test: add several test cases for the transaction feature (#351)

Author: supercaracal

README.md

@@ -211,6 +211,29 @@ end
 # either of the values
 ```
 
+You can early return out of your block with a `next` statement if you want to cancel your transaction.
+In this context, don't use `break` and `return` statements.
+
+```ruby
+# The transaction isn't executed.
+cli.multi do |tx|
+  next if some_conditions?
+
+  tx.call('SET', '{key}1', '1')
+  tx.call('SET', '{key}2', '2')
+end
+```
+
+```ruby
+# The watching state is automatically cleared with an execution of an empty transaction.
+cli.multi(watch: %w[{key}1 {key}2]) do |tx|
+  next if some_conditions?
+
+  tx.call('SET', '{key}1', '1')
+  tx.call('SET', '{key}2', '2')
+end
+```
+
 `RedisClient::Cluster#multi` is aware of redirections and node failures like ordinary calls to `RedisClient::Cluster`,
 but because you may have written non-idempotent code inside your block, the block is called once if e.g. the slot
 it is operating on moves to a different node.

test/redis_client/test_cluster.rb

@@ -240,6 +240,34 @@ def test_transaction_with_empty_block_and_watch
         assert_equal(%w[WATCH MULTI EXEC], @captured_commands.to_a.map(&:command).map(&:first))
       end
 
+      def test_transaction_with_early_return_block
+        @captured_commands.clear
+        condition = true
+        got = @client.multi do |tx|
+          next if condition
+
+          tx.call('SET', 'key', 'value')
+        end
+
+        assert_empty(got)
+        assert_empty(@captured_commands.to_a.map(&:command).map(&:first))
+        assert_nil(@client.call('GET', 'key'))
+      end
+
+      def test_transaction_with_early_return_block_in_watching
+        @captured_commands.clear
+        condition = true
+        got = @client.multi(watch: %w[key]) do |tx|
+          next if condition
+
+          tx.call('SET', 'key', 'value')
+        end
+
+        assert_empty(got)
+        assert_equal(%w[WATCH MULTI EXEC], @captured_commands.to_a.map(&:command).map(&:first))
+        assert_nil(@client.call('GET', 'key'))
+      end
+
       def test_transaction_with_only_keyless_commands
         assert_raises(::RedisClient::Cluster::Transaction::ConsistencyError) do
           @client.multi do |t|