getVersion used as connection validation in MemcachedConnector breaks redundant memcached setups

[ukautz]

• Laravel Version: 5.4.12 (since >=4)
• PHP Version: 7.0.15 (not a factor)
• Database Driver & Version: libmemcached 1.0.18 + php-memcached 3.0.2

Description:

In the MemcachedConnector class the validateConnection method, which is called after memcached servers have been added, uses Memcached::getVersion to verify that memcached servers have been added.

Now, getVersion implemented by php-memcached uses memcached_version from libmemcache. Should any memcached server fail, then memcached_version() will return MEMCACHED_SOME_ERRORS which causes getVersion to return false instead not an array of the "working servers". So any failing servers results in validateConnection throwing a \RuntimeException.

This is especially counterproductive when using a replicated memcached setup, in which one or multiple memcached servers could fail without problems - would it not be for the check and which disallows any memcached server to fail.

Both memcached_version from libmemcached as well as getVersion from php-memcached act as intended (as I understand it), so it's not a bug with either. Just using getVersion to validate the connection is not the intended usage. As far as I can tell this is the case with any available libmemcached, at least since 1.0.0. Also php-memcached, at least since 2.0.0.

Steps To Reproduce:

• Setup memcached as cache in Laravel using more than one memcached servers
• Write a controller, which uses \Cache::(put|get|...) so that MemcachedConnector is created
• Simulate an outage in one of the memcached servers
• See the \RuntimeException

Here a short test setup. Laravel configured as above is available locally under http://localhost:8080/, then:

# start up memcached
$ memcached -p 11211 &
$ memcached -p 11212 &

# issue a request to your Laravel App, while both servers are up
$ curl http://localhost:8080/

# simulate outage of _one_ of the memcached servers
$ iptables -I INPUT -p tcp --destination-port 11211 -j DROP

# issue another request, see the exception
$ curl http://localhost:8080/

Solution (idea)

Either remove the getVersion check, make it controllable (disable/enable) via configuration or use set("some-special-key", ..) + get("some-special-key"). The latter is of course more expensive than getVersion.

I can provide a patch, if you let me know which of the solution ideas you prefer.

[themsaid]

Solved in #19192