Re: [capnproto] Detect closed connection using StreamFdMessageReader?

['Kenton Varda' via Cap'n Proto]

So, StreamFdMessageReader was designed to read one message from an FD. It
doesn't really have any way to say "try to read a message, but let me know
if it's EOF".

Things you could do instead:
- Use poll() to check for POLLHUP before constructing
StreamFdMessageReader. If POLLHUP is indicated, use ioctl FIONREAD to check
if there are any bytes waiting to be read. If there are not, then you're at
EOF, so don't create a StreamFdMessageReader at all.
- Wrap the FD in an kj::FdInputStream, and wrap that in
kj::BufferedInputStreamWrapper. That lets you call `tryGetReadBuffer()`
which will return null on EOF. `tryGetReadBuffer` doesn't actually advance
the read pointer, it "peeks" at the next bytes in the stream, so if it
returns a non-empty array, you can then use InputStreamMessageReader parse
the message.
- Use KJ's async I/O, in which case you can use capnp::tryReadMessage()
from capnp/serialize-async.h. This attempts to read a message but returns
null on EOF.

Admittedly, these are all work-arounds for a missing API. We really should
have a way to use StreamFdMessageReader but have it tell you if the stream
was EOF...

-Kenton

On Thu, Feb 3, 2022 at 10:47 AM SR D <
software.research.developm...@gmail.com> wrote:

> Hi,
>
> I use capnp::StreamFdMessageReader(fd) to read a message on a file
> descriptor created using network sockets.
>
> And it works fine. However, I'm trying to detect if the client socket has
> closed. Normally, if a read() call returns 0, this would show the client
> connection has closed.
>
> How would I go about this by using StreamFdMessageReader?
>
>
> --
> You received this message because you are subscribed to the Google Groups
> "Cap'n Proto" group.
> To unsubscribe from this group and stop receiving emails from it, send an
> email to capnproto+unsubscr...@googlegroups.com.
> To view this discussion on the web visit
> https://groups.google.com/d/msgid/capnproto/c5b44e93-dde3-41ec-bced-7d839fde6185n%40googlegroups.com
> 
> .
>

-- 
You received this message because you are subscribed to the Google Groups 
"Cap'n Proto" group.
To unsubscribe from this group and stop receiving emails from it, send an email 
to capnproto+unsubscr...@googlegroups.com.
To view this discussion on the web visit 
https://groups.google.com/d/msgid/capnproto/CAJouXQnHv5ak%3D6Fzvf7m2ccAv__TO01_tyKDFp-Fan%3DMerTcXg%40mail.gmail.com.

[capnproto] Detect closed connection using StreamFdMessageReader?

[SR D]

Hi,

I use capnp::StreamFdMessageReader(fd) to read a message on a file 
descriptor created using network sockets.

And it works fine. However, I'm trying to detect if the client socket has 
closed. Normally, if a read() call returns 0, this would show the client 
connection has closed.

How would I go about this by using StreamFdMessageReader?


-- 
You received this message because you are subscribed to the Google Groups 
"Cap'n Proto" group.
To unsubscribe from this group and stop receiving emails from it, send an email 
to capnproto+unsubscr...@googlegroups.com.
To view this discussion on the web visit 
https://groups.google.com/d/msgid/capnproto/c5b44e93-dde3-41ec-bced-7d839fde6185n%40googlegroups.com.