Add Redis source code for testing

1 files changed, 932 insertions, 0 deletions

diff --git a/examples/redis-unstable/src/expire.c b/examples/redis-unstable/src/expire.c
new file mode 100644
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+++ b/examples/redis-unstable/src/expire.c
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+/* Implementation of EXPIRE (keys with fixed time to live).
+ *
+ * ----------------------------------------------------------------------------
+ *
+ * Copyright (c) 2009-Present, Redis Ltd.
+ * All rights reserved.
+ *
+ * Licensed under your choice of (a) the Redis Source Available License 2.0
+ * (RSALv2); or (b) the Server Side Public License v1 (SSPLv1); or (c) the
+ * GNU Affero General Public License v3 (AGPLv3).
+ */
+
+#include "server.h"
+#include "cluster.h"
+#include "redisassert.h"
+
+/*-----------------------------------------------------------------------------
+ * Incremental collection of expired keys.
+ *
+ * When keys are accessed they are expired on-access. However we need a
+ * mechanism in order to ensure keys are eventually removed when expired even
+ * if no access is performed on them.
+ *----------------------------------------------------------------------------*/
+
+/* Constants table from pow(0.98, 1) to pow(0.98, 16). 
+ * Help calculating the db->avg_ttl. */
+static double avg_ttl_factor[16] = {0.98, 0.9604, 0.941192, 0.922368, 0.903921, 0.885842, 0.868126, 0.850763, 0.833748, 0.817073, 0.800731, 0.784717, 0.769022, 0.753642, 0.738569, 0.723798};
+
+/* Helper function for the activeExpireCycle() function.
+ * This function will try to expire the key-value entry that is stored in the 
+ * hash table entry 'de' of the 'expires' hash table of a Redis database.
+ *
+ * If the key is found to be expired, it is removed from the database and
+ * 1 is returned. Otherwise no operation is performed and 0 is returned.
+ *
+ * When a key is expired, server.stat_expiredkeys is incremented.
+ *
+ * The parameter 'now' is the current time in milliseconds as is passed
+ * to the function to avoid too many gettimeofday() syscalls. */
+int activeExpireCycleTryExpire(redisDb *db, kvobj *kv, long long now) {
+    if (now < kvobjGetExpire(kv))
+        return 0;
+
+    enterExecutionUnit(1, 0);
+    sds key = kvobjGetKey(kv);
+    robj *keyobj = createStringObject(key,sdslen(key));
+    deleteExpiredKeyAndPropagate(db,keyobj);
+    decrRefCount(keyobj);
+    exitExecutionUnit();
+    /* Propagate the DEL command */
+    postExecutionUnitOperations();
+    return 1;
+}
+
+/* Try to expire a few timed out keys. The algorithm used is adaptive and
+ * will use few CPU cycles if there are few expiring keys, otherwise
+ * it will get more aggressive to avoid that too much memory is used by
+ * keys that can be removed from the keyspace.
+ *
+ * Every expire cycle tests multiple databases: the next call will start
+ * again from the next db. No more than CRON_DBS_PER_CALL databases are
+ * tested at every iteration.
+ *
+ * The function can perform more or less work, depending on the "type"
+ * argument. It can execute a "fast cycle" or a "slow cycle". The slow
+ * cycle is the main way we collect expired cycles: this happens with
+ * the "server.hz" frequency (usually 10 hertz).
+ *
+ * However the slow cycle can exit for timeout, since it used too much time.
+ * For this reason the function is also invoked to perform a fast cycle
+ * at every event loop cycle, in the beforeSleep() function. The fast cycle
+ * will try to perform less work, but will do it much more often.
+ *
+ * The following are the details of the two expire cycles and their stop
+ * conditions:
+ *
+ * If type is ACTIVE_EXPIRE_CYCLE_FAST the function will try to run a
+ * "fast" expire cycle that takes no longer than ACTIVE_EXPIRE_CYCLE_FAST_DURATION
+ * microseconds, and is not repeated again before the same amount of time.
+ * The cycle will also refuse to run at all if the latest slow cycle did not
+ * terminate because of a time limit condition.
+ *
+ * If type is ACTIVE_EXPIRE_CYCLE_SLOW, that normal expire cycle is
+ * executed, where the time limit is a percentage of the REDIS_HZ period
+ * as specified by the ACTIVE_EXPIRE_CYCLE_SLOW_TIME_PERC define. In the
+ * fast cycle, the check of every database is interrupted once the number
+ * of already expired keys in the database is estimated to be lower than
+ * a given percentage, in order to avoid doing too much work to gain too
+ * little memory.
+ *
+ * The configured expire "effort" will modify the baseline parameters in
+ * order to do more work in both the fast and slow expire cycles.
