Fix unnecessary writes with multiple memcache servers

Fixes a bug in Memcache.php that would cause every fetch to also
result in a push if multiple memcache servers were being used.

If multiple memcache servers are in use, the code needs to check
if they aren't out of sync. If they are, the most recent data needs
to be pushed to all mirrors.

The original code compared the parsed data structures using ===,
but that always fails because that just does a pointer comparison
and not a deep comparison.

Changed the code to compare the original unparsed values instead.
Also added a few debug-level log statements.

lib/SimpleSAML/Memcache.php

@@ -31,7 +31,9 @@ class SimpleSAML_Memcache {
 	 * @return The data stored with the given key, or NULL if no data matching the key was found.
 	 */
 	public static function get($key) {
+		SimpleSAML_Logger::debug("loading key $key from memcache");
 
+		$latestInfo = NULL;
 		$latestTime = 0.0;
 		$latestData = NULL;
 		$mustUpdate = FALSE;
@@ -69,15 +71,15 @@ class SimpleSAML_Memcache {
 				continue;
 			}
 
-
-			if($latestTime === 0.0) {
-				/* First data found. */
+			if($latestInfo === NULL) {
+				/* First info found. */
+				$latestInfo = $serializedInfo;
 				$latestTime = $info['timestamp'];
 				$latestData = $info['data'];
 				continue;
 			}
 
-			if($info['timestamp'] === $latestTime && $info['data'] === $latestData) {
+			if($info['timestamp'] === $latestTime && $serializedInfo === $latestInfo) {
 				/* This data matches the data from the other server(s). */
 				continue;
 			}
@@ -91,18 +93,21 @@ class SimpleSAML_Memcache {
 
 			/* Update if data in $info is newer than $latestData. */
 			if($latestTime < $info['timestamp']) {
+				$latestInfo = $serializedInfo;
 				$latestTime = $info['timestamp'];
 				$latestData = $info['data'];
 			}
 		}
 
-		if($latestTime === 0.0) {
+		if($latestData === NULL) {
 			/* We didn't find any data matching the key. */
+			SimpleSAML_Logger::debug("key $key not found in memcache");
 			return NULL;
 		}
 
 		if($mustUpdate) {
 			/* We found data matching the key, but some of the servers need updating. */
+			SimpleSAML_Logger::debug("Memcache servers out of sync for $key, forcing sync");
 			self::set($key, $latestData);
 		}
 
@@ -118,6 +123,7 @@ class SimpleSAML_Memcache {
 	 * @param int|NULL $expire  The expiration timestamp of the data.
 	 */
 	public static function set($key, $value, $expire = NULL) {
+		SimpleSAML_Logger::debug("saving key $key to memcache");
 		$savedInfo = array(
 			'timestamp' => microtime(TRUE),
 			'data' => $value
@@ -143,6 +149,7 @@ class SimpleSAML_Memcache {
 	 */
 	public static function delete($key) {
 		assert('is_string($key)');
+		SimpleSAML_Logger::debug("deleting key $key from memcache");
 
 		/* Store this object to all groups of memcache servers. */
 		foreach(self::getMemcacheServers() as $server) {