Ticket #76 (closed defect)

Opened 8 years ago

Last modified 8 years ago

libssh2_poll detects data on channel, but no data available

Reported by: anonymous Owned by: bagder
Priority: low Milestone:
Component: Version:
Keywords: Cc: bagder, thomaspu
Blocked By: Blocks:


I've got a small home-brewed SSH client and I've encountered a problem in the library with libssh2_poll() when checking for data on a channel.

Simply put, here's the setup. I have a thread that polls for data on a channel every few usec. When the libssh2_poll function detects data available to be read on the channel I'm interested in, I immediately try to read from that channel. Trouble is, if its a small piece of data that I'm expecting, the read fails while subsequent polls still think that there is data there.

So In my SSH client with a pty, I do something like hit the "z" key. This should send a single character over the lines to the remote SSH server plus any SSH/TCP/IP.... overhead. Now the remote SSH server will echo back the key plus any additional terminal formatting.

As soon as I hit my "z" key, libssh2_poll alerts me that data is available to be read from the channel, but attempting to read from the channel doesn't read any data...polling keeps saying theres data, I read nothing... repeat a lot of times...

If I keep hitting keystrokes, eventually the channel read picks up the data. So it seems like libssh2 holds onto small packets for some reason.

Change History

comment:1 Changed 8 years ago by thomaspu

This one was written up by me, I just forgot to log in again!

comment:2 Changed 8 years ago by thomaspu

I think the problem stems around line 1525 of channel.c. Looks like if we call the channel_read function, we expect to read data from a call to libssh2_packet_read() and if we don't read anything from that function call, we don;t read anything period. However, the code doesn't check the packet list to see if there's packets in it waiting to be read.

comment:3 Changed 8 years ago by bagder

libssh2_poll() seems to be fundamentally flawed.

I suggest we just call it deprecated and tell people to use poll() instead!

Can you (or anyone else) provide a use case where the normal poll() or select() isn't good enough or better?

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