Implements [B]Z[REV]POP and the respective unit tests

An implementation of the
[Ze POP Redis Module](https://github.com/itamarhaber/zpop) as core
Redis commands.

Fixes #1861.

src/t_zset.c

@@ -3068,3 +3068,147 @@ void zscanCommand(client *c) {
         checkType(c,o,OBJ_ZSET)) return;
     scanGenericCommand(c,o,cursor);
 }
+
+/* This command implements the generic zpop operation, used by:
+ * ZPOP, ZREVPOP, BZPOP and BZREVPOP */
+void genericZpopCommand(client *c, robj **keyv, int keyc, int reverse) {
+    int idx;
+    robj *key;
+    robj *zobj;
+    sds ele;
+    double score;
+    char *events[2] = {"zpop","zrevpop"};
+
+    // Check type and break on the first error, otherwise identify candidate
+    idx = 0;
+    while (idx < keyc) {
+        key = keyv[idx++];
+        zobj = lookupKeyWrite(c->db,key);
+        if (!zobj) continue;
+        if (checkType(c,zobj,OBJ_ZSET)) return;
+        break;
+    }
+
+    // No candidate for zpopping, return empty
+    if (!zobj) {
+        addReply(c,shared.emptymultibulk);
+        return;
+    }
+
+    if (zobj->encoding == OBJ_ENCODING_ZIPLIST) {
+        unsigned char *zl = zobj->ptr;
+        unsigned char *eptr, *sptr;
+        unsigned char *vstr;
+        unsigned int vlen;
+        long long vlong;
+
+        // Get the first or last element in the sorted set
+        eptr = ziplistIndex(zl,reverse ? -2 : 0);
+        serverAssertWithInfo(c,zobj,eptr != NULL);
+        
+        // There must be an element in the sorted set
+        serverAssertWithInfo(c,zobj,eptr != NULL);
+        serverAssertWithInfo(c,zobj,ziplistGet(eptr,&vstr,&vlen,&vlong));
+        if (vstr == NULL)
+            ele = sdsfromlonglong(vlong);
+        else
+            ele = sdsnewlen(vstr,vlen);
+
+        // Get the score
+        sptr = ziplistNext(zl,eptr);
+        serverAssertWithInfo(c,zobj,sptr != NULL);
+        score = zzlGetScore(sptr);
+    } else if (zobj->encoding == OBJ_ENCODING_SKIPLIST) {
+        zset *zs = zobj->ptr;
+        zskiplist *zsl = zs->zsl;
+        zskiplistNode *ln;
+
+        // Get the first or last element in the sorted set
+        ln = (reverse ? zsl->tail : zsl->header->level[0].forward);
+
+        // There must be an element in the sorted set
+        serverAssertWithInfo(c,zobj,ln != NULL);
+        ele = sdsdup(ln->ele);
+        score = ln->score;
+    } else {
+        serverPanic("Unknown sorted set encoding");
+    }
+
+    // Remove the element
+    serverAssertWithInfo(c,zobj,zsetDel(zobj,ele));
+    server.dirty++;
+    signalModifiedKey(c->db,key);
+    notifyKeyspaceEvent(NOTIFY_ZSET,events[reverse],key,c->db->id);
+
+    // Remove the key, if indeed needed
+    if (zsetLength(zobj) == 0) {
+        dbDelete(c->db,key);
+        notifyKeyspaceEvent(NOTIFY_GENERIC,"del",key,c->db->id);
+    }
+
+    addReplyMultiBulkLen(c,3);
+    addReplyBulk(c,key);
+    addReplyDouble(c,score);
+    addReplyBulkCBuffer(c,ele,sdslen(ele));
+    sdsfree(ele);
+}
+
+// ZPOP key [key ...]
+void zpopCommand(client *c) {
+    genericZpopCommand(c,&c->argv[1],c->argc-1,0);
+}
+
+// ZREVPOP key [key ...]
+void zrevpopCommand(client *c) {
+    genericZpopCommand(c,&c->argv[1],c->argc-1,1);
+}
+
+/* Blocking Z[REV]POP */
+void blockingGenericZpopCommand(client *c, int reverse) {
+    robj *o;
+    mstime_t timeout;
+    int j;
+
+    if (getTimeoutFromObjectOrReply(c,c->argv[c->argc-1],&timeout,UNIT_SECONDS)
+        != C_OK) return;
+
+    for (j = 1; j < c->argc-1; j++) {
+        o = lookupKeyWrite(c->db,c->argv[j]);
+        if (o != NULL) {
+            if (o->type != OBJ_ZSET) {
+                addReply(c,shared.wrongtypeerr);
+                return;
+            } else {
+                if (zsetLength(o) != 0) {
+                    /* Non empty zset, this is like a normal Z[REV]POP. */
+                    genericZpopCommand(c,&c->argv[j],1,reverse);
+                    /* Replicate it as an Z[REV]POP instead of BZ[REV]POP. */
+                    rewriteClientCommandVector(c,2,
+                        reverse ? shared.zrevpop : shared.zpop,
+                        c->argv[j]);
+                    return;
+                }
+            }
+        }
+    }
+
+    /* If we are inside a MULTI/EXEC and the zset is empty the only thing
+     * we can do is treating it as a timeout (even with timeout 0). */
+    if (c->flags & CLIENT_MULTI) {
+        addReply(c,shared.nullmultibulk);
+        return;
+    }
+
+    /* If the keys do not exist we must block */
+    blockForKeys(c,BLOCKED_ZSET,c->argv + 1,c->argc - 2,timeout,NULL,NULL);
+}
+
+// BZPOP key [key ...] timeout
+void bzpopCommand(client *c) {
+    blockingGenericZpopCommand(c,0);
+}
+
+// BZREVPOP key [key ...] timeout
+void bzrevpopCommand(client *c) {
+    blockingGenericZpopCommand(c,1);
+}