Make exists variadic.

`exists` now return an Integer if passed multiple keys.

If passed a single key it will still return a Boolean, but print a deprecation warning.

To get a Boolean use `exists?` instead.

To opt-in to the new behavior, set `Redis.exists_returns_integer = true`.

lib/redis.rb

@@ -4,6 +4,10 @@
 require_relative "redis/errors"
 
 class Redis
+  class << self
+    attr_accessor :exists_returns_integer
+  end
+
   def self.current
     @current ||= Redis.new
   end
@@ -550,13 +554,30 @@ def unlink(*keys)
     end
   end
 
-  # Determine if a key exists.
+  # Determine how many of the keys exists.
   #
-  # @param [String] key
-  # @return [Boolean]
-  def exists(key)
+  # @param [String, Array<String>] keys
+  # @return [Integer]
+  def exists(*keys)
+    if !Redis.exists_returns_integer && keys.size == 1
+      message = "`Redis#exists(key)` will return an Integer in redis-rb 4.3, if you want to keep the old behavior, " \
+        "use `exists?` instead. To opt-in to the new behavior now you can set Redis.exists_returns_integer = true. " \
+        "(#{::Kernel.caller(1, 1).first})\n"
+
+      if defined?(::Warning)
+        ::Warning.warn(message)
+      else
+        $stderr.puts(message)
+      end
+      exists?(*keys)
+    else
+      _exists(*keys)
+    end
+  end
+
+  def _exists(*keys)
     synchronize do |client|
-      client.call([:exists, key], &Boolify)
+      client.call([:exists, *keys])
     end
   end
 

lib/redis/distributed.rb

@@ -171,8 +171,24 @@ def unlink(*args)
     end
 
     # Determine if a key exists.
-    def exists(key)
-      node_for(key).exists(key)
+    def exists(*args)
+      if !Redis.exists_returns_integer && args.size == 1
+        message = "`Redis#exists(key)` will return an Integer in redis-rb 4.3, if you want to keep the old behavior, " \
+          "use `exists?` instead. To opt-in to the new behavior now you can set Redis.exists_returns_integer = true. " \
+          "(#{::Kernel.caller(1, 1).first})\n"
+
+        if defined?(::Warning)
+          ::Warning.warn(message)
+        else
+          $stderr.puts(message)
+        end
+        exists?(*args)
+      else
+        keys_per_node = args.group_by { |key| node_for(key) }
+        keys_per_node.inject(0) do |sum, (node, keys)|
+          sum + node._exists(*keys)
+        end
+      end
     end
 
     # Determine if any of the keys exists.

test/lint/value_types.rb

@@ -11,6 +11,30 @@ def test_exists
       assert_equal true,  r.exists("foo")
     end
 
+    def test_exists_integer
+      previous_exists_returns_integer = Redis.exists_returns_integer
+      Redis.exists_returns_integer = true
+      assert_equal 0, r.exists("foo")
+
+      r.set("foo", "s1")
+
+      assert_equal 1,  r.exists("foo")
+    ensure
+      Redis.exists_returns_integer = previous_exists_returns_integer
+    end
+
+    def test_variadic_exists
+      assert_equal 0, r.exists("{1}foo", "{1}bar")
+
+      r.set("{1}foo", "s1")
+
+      assert_equal 1, r.exists("{1}foo", "{1}bar")
+
+      r.set("{1}bar", "s2")
+
+      assert_equal 2, r.exists("{1}foo", "{1}bar")
+    end
+
     def test_exists?
       assert_equal false, r.exists?("{1}foo", "{1}bar")