CURL review request

[Jonathan M Davis]

On Tuesday, August 16, 2011 12:32 Martin Nowak wrote:
> On Tue, 16 Aug 2011 20:48:51 +0200, jdrewsen <jdrewsen at nospam.com> wrote:
> > Den 16-08-2011 18:55, Martin Nowak skrev:
> >> On Tue, 16 Aug 2011 15:13:40 +0200, dsimcha <dsimcha at yahoo.com> wrote:
> >>> On 8/16/2011 7:48 AM, Jonas Drewsen wrote:
> >>>> Hi all,
> >>>> 
> >>>> This is a review request for the curl wrapper. Please read the "known
> >>>> issues" in the top of the source file and if possible suggest a
> >>>> solution.
> >>>> 
> >>>> We also need somebody for running the review process. Anyone?
> >>>> 
> >>>> Code:
> >>>> https://github.com/jcd/phobos/blob/curl-wrapper/etc/curl.d
> >>>> Docs:
> >>>> http://freeze.steamwinter.com/D/web/phobos/etc_curl.html
> >>>> 
> >>>> Demolish!
> >>>> 
> >>>> /Jonas
> >>> 
> >>> From a quick look, this looks very well thought out. I'll review it
> >>> more thoroughly when I have more time. A few questions/comments from a
> >>> quick look at the docs:
> >>> 
> >>> Does the async stuff use D threads, or does Curl have its own async
> >>> API?
> >>> 
> >>> In your examples for postData, you have onReceive a ubyte[] and write
> >>> it out to console. Did you mean to cast this to some kind of string?
> >>> 
> >>> For onReceive, what's the purpose of the return value?
> >>> 
> >>> If/when this module makes it into Phobos, are we going to start
> >>> including a libcurl binary with DMD distributions so that std.curl
> >>> feels truly **standard** and requires zero extra configuration?
> >> 
> >> I was also wondering about the async handling. In the long-term I'd like
> >> to see a bigger picture for async handling in phobos (offering some kind
> >> of futures, maybe event-loops etc.).
> >> Though this is not a requirement for the curl wrapper now.
> >> std.parallelism also has some kind of this stuff and file reading would
> >> benefit from it too.
> > 
> > This has been discussed before and I also think this is very important.
> > But before that I think some kind of package management should be
> > prioritized (A DIP11 implementaion or a more traditional solution).
> > 
> >> One thing I spotted at a quick glance, sending to be filled buffers to
> >> another thread should not be done by casting to shared not immutable.
> > 
> > I'm not entirely sure what you mean. There is no use of shared buffers
> > in the wrapper. I do cast the buffer between mutable/immutable because
> > only immutable or by value data can be passed using std.concurrency.
> > Since the buffers are only used by the thread that currently has the
> > buffer this is safe. I've previously asked for a non-cast solution (ie.
> > some kind of move between threads semantic for std.concurrency) but was
> > advised that this was the way to do it.
> > 
> >> martin
> 
> Pardon the typo. What I meant is that AFAIK casting from immutable to
> mutable has undefined behavior.
> The intended method for sending a uint[] buffer to another thread is to
> cast that
> buffer to shared (cast(shared(uint[])) and casting away the shared on the
> receiving side.
> It is allowed to send shared data using std.concurrency.

Casting away immutability and then altering data is undefined. Actually 
casting it away is defined. So, if you have data in one thread that you know 
is unique, you can cast it to immutable (or std.exception.assumeUnique to do 
it) and then send it to another thread. On that thread, you can then cast it 
to mutable and alter it.

However, you're circumventing the type system when you do this. So, you have 
to be very careful. You're throwing away the guarantees that the compiler 
makes with regards to const and immutable. It _is_ guaranteed to work though. 
And I'm not sure that there's really any difference between casting to shared 
and back and casting to immutable and back. In both cases, you're 
circumventing the type system. The main difference would be that if you 
screwed up with immutable and cast away immutable on something that really was 
immutable rather than something that you cast to immutable just to send it to 
another thread, then you could a segfault when you tried to alter it, since it 
could be in ROM.

- Jonathan M Davis