+ */
+
+#define ACTIVE_EXPIRE_CYCLE_KEYS_PER_LOOP 20 /* Keys for each DB loop. */
+#define ACTIVE_EXPIRE_CYCLE_FAST_DURATION 1000 /* Microseconds. */
+#define ACTIVE_EXPIRE_CYCLE_SLOW_TIME_PERC 25 /* Max % of CPU to use. */
+#define ACTIVE_EXPIRE_CYCLE_ACCEPTABLE_STALE 10 /* % of stale keys after which
+                                                   we do extra efforts. */
+
+#define HFE_DB_BASE_ACTIVE_EXPIRE_FIELDS_PER_SEC 10000
+
+/* Data used by the expire dict scan callback. */
+typedef struct {
+    redisDb *db;
+    long long now;
+    unsigned long sampled; /* num keys checked */
+    unsigned long expired; /* num keys expired */
+    long long ttl_sum; /* sum of ttl for key with ttl not yet expired */
+    int ttl_samples; /* num keys with ttl not yet expired */
+} expireScanData;
+
+void expireScanCallback(void *privdata, const dictEntry *de, dictEntryLink plink) {
+    UNUSED(plink);
+    kvobj *kv = dictGetKV(de);
+    expireScanData *data = privdata;
+    long long ttl  = kvobjGetExpire(kv) - data->now;
+    if (activeExpireCycleTryExpire(data->db, kv, data->now)) {
+        data->expired++;
+    }
+    if (ttl > 0) {
+        /* We want the average TTL of keys yet not expired. */
+        data->ttl_sum += ttl;
+        data->ttl_samples++;
+    }
+    data->sampled++;
+}
+
+static inline int expirySamplingShouldSkipDict(dict *d, int didx) {
+    long long numkeys = dictSize(d);
+    unsigned long buckets = dictBuckets(d);
+    /* When there are less than 1% filled buckets, sampling the key
+     * space is expensive, so stop here waiting for better times...
+     * The dictionary will be resized asap. */
+    if (buckets > DICT_HT_INITIAL_SIZE && (numkeys * 100/buckets < 1)) {
+        return 1;
+    }
+
+    /* During atomic slot migration, keys that are being imported are in an
+     * intermediate state. we cannot expire them and therefore skip them. */
+    if (!clusterCanAccessKeysInSlot(didx)) return 1;
+
+    return 0;
+}
+
+/* SubexpireCtx passed to activeSubexpiresCb() */
+typedef struct SubexpireCtx {
+    uint32_t fieldsToExpireQuota;
+    redisDb *db;
+    int slot;
+} SubexpireCtx;
+
+/*
+ * Active sub-expiration callback
+ *
+ * Called by activeSubexpires() for each key registered in the subexpires DB
+ * with an expiration-time on its "elements"  that are less than or equal current
+ * time.
+ *
+ * This callback performs the following actions for each hash:
+ * - Delete expired fields as by calling ebExpire(hash)
+ * - If afterward there are future fields to expire, it will update the hash in
+ *   HFE DB with the next hash-field minimum expiration time by returning
+ *   ACT_UPDATE_EXP_ITEM.
+ * - If the hash has no more fields to expire, it is removed from the HFE DB
+ *   by returning ACT_REMOVE_EXP_ITEM.
+ * - If hash has no more fields afterward, it will remove the hash from keyspace.
+ */
+static ExpireAction activeSubexpiresCb(eItem item, void *ctx) {
+    SubexpireCtx *subexCtx = ctx;
+
+    /* If no more quota left for this callback, stop */
+    if (subexCtx->fieldsToExpireQuota == 0)
+        return ACT_STOP_ACTIVE_EXP;
+
+    kvobj *kv = (kvobj *) item;
+
+    /* currently we only support hash type sub-expire */
+    assert(kv->type == OBJ_HASH);
+    uint64_t nextExpTime = hashTypeActiveExpire(subexCtx->db,kv,
+                                          &subexCtx->fieldsToExpireQuota, 0);
+
+    /* If hash has no more fields to expire or got deleted, indicate
+     * to remove it from HFE DB to the caller ebExpire() */
+    if (nextExpTime == EB_EXPIRE_TIME_INVALID || nextExpTime == 0) {
+        return ACT_REMOVE_EXP_ITEM;
+    } else {
+        /* Hash has more fields to expire. Update next expiration time of the hash
+         * and indicate to add it back to global HFE DS */
+        ebSetMetaExpTime(hashGetExpireMeta(item), nextExpTime);
+        return ACT_UPDATE_EXP_ITEM;
+    }
+}
+
+/* DB active expire and update hashes with time-expiration on fields.
+ *
+ * The callback function activeSubexpiresCb() is invoked for each hash registered
+ * in the subexpires DB with an expiration-time less than or equal to the
+ * current time. This callback performs the following actions for each hash:
+ * - If the hash has one or more fields to expire, it will delete those fields.
+ * - If there are more fields to expire, it will update the hash with the next
+ *   expiration time in subexpires DB.
+ * - If the hash has no more fields to expire, it is removed from the subexpires DB.
+ * - If the hash has no more fields, it is removed from the main DB.
+ *
+ * Returns number of fields active-expired.
