Ensure all Cache store have consistent TTL behavior on increment

Make sure they all increment the counter but don't update the TTL.

activesupport/lib/active_support/cache/file_store.rb

@@ -211,17 +211,22 @@ module ActiveSupport
         # If the key is not found it is created and set to +amount+.
         def modify_value(name, amount, options)
           file_name = normalize_key(name, options)
+          options = merged_options(options)
+          key = normalize_key(name, options)
+          version = normalize_version(name, options)
+          amount = Integer(amount)
 
           lock_file(file_name) do
-            options = merged_options(options)
+            entry = read_entry(key, **options)
 
-            if num = read(name, options)
-              num = num.to_i + amount
-              write(name, num, options)
-              num
-            else
-              write(name, Integer(amount), options)
+            if !entry || entry.expired? || entry.mismatched?(version)
+              write(name, amount, options)
               amount
+            else
+              num = entry.value.to_i + amount
+              entry = Entry.new(num, expires_at: entry.expires_at, version: entry.version)
+              write_entry(key, entry)
+              num
             end
           end
         end

activesupport/test/cache/behaviors/cache_increment_decrement_behavior.rb

@@ -30,4 +30,18 @@ module CacheIncrementDecrementBehavior
     missing = @cache.decrement(SecureRandom.alphanumeric, 100)
     assert_equal @cache.is_a?(ActiveSupport::Cache::MemCacheStore) ? 0 : -100, missing
   end
+
+  def test_ttl_isnt_updated
+    key = SecureRandom.uuid
+
+    assert_equal 1, @cache.increment(key, 1, expires_in: 1)
+    assert_equal 2, @cache.increment(key, 1, expires_in: 5000)
+
+    # having to sleep two seconds in a test is bad, but we're testing
+    # a wide range of backends with different TTL mecanisms, most without
+    # subsecond granularity, so this is the only reliable way.
+    sleep 2
+
+    assert_nil @cache.read(key, raw: true)
+  end
 end