Function trimReplyUnusedTailSpace

src/networking.cpp:668–691

Sometimes we are forced to create a new reply node, and we can't append to * the previous one, when that happens, we wanna try to trim the unused space * at the end of the last reply node which we won't use anymore. */

 * the previous one, when that happens, we wanna try to trim the unused space
 * at the end of the last reply node which we won't use anymore. */
void trimReplyUnusedTailSpace(client *c) {
    listNode *ln = listLast(c->reply);
    clientReplyBlock *tail = ln? (clientReplyBlock*)listNodeValue(ln): NULL;

    /* Note that 'tail' may be NULL even if we have a tail node, becuase when
     * addReplyDeferredLen() is used */
    if (!tail) return;

    /* We only try to trim the space is relatively high (more than a 1/4 of the
     * allocation), otherwise there's a high chance realloc will NOP.
     * Also, to avoid large memmove which happens as part of realloc, we only do
     * that if the used part is small.  */
    if (tail->size - tail->used > tail->size / 4 &&
        tail->used < PROTO_REPLY_CHUNK_BYTES)
    {
        size_t old_size = tail->size;
        tail = (clientReplyBlock*)zrealloc(tail, tail->used + sizeof(clientReplyBlock));
        /* take over the allocation's internal fragmentation (at least for
         * memory usage tracking) */
        tail->size = zmalloc_usable_size(tail) - sizeof(clientReplyBlock);
        c->reply_bytes = c->reply_bytes + tail->size - old_size;
        listNodeValue(ln) = tail;
    }
}

/* Adds an empty object to the reply list that will contain the multi bulk
 * length, which is not known when this function is called. */