+ */
+uint64_t activeSubexpires(redisDb *db, int slot, uint32_t maxFieldsToExpire) {
+    SubexpireCtx ctx = { .db = db, .fieldsToExpireQuota = maxFieldsToExpire, .slot = slot };
+    ExpireInfo info = {
+            .maxToExpire = UINT64_MAX, /* Only maxFieldsToExpire play a role */
+            .onExpireItem = activeSubexpiresCb,
+            .ctx = &ctx,
+            .now = commandTimeSnapshot(),
+            .itemsExpired = 0};
+
+    estoreActiveExpire(db->subexpires, slot, &info);
+
+    /* Return number of fields active-expired */
+    return maxFieldsToExpire - ctx.fieldsToExpireQuota;
+}
+
+/* Active expiration Cycle for hash-fields.
+ *
+ * Note that releasing fields is expected to be more predictable and rewarding
+ * than releasing keys because it is stored in `ebuckets` DS which optimized for
+ * active expiration and in addition the deletion of fields is simple to handle. */
+static inline void activeSubexpiresCycle(int type) {
+    /* Remember current db across calls */
+    static unsigned int currentDb = 0;
+    static int currentSlot = -1;
+
+    /* Tracks the count of fields actively expired for the current database.
+     * This count continues as long as it fails to actively expire all expired
+     * fields of currentDb, indicating a possible need to adjust the value of
+     * maxToExpire. */
+    static uint64_t activeExpirySequence = 0;
+    /* Threshold for adjusting maxToExpire */
+    const uint32_t EXPIRED_FIELDS_TH = 1000000;
+
+    redisDb *db = server.db + currentDb;
+
+    /* If db is empty, move to next db and return */
+    if (estoreIsEmpty(db->subexpires)) {
+        activeExpirySequence = 0;
+        currentDb = (currentDb + 1) % server.dbnum;
+        return;
+    }
+    if (currentSlot == -1)
+        currentSlot = estoreGetFirstNonEmptyBucket(db->subexpires);
+
+    /* During atomic slot migration, keys that are being imported are in an
+     * intermediate state. We cannot expire them and therefore skip them. */
+    if (!clusterCanAccessKeysInSlot(currentSlot)) {
+        /* Move to next non-empty subexpires slot */
+        currentSlot = estoreGetNextNonEmptyBucket(db->subexpires, currentSlot);
+        if (currentSlot == -1)
+            currentDb = (currentDb + 1) % server.dbnum; /* Move to next db */
+        return;
+    }
+
+    /* Maximum number of fields to actively expire on a single call */
+    uint32_t maxToExpire = HFE_DB_BASE_ACTIVE_EXPIRE_FIELDS_PER_SEC / server.hz;
+
+    /* If running for a while and didn't manage to active-expire all expired fields of
+     * currentDb (i.e. activeExpirySequence becomes significant) then adjust maxToExpire */
+    if ((activeExpirySequence > EXPIRED_FIELDS_TH) && (type == ACTIVE_EXPIRE_CYCLE_SLOW)) {
+        /* maxToExpire is multiplied by a factor between 1 and 32, proportional to
+         * the number of times activeExpirySequence exceeded EXPIRED_FIELDS_TH */
+        uint64_t factor = activeExpirySequence / EXPIRED_FIELDS_TH;
+        maxToExpire *= (factor<32) ? factor : 32;
+    }
+
+    if (activeSubexpires(db, currentSlot, maxToExpire) == maxToExpire) {
+        /* active-expire reached maxToExpire limit */
+        activeExpirySequence += maxToExpire;
+    } else {
+        /* Managed to active-expire all expired fields of currentDb */
+        activeExpirySequence = 0;
+        /* Move to next non-empty subexpires slot */
+        currentSlot = estoreGetNextNonEmptyBucket(db->subexpires, currentSlot);
+        if (currentSlot == -1)
+            currentDb = (currentDb + 1) % server.dbnum;
+    }
+}
+
+void activeExpireCycle(int type) {
+    /* Adjust the running parameters according to the configured expire
+     * effort. The default effort is 1, and the maximum configurable effort
+     * is 10. */
+    unsigned long
+    effort = server.active_expire_effort-1, /* Rescale from 0 to 9. */
+    config_keys_per_loop = ACTIVE_EXPIRE_CYCLE_KEYS_PER_LOOP +
+                           ACTIVE_EXPIRE_CYCLE_KEYS_PER_LOOP/4*effort,
+    config_cycle_fast_duration = ACTIVE_EXPIRE_CYCLE_FAST_DURATION +
+                                 ACTIVE_EXPIRE_CYCLE_FAST_DURATION/4*effort,
+    config_cycle_slow_time_perc = ACTIVE_EXPIRE_CYCLE_SLOW_TIME_PERC +
+                                  2*effort,
+    config_cycle_acceptable_stale = ACTIVE_EXPIRE_CYCLE_ACCEPTABLE_STALE-
+                                    effort;
+
+    /* This function has some global state in order to continue the work
+     * incrementally across calls. */
+    static unsigned int current_db = 0; /* Next DB to test. */
+    static int timelimit_exit = 0;      /* Time limit hit in previous call? */
+    static long long last_fast_cycle = 0; /* When last fast cycle ran. */
+
+    int j, iteration = 0;
+    int dbs_per_call = CRON_DBS_PER_CALL;
+    int dbs_performed = 0;
+    long long start = ustime(), timelimit, elapsed;
+
+    /* If 'expire' action is paused, for whatever reason, then don't expire any key.
