RUBY-3296 Write to log when non-genuine host is detected

lib/mongo/cluster.rb

@@ -186,6 +186,8 @@ def initialize(seeds, monitoring, options = Options::Redacted.new)
         recreate_topology(topology, opening_topology)
       end
 
+      possibly_warn_about_compatibility!
+
       if load_balanced?
         # We are required by the specifications to produce certain SDAM events
         # when in load-balanced topology.
@@ -1082,6 +1084,30 @@ def recreate_topology(new_topology_template, previous_topology)
         Monitoring::Event::TopologyChanged.new(previous_topology, @topology)
       )
     end
+
+    COSMOSDB_HOST_PATTERNS = %w[ .cosmos.azure.com ]
+    COSMOSDB_LOG_MESSAGE = 'You appear to be connected to a CosmosDB cluster. ' \
+      'For more information regarding feature compatibility and support please visit ' \
+      'https://www.mongodb.com/supportability/cosmosdb'
+
+    DOCUMENTDB_HOST_PATTERNS = %w[ .docdb.amazonaws.com .docdb-elastic.amazonaws.com ]
+    DOCUMENTDB_LOG_MESSAGE = 'You appear to be connected to a DocumentDB cluster. ' \
+      'For more information regarding feature compatibility and support please visit ' \
+      'https://www.mongodb.com/supportability/documentdb'
+
+    # Compares the server hosts with address suffixes of known services
+    # that provide limited MongoDB API compatibility, and warns about them.
+    def possibly_warn_about_compatibility!
+      if topology.server_hosts_match_any?(COSMOSDB_HOST_PATTERNS)
+        log_info COSMOSDB_LOG_MESSAGE
+        return
+      end
+
+      if topology.server_hosts_match_any?(DOCUMENTDB_HOST_PATTERNS)
+        log_info DOCUMENTDB_LOG_MESSAGE
+        return
+      end
+    end
   end
 end
 

lib/mongo/cluster/topology/base.rb

@@ -211,6 +211,22 @@ def new_max_set_version(description)
           end
         end
 
+        # Compares each server address against the list of patterns.
+        #
+        # @param [ Array<String> ] patterns the URL suffixes to compare
+        #   each server against.
+        #
+        # @return [ true | false ] whether any of the addresses match any of
+        #   the patterns or not.
+        #
+        # @api private
+        def server_hosts_match_any?(patterns)
+          server_descriptions.any? do |addr_spec, _desc|
+            addr, _port = addr_spec.split(/:/)
+            patterns.any? { |pattern| addr.end_with?(pattern) }
+          end
+        end
+
         private
 
         # Validates and/or transforms options as necessary for the topology.

spec/mongo/cluster_spec.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require 'spec_helper'
+require 'support/recording_logger'
 
 # let these existing styles stand, rather than going in for a deep refactoring
 # of these specs.
@@ -84,6 +85,41 @@
         )
       end
     end
+
+    context 'when a non-genuine host is detected' do
+      before { described_class.new(host_names, monitoring, logger: logger, monitoring_io: false) }
+
+      let(:logger) { RecordingLogger.new }
+
+      shared_examples 'an action that logs' do
+        it 'writes a warning to the log' do
+          expect(logger.lines).to include(a_string_matching(expected_log_output))
+        end
+      end
+
+      context 'when CosmosDB is detected' do
+        let(:host_names) { %w[ xyz.cosmos.azure.com ] }
+        let(:expected_log_output) { %r{https://www.mongodb.com/supportability/cosmosdb} }
+
+        it_behaves_like 'an action that logs'
+      end
+
+      context 'when DocumentDB is detected' do
+        let(:expected_log_output) { %r{https://www.mongodb.com/supportability/documentdb} }
+
+        context 'with docdb uri' do
+          let(:host_names) { [ 'xyz.docdb.amazonaws.com' ] }
+
+          it_behaves_like 'an action that logs'
+        end
+
+        context 'with docdb-elastic uri' do
+          let(:host_names) { [ 'xyz.docdb-elastic.amazonaws.com' ] }
+
+          it_behaves_like 'an action that logs'
+        end
+      end
+    end
   end
 
   describe '#==' do

spec/support/recording_logger.rb

@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+# rubocop:todo all
+
+require 'stringio'
+
+# A "Logger-alike" class, quacking like ::Logger, used for recording messages
+# as they are written to the log
+class RecordingLogger < Logger
+  def initialize(*args, **kwargs)
+    @buffer = StringIO.new
+    super(@buffer, *args, **kwargs)
+  end
+
+  # Accesses the raw contents of the log
+  #
+  # @return [ String ] the raw contents of the log
+  def contents
+    @buffer.string
+  end
+
+  # Returns the contents of the log as individual lines.
+  #
+  # @return [ Array<String> ] the individual log lines
+  def lines
+    contents.split(/\n/)
+  end
+end