Comment by numeromancer

NB: In the following function:

    /* Like sdscatpritf() but gets va_list instead of being variadic. */
    sds sdscatvprintf(sds s, const char *fmt, va_list ap) {
        va_list cpy;
        char *buf, *t;
        size_t buflen = 16;
    
        while(1) {
            buf = malloc(buflen);
            if (buf == NULL) return NULL;
            buf[buflen-2] = '\0';
            va_copy(cpy,ap);
            vsnprintf(buf, buflen, fmt, cpy);
            va_end(cpy); // <--- add this ----
            if (buf[buflen-2] != '\0') {
                free(buf);
                buflen *= 2;
                continue;
            }
            break;
        }
        t = sdscat(s, buf);
        free(buf);
        return t;
    }

From the `man va_copy` on my system:

    Each  invocation  of  va_copy()  must  be  matched  by  
    a  corresponding invocation of va_end() in the same
    function.

This is not likely to be a problem in most systems, but it can't hurt to be formally correct.