+     * Typically, at the end of the pause we will properly expire the key OR we
+     * will have failed over and the new primary will send us the expire. */
+    if (isPausedActionsWithUpdate(PAUSE_ACTION_EXPIRE)) return;
+
+    if (type == ACTIVE_EXPIRE_CYCLE_FAST) {
+        /* Don't start a fast cycle if the previous cycle did not exit
+         * for time limit, unless the percentage of estimated stale keys is
+         * too high. Also never repeat a fast cycle for the same period
+         * as the fast cycle total duration itself. */
+        if (!timelimit_exit &&
+            server.stat_expired_stale_perc < config_cycle_acceptable_stale)
+            return;
+
+        if (start < last_fast_cycle + (long long)config_cycle_fast_duration*2)
+            return;
+
+        last_fast_cycle = start;
+    }
+
+    /* We usually should test CRON_DBS_PER_CALL per iteration, with
+     * two exceptions:
+     *
+     * 1) Don't test more DBs than we have.
+     * 2) If last time we hit the time limit, we want to scan all DBs
+     * in this iteration, as there is work to do in some DB and we don't want
+     * expired keys to use memory for too much time. */
+    if (dbs_per_call > server.dbnum || timelimit_exit)
+        dbs_per_call = server.dbnum;
+
+    /* We can use at max 'config_cycle_slow_time_perc' percentage of CPU
+     * time per iteration. Since this function gets called with a frequency of
+     * server.hz times per second, the following is the max amount of
+     * microseconds we can spend in this function. */
+    timelimit = config_cycle_slow_time_perc*1000000/server.hz/100;
+    timelimit_exit = 0;
+    if (timelimit <= 0) timelimit = 1;
+
+    if (type == ACTIVE_EXPIRE_CYCLE_FAST)
+        timelimit = config_cycle_fast_duration; /* in microseconds. */
+
+    /* Accumulate some global stats as we expire keys, to have some idea
+     * about the number of keys that are already logically expired, but still
+     * existing inside the database. */
+    long total_sampled = 0;
+    long total_expired = 0;
+
+    /* Try to smoke-out bugs (server.also_propagate should be empty here) */
+    serverAssert(server.also_propagate.numops == 0);
+
+    /* Stop iteration when one of the following conditions is met:
+     *
+     * 1) We have checked a sufficient number of databases with expiration time.
+     * 2) The time limit has been exceeded.
+     * 3) All databases have been traversed. */
+    for (j = 0; dbs_performed < dbs_per_call && timelimit_exit == 0 && j < server.dbnum; j++) {
+        /* Scan callback data including expired and checked count per iteration. */
+        expireScanData data;
+        data.ttl_sum = 0;
+        data.ttl_samples = 0;
+
+        redisDb *db = server.db+(current_db % server.dbnum);
+        data.db = db;
+
+        int db_done = 0; /* The scan of the current DB is done? */
+        int update_avg_ttl_times = 0, repeat = 0;
+
+        /* Increment the DB now so we are sure if we run out of time
+         * in the current DB we'll restart from the next. This allows to
+         * distribute the time evenly across DBs. */
+        current_db++;
+
+        /* Interleaving sub-expiration with key expiration. Better call it before
+         * handling expired keys because ebuckets is optimized for active expiration */
+        activeSubexpiresCycle(type);
+
+        if (kvstoreSize(db->expires))
+            dbs_performed++;
+
+        /* Continue to expire if at the end of the cycle there are still
+         * a big percentage of keys to expire, compared to the number of keys
+         * we scanned. The percentage, stored in config_cycle_acceptable_stale
+         * is not fixed, but depends on the Redis configured "expire effort". */
+        do {
+            unsigned long num;
+            iteration++;
+
+            /* If there is nothing to expire try next DB ASAP. */
+            if ((num = kvstoreSize(db->expires)) == 0) {
+                db->avg_ttl = 0;
+                break;
+            }
+            data.now = mstime();
+
+            /* The main collection cycle. Scan through keys among keys
+             * with an expire set, checking for expired ones. */
+            data.sampled = 0;
+            data.expired = 0;
+
+            if (num > config_keys_per_loop)
+                num = config_keys_per_loop;
+
+            /* Here we access the low level representation of the hash table
+             * for speed concerns: this makes this code coupled with dict.c,
+             * but it hardly changed in ten years.
