On Mon, Aug 28, 2023 at 14:32 Daniel Stenberg via curl-library < curl-library@lists.haxx.se> wrote:
> On Mon, 28 Aug 2023, Paul Fotheringham wrote: > > > That thread seemed to peter out without a reply to Brad's question. I > too > > would benefit from an example of why calling libcurl functions from > within > > the callback is tricky to handle. I'm not saying it's not tricky to > handle, > > I just ask from a position of ignorance. > > First out, we already deny most libcurl functions from getting called from > their callbacks precisely because of the challenges. > > When libcurl calls a callback it is "in the middle of something", while > when > it returns back to the caller, it has finished doing it and stores the > state > correctly somewhere in order to be able to continue from there at the next > invokation. > > When a callback calls libcurl *back* in a recursive manner, it is hard to > make > sure that alls states, pointers and variables are handled correctly. Since > the > recursively called function changes internals and then returns back to the > callback that returns back into libcurl again ... into another context > which > may have local state or variables that now no longer actually are correct > because things have been changed. > > It is of course quite *possible* to make this work (it is just code after > all) > but it requires deliberate attention to this and quite a lot of testing to > make sure lots of edge cases are covered. And we don't do nor have that. > As I > said: we prevent the recursive call instead to protect the application > from > problems. > > By avoiding recursive calls into the library our lives are much easier. > That all makes sense - What I was thinking of as the strength of my original proposal is up-to-the-moment canonical truth (because calls are dispatched from within curl itself with its own known state); it might be starting to get clunky (food for discussion) - but perhaps the contract 1) whitelists known-good functions, or 2) CURLM* and CURL* handles are for reading purposes only, and subsequent ops are relegated to (eg) querying the callers own userdata upon return of curlm_easy_iter() and operating in “typical scope” rather than some arbitrary callback depth. -bch > -- > > / daniel.haxx.se > | Commercial curl support up to 24x7 is available! > | Private help, bug fixes, support, ports, new features > | https://curl.se/support.html > -- > Unsubscribe: https://lists.haxx.se/mailman/listinfo/curl-library > Etiquette: https://curl.se/mail/etiquette.html >
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