Add Memcached ascii authentication support

This is great! Please find an updated patch after more testing.

- renamed all related functions and variables to include ascii_auth (to differentiate from sasl auth)
- added additional documentation
- perform additional tests on admin Status Report

Uploading a new version of the patch, as the previous wasn't properly checking the memcached version.

I ran some load tests on this patch. Enabling ASCII protocol authentication on the server and in the module results in ~1.25% performance overhead. This seems primarily due to an ~11.25% increase in memcache GET requests. Very acceptable overhead.

1. +++ b/README.txt
   @@ -686,6 +690,31 @@ memcache_sasl_username and memcache_sasl_password in settings.php. For example:
   +ASCII protocol authentication requires Memcached version 1.5.15 or greater
   

   I suggest to add that at least 1.6.4 is recommended to avoid that one bug.

2. +++ b/dmemcache.inc
   @@ -1265,3 +1322,221 @@ function _dmemcache_write_debug($action, $bin, $key, $rc) {
   +  // Login to the server.
   +  $rc = $mc->setByKey($full_key, DRUPAL_MEMCACHE_ASCII_AUTH_KEY, $memcache_ascii_auth);
   +  if (!$rc) {
   

   You are missing an essential interdiff - I will post it again.

3. +++ b/dmemcache.inc
   @@ -1265,3 +1322,221 @@ function _dmemcache_write_debug($action, $bin, $key, $rc) {
   +  // Set the lifetime key.
   +  $mc->setByKey($full_key, DRUPAL_MEMCACHE_ASCII_AUTH_LIFETIME_KEY, TRUE);
   

   I still don't get why setByKey returns success when it fails to set the key. [or maybe it can fail to set the key even though the connection is authorized, because any set on an unauthorized connection should give back error 9 CLIENT ERROR].

4. +++ b/memcache.install
   @@ -113,6 +114,40 @@ function memcache_requirements($phase) {
   +        // Confirm ASCII authentication works on all memcached servers.
   +        $memcache_bins = variable_get('memcache_bins', array('cache' => 'default'));
   +        foreach ($memcache_bins as $bin => $_) {
   +          if ($mc = dmemcache_object($bin)) {
   +            $ascii_auth =  _dmemcache_reset_ascii_auth($mc, TRUE);
   +            if ($ascii_auth !== TRUE) {
   

   IIRC - you also wanted to check if ascii auth is even enabled.

   The easiest is to create a new Memcached() instance for each connection in the getServerList() and see that a simple set on a dummy key fails.

I suggest to add that at least 1.6.4 is recommended to avoid that one bug.

I've updated the README, and I also added a Warning on the Status Page if <1.6.4 is used.

You are missing an essential interdiff - I will post it again.

Merged in.

I still don't get why setByKey returns success when it fails to set the key. [or maybe it can fail to set the key even though the connection is authorized, because any set on an unauthorized connection should give back error 9 CLIENT ERROR].

It likely is specific to have 1 failed among 2+ servers in a given bin?

I did not address 4, but additional checks are certainly welcome as silent errors are very difficult to debug.

Note, this patch was load tested with Goose using this example:
https://github.com/tag1consulting/goose/blob/master/examples/drupal_load...

The raw command used was:
cargo run --release --example drupal_loadtest -- --host http://local.dev/ -c10 -t15m -v --only-summary

And the collected results:
https://docs.google.com/spreadsheets/d/1aRns-fiXepYhz-lA2y2aJNgFuWjy67OS...

This has been committed. Marking as to-be-ported to the 8.x/9.x version.