+             *
+             * Note that certain places of the hash table may be empty,
+             * so we want also a stop condition about the number of
+             * buckets that we scanned. However scanning for free buckets
+             * is very fast: we are in the cache line scanning a sequential
+             * array of NULL pointers, so we can scan a lot more buckets
+             * than keys in the same time. */
+            long max_buckets = num*20;
+            long checked_buckets = 0;
+
+            int origin_ttl_samples = data.ttl_samples;
+
+            while (data.sampled < num && checked_buckets < max_buckets) {
+                db->expires_cursor = kvstoreScan(db->expires, db->expires_cursor, -1, expireScanCallback, expirySamplingShouldSkipDict, &data);
+                if (db->expires_cursor == 0) {
+                    db_done = 1;
+                    break;
+                }
+                checked_buckets++;
+            }
+            total_expired += data.expired;
+            total_sampled += data.sampled;
+
+            /* If find keys with ttl not yet expired, we need to update the average TTL stats once. */
+            if (data.ttl_samples - origin_ttl_samples > 0) update_avg_ttl_times++;
+
+            /* We don't repeat the cycle for the current database if the db is done
+             * for scanning or an acceptable number of stale keys (logically expired
+             * but yet not reclaimed). */
+            repeat = db_done ? 0 : (data.sampled == 0 || (data.expired * 100 / data.sampled) > config_cycle_acceptable_stale);
+
+            /* We can't block forever here even if there are many keys to
+             * expire. So after a given amount of microseconds return to the
+             * caller waiting for the other active expire cycle. */
+            if ((iteration & 0xf) == 0 || !repeat) { /* Update the average TTL stats every 16 iterations or about to exit. */
+                /* Update the average TTL stats for this database, 
+                 * because this may reach the time limit. */
+                if (data.ttl_samples) {
+                    long long avg_ttl = data.ttl_sum / data.ttl_samples;
+
+                    /* Do a simple running average with a few samples.
+                     * We just use the current estimate with a weight of 2%
+                     * and the previous estimate with a weight of 98%. */
+                    if (db->avg_ttl == 0) {
+                        db->avg_ttl = avg_ttl;
+                    } else {
+                        /* The origin code is as follow.
+                         * for (int i = 0; i < update_avg_ttl_times; i++) {
+                         *   db->avg_ttl = (db->avg_ttl/50)*49 + (avg_ttl/50);
+                         * } 
+                         * We can convert the loop into a sum of a geometric progression.
+                         * db->avg_ttl = db->avg_ttl * pow(0.98, update_avg_ttl_times) + 
+                         *                  avg_ttl / 50 * (pow(0.98, update_avg_ttl_times - 1) + ... + 1) 
+                         *             = db->avg_ttl * pow(0.98, update_avg_ttl_times) + 
+                         *                  avg_ttl * (1 - pow(0.98, update_avg_ttl_times))
+                         *             = avg_ttl +  (db->avg_ttl - avg_ttl) * pow(0.98, update_avg_ttl_times) 
+                         * Notice that update_avg_ttl_times is between 1 and 16, we use a constant table 
+                         * to accelerate the calculation of pow(0.98, update_avg_ttl_times).*/
+                        db->avg_ttl = avg_ttl + (db->avg_ttl - avg_ttl) * avg_ttl_factor[update_avg_ttl_times - 1] ;
+                    }
+                    update_avg_ttl_times = 0;
+                    data.ttl_sum = 0;
+                    data.ttl_samples = 0;
+                }
+                if ((iteration & 0xf) == 0) { /* check time limit every 16 iterations. */
+                    elapsed = ustime()-start;
+                    if (elapsed > timelimit) {
+                        timelimit_exit = 1;
+                        server.stat_expired_time_cap_reached_count++;
+                        break;
+                    }
+                }
+            }
+        } while (repeat);
+    }
+
+    elapsed = ustime()-start;
+    server.stat_expire_cycle_time_used += elapsed;
+    latencyAddSampleIfNeeded("expire-cycle",elapsed/1000);
+
+    /* Update our estimate of keys existing but yet to be expired.
+     * Running average with this sample accounting for 5%. */
+    double current_perc;
+    if (total_sampled) {
+        current_perc = (double)total_expired/total_sampled;
+    } else
+        current_perc = 0;
+    server.stat_expired_stale_perc = (current_perc*0.05)+
+                                     (server.stat_expired_stale_perc*0.95);
+}
+
+/*-----------------------------------------------------------------------------
+ * Expires of keys created in writable slaves
+ *
+ * Normally slaves do not process expires: they wait the masters to synthesize
+ * DEL operations in order to retain consistency. However writable slaves are
+ * an exception: if a key is created in the slave and an expire is assigned
+ * to it, we need a way to expire such a key, since the master does not know
+ * anything about such a key.
+ *
+ * In order to do so, we track keys created in the slave side with an expire
+ * set, and call the expireSlaveKeys() function from time to time in order to
+ * reclaim the keys if they already expired.
+ *
+ * Note that the use case we are trying to cover here, is a popular one where
+ * slaves are put in writable mode in order to compute slow operations in
+ * the slave side that are mostly useful to actually read data in a more
+ * processed way. Think at sets intersections in a tmp key, with an expire so
+ * that it is also used as a cache to avoid intersecting every time.
+ *
+ * This implementation is currently not perfect but a lot better than leaking
+ * the keys as implemented in 3.2.
+ *----------------------------------------------------------------------------*/
+
+/* The dictionary where we remember key names and database ID of keys we may
+ * want to expire from the slave. Since this function is not often used we
+ * don't even care to initialize the database at startup. We'll do it once
+ * the feature is used the first time, that is, when rememberSlaveKeyWithExpire()
+ * is called.
