Raise an error when subscribing in the future

When client code tries to subscribe to a channel using a `last_id` that doesn't exist on that channel, we raise an exception.

See ae0d7cd#commitcomment-31544627 for details.

.rubocop_todo.yml

@@ -1,6 +1,6 @@
 # This configuration was generated by
 # `rubocop --auto-gen-config`
-# on 2018-12-03 22:29:14 +0000 using RuboCop version 0.60.0.
+# on 2018-12-04 18:23:24 +0000 using RuboCop version 0.60.0.
 # The point is for the user to remove these configuration records
 # one by one as the offenses are removed from the code base.
 # Note that changes in the inspected code, or installation of new
@@ -86,12 +86,12 @@ Metrics/BlockLength:
 Metrics/BlockNesting:
   Max: 4
 
-# Offense count: 1
+# Offense count: 9
 # Configuration parameters: CountComments.
 Metrics/ClassLength:
   Max: 315
 
-# Offense count: 17
+# Offense count: 18
 Metrics/CyclomaticComplexity:
   Max: 30
 
@@ -103,7 +103,7 @@ Metrics/MethodLength:
 # Offense count: 1
 # Configuration parameters: CountComments.
 Metrics/ModuleLength:
-  Max: 395
+  Max: 399
 
 # Offense count: 17
 Metrics/PerceivedComplexity:
@@ -329,7 +329,7 @@ Style/IfInsideElse:
   Exclude:
     - 'lib/message_bus.rb'
 
-# Offense count: 22
+# Offense count: 23
 # Cop supports --auto-correct.
 Style/IfUnlessModifier:
   Exclude:
@@ -540,7 +540,7 @@ Style/SpecialGlobalVars:
     - 'message_bus.gemspec'
     - 'spec/spec_helper.rb'
 
-# Offense count: 707
+# Offense count: 708
 # Cop supports --auto-correct.
 # Configuration parameters: EnforcedStyle, ConsistentQuotesInMultiline.
 # SupportedStyles: single_quotes, double_quotes
@@ -648,7 +648,7 @@ Style/ZeroLengthPredicate:
     - 'lib/message_bus/rack/middleware.rb'
     - 'spec/lib/message_bus_spec.rb'
 
-# Offense count: 218
+# Offense count: 219
 # Configuration parameters: AllowHeredoc, AllowURI, URISchemes, IgnoreCopDirectives, IgnoredPatterns.
 # URISchemes: http, https
 Metrics/LineLength:

lib/message_bus.rb

@@ -592,7 +592,12 @@ def subscribe_impl(channel, site_id, last_id, &blk)
     raise MessageBus::BusDestroyed if @destroyed
 
     if last_id >= 0
-      backlog(channel, last_id, site_id, inclusive: true).each do |m|
+      existing_messages = backlog(channel, last_id, site_id, inclusive: true)
+      if existing_messages.empty?
+        raise ArgumentError, "You tried to subscribe starting from a message that doesn't exist yet."
+      end
+
+      existing_messages.each do |m|
         yield m
       end
     end

spec/lib/message_bus_spec.rb

@@ -69,6 +69,12 @@
     data1.must_equal ['bananana', "it's so fluffy"]
     data2.must_equal ['banana', 'bananana', "it's so fluffy"]
     data3.must_equal ['banana', 'bananana', "it's so fluffy"]
+
+    lambda do
+      @bus.subscribe("/minion", 4) do |_msg| # This last_id doesn't exist yet
+        raise "Don't expect any delivery here"
+      end
+    end.must_raise ArgumentError
   end
 
   it "can transmit client_ids" do