redis: use atomic operations everywhere

This removes a lot of acrobatics in the code and does each operation
atomically using a lua script. This also reduces several round trips
to the server, and the scripts are compiled and cached server-side.

Notably, since all operations work only on a single key (except clear,
which is broken anyway and shouldn't be used), they will continue to
function and be atomic for Redis cluster.

Signed-off-by: Varun Patil <varunpatil@ucla.edu>

1 files changed, 55 insertions, 35 deletions

diff --git a/lib/private/Memcache/Redis.php b/lib/private/Memcache/Redis.php
index b6f96fffba4..4c8151b9d6c 100644
--- a/lib/private/Memcache/Redis.php
+++ b/lib/private/Memcache/Redis.php
@@ -31,6 +31,22 @@ namespace OC\Memcache;
 
 use OCP\IMemcacheTTL;
 
+/** name => [script, sha1] */
+const LUA_SCRIPTS = [
+	'dec' => [
+		'if redis.call("exists", KEYS[1]) == 1 then return redis.call("decrby", KEYS[1], ARGV[1]) else return "NEX" end',
+		'720b40cb66cef1579f2ef16ec69b3da8c85510e9',
+	],
+	'cas' => [
+		'if redis.call("get", KEYS[1]) == ARGV[1] then redis.call("set", KEYS[1], ARGV[2]) return 1 else return 0 end',
+		'94eac401502554c02b811e3199baddde62d976d4',
+	],
+	'cad' => [
+		'if redis.call("get", KEYS[1]) == ARGV[1] then return redis.call("del", KEYS[1]) else return 0 end',
+		'cf0e94b2e9ffc7e04395cf88f7583fc309985910',
+	],
+];
+
 class Redis extends Cache implements IMemcacheTTL {
 	/**
 	 * @var \Redis|\RedisCluster $cache
@@ -54,18 +70,19 @@ class Redis extends Cache implements IMemcacheTTL {
 
 	public function get($key) {
 		$result = $this->getCache()->get($this->getPrefix() . $key);
-		if ($result === false && !$this->getCache()->exists($this->getPrefix() . $key)) {
+		if ($result === false) {
 			return null;
-		} else {
-			return json_decode($result, true);
 		}
+
+		return self::decodeValue($result);
 	}
 
 	public function set($key, $value, $ttl = 0) {
+		$value = self::encodeValue($value);
 		if ($ttl > 0) {
-			return $this->getCache()->setex($this->getPrefix() . $key, $ttl, json_encode($value));
+			return $this->getCache()->setex($this->getPrefix() . $key, $ttl, $value);
 		} else {
-			return $this->getCache()->set($this->getPrefix() . $key, json_encode($value));
+			return $this->getCache()->set($this->getPrefix() . $key, $value);
 		}
 	}
 
@@ -82,6 +99,7 @@ class Redis extends Cache implements IMemcacheTTL {
 	}
 
 	public function clear($prefix = '') {
+		// TODO: this is slow and would fail with Redis cluster
 		$prefix = $this->getPrefix() . $prefix . '*';
 		$keys = $this->getCache()->keys($prefix);
 		$deleted = $this->getCache()->del($keys);
@@ -98,17 +116,14 @@ class Redis extends Cache implements IMemcacheTTL {
 	 * @return bool
 	 */
 	public function add($key, $value, $ttl = 0) {
-		// don't encode ints for inc/dec
-		if (!is_int($value)) {
-			$value = json_encode($value);
-		}
+		$value = self::encodeValue($value);
 
 		$args = ['nx'];
 		if ($ttl !== 0 && is_int($ttl)) {
 			$args['ex'] = $ttl;
 		}
 
-		return $this->getCache()->set($this->getPrefix() . $key, (string)$value, $args);
+		return $this->getCache()->set($this->getPrefix() . $key, $value, $args);
 	}
 
 	/**
@@ -130,10 +145,8 @@ class Redis extends Cache implements IMemcacheTTL {
 	 * @return int | bool
 	 */
 	public function dec($key, $step = 1) {
-		if (!$this->hasKey($key)) {
-			return false;
-		}
-		return $this->getCache()->decrBy($this->getPrefix() . $key, $step);
+		$res = $this->evalLua('dec', [$key], [$step]);
+		return ($res === 'NEX') ? false : $res;
 	}
 
 	/**
@@ -145,18 +158,10 @@ class Redis extends Cache implements IMemcacheTTL {
 	 * @return bool
 	 */
 	public function cas($key, $old, $new) {
-		if (!is_int($new)) {
-			$new = json_encode($new);
-		}
-		$this->getCache()->watch($this->getPrefix() . $key);
-		if ($this->get($key) === $old) {
-			$result = $this->getCache()->multi()
-				->set($this->getPrefix() . $key, $new)
-				->exec();
-			return $result !== false;
-		}
-		$this->getCache()->unwatch();
-		return false;
+		$old = self::encodeValue($old);
+		$new = self::encodeValue($new);
+
+		return $this->evalLua('cas', [$key], [$old, $new]) > 0;
 	}
 
 	/**
@@ -167,15 +172,9 @@ class Redis extends Cache implements IMemcacheTTL {
 	 * @return bool
 	 */
 	public function cad($key, $old) {
-		$this->getCache()->watch($this->getPrefix() . $key);
-		if ($this->get($key) === $old) {
-			$result = $this->getCache()->multi()
-				->unlink($this->getPrefix() . $key)
-				->exec();
-			return $result !== false;
-		}
-		$this->getCache()->unwatch();
-		return false;
+		$old = self::encodeValue($old);
+
+		return $this->evalLua('cad', [$key], [$old]) > 0;
 	}
 
 	public function setTTL($key, $ttl) {
@@ -185,4 +184,25 @@ class Redis extends Cache implements IMemcacheTTL {
 	public static function isAvailable(): bool {
 		return \OC::$server->getGetRedisFactory()->isAvailable();
 	}
+
+	protected function evalLua($scriptName, $keys, $args) {
+		$keys = array_map(fn ($key) => $this->getPrefix() . $key, $keys);
+		$args = array_merge($keys, $args);
+		$script = LUA_SCRIPTS[$scriptName];
+
+		$result = $this->getCache()->evalSha($script[1], $args, count($keys));
+		if (false === $result) {
+			$result = $this->getCache()->eval($script[0], $args, count($keys));
+		}
+
+		return $result;
+	}
+
+	protected static function encodeValue($value) {
+		return is_int($value) ? (string) $value : json_encode($value);
+	}
+
+	protected static function decodeValue($value) {
+		return is_numeric($value) ? (int) $value : json_decode($value, true);
+	}
 }