+ *
+ * The dictionary has an SDS string representing the key as the hash table
+ * key, while the value is a 64 bit unsigned integer with the bits corresponding
+ * to the DB where the keys may exist set to 1. Currently the keys created
+ * with a DB id > 63 are not expired, but a trivial fix is to set the bitmap
+ * to the max 64 bit unsigned value when we know there is a key with a DB
+ * ID greater than 63, and check all the configured DBs in such a case. */
+dict *slaveKeysWithExpire = NULL;
+
+/* Check the set of keys created by the master with an expire set in order to
+ * check if they should be evicted. */
+void expireSlaveKeys(void) {
+    if (slaveKeysWithExpire == NULL ||
+        dictSize(slaveKeysWithExpire) == 0) return;
+
+    int cycles = 0, noexpire = 0;
+    mstime_t start = mstime();
+    while(1) {
+        dictEntry *de = dictGetRandomKey(slaveKeysWithExpire);
+        sds keyname = dictGetKey(de);
+        uint64_t dbids = dictGetUnsignedIntegerVal(de);
+        uint64_t new_dbids = 0;
+
+        /* Check the key against every database corresponding to the
+         * bits set in the value bitmap. */
+        int dbid = 0;
+        while(dbids && dbid < server.dbnum) {
+            if ((dbids & 1) != 0) {
+                redisDb *db = server.db+dbid;
+                kvobj *kv = dbFindExpires(db, keyname);
+                int expired = kv && activeExpireCycleTryExpire(server.db+dbid, kv, start);
+
+                /* If the key was not expired in this DB, we need to set the
+                 * corresponding bit in the new bitmap we set as value.
+                 * At the end of the loop if the bitmap is zero, it means we
+                 * no longer need to keep track of this key. */
+                if (kv && !expired) {
+                    noexpire++;
+                    new_dbids |= (uint64_t)1 << dbid;
+                }
+            }
+            dbid++;
+            dbids >>= 1;
+        }
+
+        /* Set the new bitmap as value of the key, in the dictionary
+         * of keys with an expire set directly in the writable slave. Otherwise
+         * if the bitmap is zero, we no longer need to keep track of it. */
+        if (new_dbids)
+            dictSetUnsignedIntegerVal(de,new_dbids);
+        else
+            dictDelete(slaveKeysWithExpire,keyname);
+
+        /* Stop conditions: found 3 keys we can't expire in a row or
+         * time limit was reached. */
+        cycles++;
+        if (noexpire > 3) break;
+        if ((cycles % 64) == 0 && mstime()-start > 1) break;
+        if (dictSize(slaveKeysWithExpire) == 0) break;
+    }
+}
+
+/* Track keys that received an EXPIRE or similar command in the context
+ * of a writable slave. */
+void rememberSlaveKeyWithExpire(redisDb *db, sds key) {
+    if (slaveKeysWithExpire == NULL) {
+        static dictType dt = {
+            dictSdsHash,                /* hash function */
+            NULL,                       /* key dup */
+            NULL,                       /* val dup */
+            dictSdsKeyCompare,          /* key compare */
+            dictSdsDestructor,          /* key destructor */
+            NULL,                       /* val destructor */
+            NULL                        /* allow to expand */
+        };
+        slaveKeysWithExpire = dictCreate(&dt);
+    }
+    if (db->id > 63) return;
+
+    dictEntry *de = dictAddOrFind(slaveKeysWithExpire, key);
+    /* If the entry was just created, set it to a copy of the SDS string
+     * representing the key: we don't want to need to take those keys
+     * in sync with the main DB. The keys will be removed by expireSlaveKeys()
+     * as it scans to find keys to remove. */
+    if (dictGetKey(de) == key) {
+        dictSetKey(slaveKeysWithExpire, de, sdsdup(key));
+        dictSetUnsignedIntegerVal(de,0);
+    }
+
+    uint64_t dbids = dictGetUnsignedIntegerVal(de);
+    dbids |= (uint64_t)1 << db->id;
+    dictSetUnsignedIntegerVal(de,dbids);
+}
+
+/* Return the number of keys we are tracking. */
+size_t getSlaveKeyWithExpireCount(void) {
+    if (slaveKeysWithExpire == NULL) return 0;
+    return dictSize(slaveKeysWithExpire);
+}
+
+/* Remove the keys in the hash table. We need to do that when data is
+ * flushed from the server. We may receive new keys from the master with
+ * the same name/db and it is no longer a good idea to expire them.
+ *
+ * Note: technically we should handle the case of a single DB being flushed
+ * but it is not worth it since anyway race conditions using the same set
+ * of key names in a writable slave and in its master will lead to
+ * inconsistencies. This is just a best-effort thing we do. */
+void flushSlaveKeysWithExpireList(void) {
+    if (slaveKeysWithExpire) {
+        dictRelease(slaveKeysWithExpire);
+        slaveKeysWithExpire = NULL;
+    }
+}
+
+int checkAlreadyExpired(long long when) {
+    /* EXPIRE with negative TTL, or EXPIREAT with a timestamp into the past
+     * should never be executed as a DEL when load the AOF or in the context
+     * of a slave instance.
