Re: [Wget-dev] wget2 | WIP: Add c-ares support for quickly fill DNS cache in background (!444)

[Tim Rühsen]

Tim Rühsen started a new discussion on async_dns/async_dns.c: 
https://gitlab.com/gnuwget/wget2/merge_requests/444#note_207965090

> +static int MAXWAITING; // Max. number of parallel DNS queries
> +static int MAXTRIES; // Max. number of tries per domain
> +static int TIMEOUT; // Max. number of ms for first try
> +
> +static int nwaiting;
> +
> +static int resolved;
> +static int failed;
> +
> +// create fname & set first row (needed for further processing with R)
> +static void prepare_file(const char *fname)
> +{
> +     struct stat st;
> +     FILE *fp;
> +
> +     if ((stat(fname, &st) || !st.st_size) && (fp = fopen(fname, "a"))) {

This is a typical TOCTOU issue (see 
https://en.wikipedia.org/wiki/Time-of-check_to_time-of-use).

Try to avoid, e.g. first `fopen(fname, "a")` and then check with ftell(fp) if 
you are a position 0 (=empty file) or not.

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