[lldb-dev] Getting ConnectionFileDescriptor working on Windows

[Zachary Turner]

The main reason for wanting to change things is because there is no select
on windows (technically there is, but it works completely differently), and
because just a file descriptor isn't enough information to make
ConnectionFileDescriptor work for all the required types of objects.  The
implementation details are completely different whether you have a pipe,
socket, disk file, or something else.

I think the way to do everything with the lowest possible impact change on
posix is the following (this was mentioned earlier in the thread but since
you started with the OP i'll rehash the summary):

1) Make separate classes for Windows.  ConnectionGenericFile,
ConnectionPipe, ConnectionSocket, ConnectionSocketAccept, etc.
2) On Posix, keep ConnectionFileDescriptor, but add ConnectionSocketAccept
which derives from ConnectionFileDescriptor and the accept-specific stuff
goes in the derived class.  No other posix changes.  Might need to change
the name from ConnectionFileDescriptor to ConnectionGenericFile, for
syntax-parity so that any platform can instantiate it regardless of the
platform.
3) In Connection (the base interface), add a static method called
CreateGenericConnection which takes a connection string.  Essentially this
is a factory method.  This function has an #ifdef _WIN32 which will look at
the connection string and create the approprioately derived type.  On
Windows this will create either a ConnectionGenericFile, ConnectionPipe,
ConnectionSocket, ConnectionSocketAccept.  On non-windows it will create
either a ConnectionFileDescriptor or a ConnectionSocketAccept.
4) Any call-sites that currently instantiate a CFD and call Connect() with
a string will be updated to use the factory method.
5) Any call-sites that instantiate a CFD with the fd constructor where the
fd comes from the user, or where there's not enough information to know
specifically what type it is will use ConnectionGenericFile (windows) or
ConnectionFileDescriptor (posix).
6) Any call-sites that instantiate a CFD with the fd constructor but where
we know what type it is will explicitly instantiate the derived type.

The creation of the ConnectionSocketAccept on posix is necessary because it
doesn't have the same interface as the others.  It has GetBoundPort().  But
we only use this in cases where we know in advance it's a listening socket,
so there should be no issue here.

On Mon, Oct 6, 2014 at 1:11 PM, Greg Clayton <gclayton at apple.com> wrote:

> NOTE: I haven't read the other responses to this thread, so I am going to
> start with this email and work my way through. More comments below.
> > On Oct 1, 2014, at 1:44 PM, Zachary Turner <zturner at google.com> wrote:
> >
> > I'm trying to get ConnectionFileDescriptor working on Windows.  Because
> select() has completely different semantics on Windows as on posix, it
> won't work for this purpose, so It hink the best approach is to re-write
> ConnectionFileDescriptor for windows.  I've already submitted a patch that
> moves CFD to Host so that I can provide a different implementation for
> Windows.
> >
> > However, I'm not sure if just providing a Windows implementation is
> sufficient, due to the very nature of this class.  In short, the things you
> can do with a generic file descriptor on Windows are very different than
> the things you can do with a file descriptor on posix.  And unfortunately,
> certain assumptions about what you can do with a file descriptor are baked
> into the code that uses CFD as well as CFD itself (after all, its design
> alone implies that all these different use cases of file descriptors are
> interchangeable).
> >
> > One of the more awkward things to deal with is that CFD provides a
> constructor that takes a file descriptor.  Inside, it *assumes* that this
> fd represents an actual file, but in practice we sometimes give it other
> things to this constructor.
> >
> > I've broken down all the cases of where we use this constructor as
> follows:
> >
> > SBCommunication::AdoptFileDesriptor - passes in whatever fd the user
> decides to give it, and treats it as a file.
>
> This is in the public API, so this needs to stay here for non windows
> builds. On windows we could add a new windows only API that uses a HANDLE
> if needed, or abstract this in some way through the public interface.
>
> >
> > ScriptInterpreterPython::ExecuteOneLine - passes in the fd of a pipe,
> CFD doesn't have any way to know it's a pipe though, and treats it as a
> file.
>
> This makes generic use of the CFD that uses a FD. Since we need to
> interface with the native python layer, we will need to do whatever we need
> to do to give python a file handle it can use for in/out/err. This probably
> can be special cased for windows to make it work as needed.
> >
> > GDBRemoteCommunicationServer::SetSTDIOFileDescriptor - Not sure, I think
> this is always an actual file, but maybe not.
> >
> > Process::SetSTDIOFileDescriptor - Same as before, I think this is always
> an actual file, but maybe not.
>
> These are all internal, so we can re-architect as needed.
>
> >
> > I'm not really sure how to deal with this on Windows.  We need to know
> what it *actually* is.  Furthermore, file descriptors in general don't
> provide the necessary functionality required to be able to implement
> interrupting as used by CFD.  For that we need the native OS type for
> whatever these objects are.  For sockets that means an object of type
> SOCKET.  For pipes and files it means a HANDLE.  "Here's an fd, do
> something with it" just isn't enough of, or the right kind of information
> to be able to do the right thing.
>
> the lldb_private::Connection class is currently a very simple abstraction
> for reading/writing something that would go over a wire to talk to a remote
> debug client, but it doesn't provide interruption.
> >
> > AFAICT I only need to deal with the above 4 cases.
> >
> > The first case I can deal with by #ifdefing it out on Windows returning
> an error that says "sorry, not supported on Windows".
>
> Or adding a new windows specific "HANDLE" variation for windows only and
> then make sure it interfaces with the internal classes (whatever we do with
> CFD).
>
> >
> > For the second case, I'm not sure what to do because I don't actually
> understand how this function works or why this level of complexity is
> required.
>
> Again, it will need to interface with the native python layer on windows.
> I don't know what that looks like or if they change any files around.
> Sometimes the current debugger has the IOHandler to to stdin/out/err and it
> needs to just use that, other times we are executing python code and want
> to catch the output so we can copy it into the result object in case the
> python code prints any output. So it needs to be generic enough to work.
> Everything right now on the IOHandler stack uses "FILE *". I really don't
> want to change that. Windows has an abstraction for "FILE *" that works, so
> I would just try to make things work on windows using the "FILE *", or
> there will be very major re-architecture required.
>
> The main reason for you wanting to change things is because there is no
> select on windows?
>
> >
> > For the third and fourth cases, maybe I can just assume they're files?
>
> Both of these two could just be changed to accept a
> lldb_private::Connection subclass. Easy fix.
>
> >
> > I was thinking about splitting CFD into multiple classes, one for each
> type of descriptor.  A FileConnection, PipeConnection, TcpSocketConnection,
> ListeningConnection, etc.  Then, the caller would have to instantiate the
> right kind.  This has its own set of problems, but at least seems to solve
> the problem of requiring the creator of the CFD to specify what they're
> actually giving us.  On posix in cases where it's user specified or don't
> know, it seems we could just create a FileConnection and that would be
> equivalent to the current behavior, since it seems to treat everything the
> same anyway.
>
> Feel free to do this on windows, but please leave the POSIX stuff alone.
> All file descriptor calls mostly just work with each other. There is no
> need to split these out as they are all related. They could be split out,
> but then a very intelligent base file descriptor class would be needed and
> then just the special cases can be broken out. I would rather not have to
> go to 5 different classes and change all of the implementations if they are
> all the same (open, close, read/write, etc).
>
>
>
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