+     *
+     * Instead we add the already expired key to the database with expire time
+     * (possibly in the past) and wait for an explicit DEL from the master. */
+    if (server.current_client && server.current_client->flags & CLIENT_MASTER) return 0;
+    return (when <= commandTimeSnapshot() && !server.loading && !server.masterhost);
+}
+
+#define EXPIRE_NX (1<<0)
+#define EXPIRE_XX (1<<1)
+#define EXPIRE_GT (1<<2)
+#define EXPIRE_LT (1<<3)
+
+/* Parse additional flags of expire commands
+ *
+ * Supported flags:
+ * - NX: set expiry only when the key has no expiry
+ * - XX: set expiry only when the key has an existing expiry
+ * - GT: set expiry only when the new expiry is greater than current one
+ * - LT: set expiry only when the new expiry is less than current one */
+int parseExtendedExpireArgumentsOrReply(client *c, int *flags) {
+    int nx = 0, xx = 0, gt = 0, lt = 0;
+
+    int j = 3;
+    while (j < c->argc) {
+        char *opt = c->argv[j]->ptr;
+        if (!strcasecmp(opt,"nx")) {
+            *flags |= EXPIRE_NX;
+            nx = 1;
+        } else if (!strcasecmp(opt,"xx")) {
+            *flags |= EXPIRE_XX;
+            xx = 1;
+        } else if (!strcasecmp(opt,"gt")) {
+            *flags |= EXPIRE_GT;
+            gt = 1;
+        } else if (!strcasecmp(opt,"lt")) {
+            *flags |= EXPIRE_LT;
+            lt = 1;
+        } else {
+            addReplyErrorFormat(c, "Unsupported option %s", opt);
+            return C_ERR;
+        }
+        j++;
+    }
+
+    if ((nx && xx) || (nx && gt) || (nx && lt)) {
+        addReplyError(c, "NX and XX, GT or LT options at the same time are not compatible");
+        return C_ERR;
+    }
+
+    if (gt && lt) {
+        addReplyError(c, "GT and LT options at the same time are not compatible");
+        return C_ERR;
+    }
+
+    return C_OK;
+}
+
+/*-----------------------------------------------------------------------------
+ * Expires Commands
+ *----------------------------------------------------------------------------*/
+
+/* This is the generic command implementation for EXPIRE, PEXPIRE, EXPIREAT
+ * and PEXPIREAT. Because the command second argument may be relative or absolute
+ * the "basetime" argument is used to signal what the base time is (either 0
+ * for *AT variants of the command, or the current time for relative expires).
+ *
+ * unit is either UNIT_SECONDS or UNIT_MILLISECONDS, and is only used for
+ * the argv[2] parameter. The basetime is always specified in milliseconds.
+ *
+ * Additional flags are supported and parsed via parseExtendedExpireArguments */
+void expireGenericCommand(client *c, long long basetime, int unit) {
+    robj *key = c->argv[1], *param = c->argv[2];
+    long long when; /* unix time in milliseconds when the key will expire. */
+    long long current_expire = -1;
+    int flag = 0;
+
+    /* checking optional flags */
+    if (parseExtendedExpireArgumentsOrReply(c, &flag) != C_OK) {
+        return;
+    }
+
+    if (getLongLongFromObjectOrReply(c, param, &when, NULL) != C_OK)
+        return;
+
+    /* EXPIRE allows negative numbers, but we can at least detect an
+     * overflow by either unit conversion or basetime addition. */
+    if (unit == UNIT_SECONDS) {
+        if (when > LLONG_MAX / 1000 || when < LLONG_MIN / 1000) {
+            addReplyErrorExpireTime(c);
+            return;
+        }
+        when *= 1000;
+    }
+
+    if (when > LLONG_MAX - basetime) {
+        addReplyErrorExpireTime(c);
+        return;
+    }
+    when += basetime;
+
+    /* No key, return zero. */
+    kvobj *kv = lookupKeyWrite(c->db,key); 
+    if (kv == NULL) {
+        addReply(c,shared.czero);
+        return;
+    }
+
+    if (flag) {
+        current_expire = kvobjGetExpire(kv);
+
+        /* NX option is set, check current expiry */
+        if (flag & EXPIRE_NX) {
+            if (current_expire != -1) {
+                addReply(c,shared.czero);
+                return;
+            }
+        }
+
+        /* XX option is set, check current expiry */
+        if (flag & EXPIRE_XX) {
+            if (current_expire == -1) {
+                /* reply 0 when the key has no expiry */
+                addReply(c,shared.czero);
+                return;
+            }
+        }
+
+        /* GT option is set, check current expiry */
+        if (flag & EXPIRE_GT) {
+            /* When current_expire is -1, we consider it as infinite TTL,
+             * so expire command with gt always fail the GT. */
+            if (when <= current_expire || current_expire == -1) {
+                /* reply 0 when the new expiry is not greater than current */
