        raise(ArgumentError,
-          "With the default #{prefix}#{feature}.elasticsearch.username, " +
-          "#{prefix}#{feature}.elasticsearch.password must be set"
-        )
-      end
-
       if authentication_count > 1
         raise(ArgumentError, "Multiple authentication options are specified, please only use one of #{prefix}#{feature}.elasticsearch.username/password, #{prefix}#{feature}.elasticsearch.cloud_auth or #{prefix}#{feature}.elasticsearch.api_key")
       end

x-pack/spec/config_management/elasticsearch_source_spec.rb

@@ -151,8 +151,12 @@
           }
         end
 
-        it "should raise an ArgumentError" do
-          expect { described_class.new(system_settings) }.to raise_error(ArgumentError)
+        it "will rely on username and password settings" do
+          # since cloud_id and cloud_auth are simply containers for host and username/password
+          # both could be set independently and if cloud_auth is not set then authn will be done
+          # using the provided username/password settings, which can be set or not if not auth is
+          # required.
+          expect { described_class.new(system_settings) }.to_not raise_error
         end
       end
     end

x-pack/spec/helpers/elasticsearch_options_spec.rb

@@ -107,10 +107,11 @@
         }
       end
 
-      it "fails without password" do
-        expect {
-          test_class.es_options_from_settings_or_modules('monitoring', system_settings)
-        }.to raise_error(ArgumentError, /password must be set/)
+      it "ignores the implicit default username when no password is set" do
+        # when no explicit password is set then the default/implicit username should be ignored
+        es_options = test_class.es_options_from_settings_or_modules('monitoring', system_settings)
+        expect(es_options).to_not include("user")
+        expect(es_options).to_not include("password")
       end
 
       context "with cloud_auth" do
@@ -296,13 +297,10 @@
       end
 
       context "when cloud_auth is not set" do
-
-        it "raises for invalid configuration" do
-          # if not other authn is provided it will assume basic auth using the default username
-          # but the password is missing.
-          expect {
-            test_class.es_options_from_settings_or_modules('monitoring', system_settings)
-          }.to raise_error(ArgumentError, /With the default.*?username,.*?password must be set/)
+        it "does not use authentication and ignores the default username" do
+          es_options = test_class.es_options_from_settings_or_modules('monitoring', system_settings)
+          expect(es_options).to include("cloud_id")
+          expect(es_options.keys).to_not include("hosts", "user", "password")
         end
 
         context 'username and password set' do