+                addReply(c,shared.czero);
+                return;
+            }
+        }
+
+        /* LT option is set, check current expiry */
+        if (flag & EXPIRE_LT) {
+            /* When current_expire -1, we consider it as infinite TTL,
+             * but 'when' can still be negative at this point, so if there is
+             * an expiry on the key and it's not less than current, we fail the LT. */
+            if (current_expire != -1 && when >= current_expire) {
+                /* reply 0 when the new expiry is not less than current */
+                addReply(c,shared.czero);
+                return;
+            }
+        }
+    }
+
+    if (checkAlreadyExpired(when)) {
+        robj *aux;
+
+        int deleted = dbGenericDelete(c->db,key,server.lazyfree_lazy_expire,DB_FLAG_KEY_EXPIRED);
+        serverAssertWithInfo(c,key,deleted);
+        server.dirty++;
+
+        /* Replicate/AOF this as an explicit DEL or UNLINK. */
+        aux = server.lazyfree_lazy_expire ? shared.unlink : shared.del;
+        rewriteClientCommandVector(c,2,aux,key);
+        keyModified(c,c->db,key,NULL,1);
+        notifyKeyspaceEvent(NOTIFY_GENERIC,"del",key,c->db->id);
+        addReply(c, shared.cone);
+        return;
+    } else {
+        kv = setExpire(c,c->db,key,when); /* might realloc kv */
+        addReply(c,shared.cone);
+        /* Propagate as PEXPIREAT millisecond-timestamp
+         * Only rewrite the command arg if not already PEXPIREAT */
+        if (c->cmd->proc != pexpireatCommand) {
+            rewriteClientCommandArgument(c,0,shared.pexpireat);
+        }
+
+        /* Avoid creating a string object when it's the same as argv[2] parameter  */
+        if (basetime != 0 || unit == UNIT_SECONDS) {
+            robj *when_obj = createStringObjectFromLongLong(when);
+            rewriteClientCommandArgument(c,2,when_obj);
+            decrRefCount(when_obj);
+        }
+
+        keyModified(c,c->db,key,kv,1);
+        notifyKeyspaceEvent(NOTIFY_GENERIC,"expire",key,c->db->id);
+        server.dirty++;
+        return;
+    }
+}
+
+/* EXPIRE key seconds [ NX | XX | GT | LT] */
+void expireCommand(client *c) {
+    expireGenericCommand(c,commandTimeSnapshot(),UNIT_SECONDS);
+}
+
+/* EXPIREAT key unix-time-seconds [ NX | XX | GT | LT] */
+void expireatCommand(client *c) {
+    expireGenericCommand(c,0,UNIT_SECONDS);
+}
+
+/* PEXPIRE key milliseconds [ NX | XX | GT | LT] */
+void pexpireCommand(client *c) {
+    expireGenericCommand(c,commandTimeSnapshot(),UNIT_MILLISECONDS);
+}
+
+/* PEXPIREAT key unix-time-milliseconds [ NX | XX | GT | LT] */
+void pexpireatCommand(client *c) {
+    expireGenericCommand(c,0,UNIT_MILLISECONDS);
+}
+
+/* Implements TTL, PTTL, EXPIRETIME and PEXPIRETIME */
+void ttlGenericCommand(client *c, int output_ms, int output_abs) {
+    long long expire, ttl = -1;
+
+    /* If the key does not exist at all, return -2 */
+    kvobj *kv = lookupKeyReadWithFlags(c->db,c->argv[1],LOOKUP_NOTOUCH);
+    if (kv == NULL) {
+        addReplyLongLong(c,-2);
+        return;
+    }
+
+    /* The key exists. Return -1 if it has no expire, or the actual
+     * TTL value otherwise. */
+    expire = kvobjGetExpire(kv);
+    if (expire != -1) {
+        ttl = output_abs ? expire : expire-commandTimeSnapshot();
+        if (ttl < 0) ttl = 0;
+    }
+    if (ttl == -1) {
+        addReplyLongLong(c,-1);
+    } else {
+        addReplyLongLong(c,output_ms ? ttl : ((ttl+500)/1000));
+    }
+}
+
+/* TTL key */
+void ttlCommand(client *c) {
+    ttlGenericCommand(c, 0, 0);
+}
+
+/* PTTL key */
+void pttlCommand(client *c) {
+    ttlGenericCommand(c, 1, 0);
+}
+
+/* EXPIRETIME key */
+void expiretimeCommand(client *c) {
+    ttlGenericCommand(c, 0, 1);
+}
+
+/* PEXPIRETIME key */
+void pexpiretimeCommand(client *c) {
+    ttlGenericCommand(c, 1, 1);
+}
+
+/* PERSIST key */
+void persistCommand(client *c) {
+    kvobj *kv;
+    if ((kv = lookupKeyWrite(c->db,c->argv[1]))) {
+        if (removeExpire(c->db,c->argv[1])) {
+            keyModified(c,c->db,c->argv[1],kv,1);
+            notifyKeyspaceEvent(NOTIFY_GENERIC,"persist",c->argv[1],c->db->id);
+            addReply(c,shared.cone);
+            server.dirty++;
+        } else {
+            addReply(c,shared.czero);
+        }
+    } else {
+        addReply(c,shared.czero);
+    }
+}
+
+/* TOUCH key1 [key2 key3 ... keyN] */
+void touchCommand(client *c) {
+    int touched = 0;
+    for (int j = 1; j < c->argc; j++)
+        if (lookupKeyRead(c->db,c->argv[j]) != NULL) touched++;
+    addReplyLongLong(c,touched);
+}