Bugs item #1860776, was opened at 2007-12-30 06:03
Message generated for change (Comment added) made by bagder
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Category: misc
Group: None
>Status: Closed
>Resolution: Fixed
Priority: 5
Private: No
Submitted By: jls (apjenkins)
Assigned to: Daniel Stenberg (bagder)
Summary: non-blocking, normal and stderr stream read bug
Initial Comment:
I have found a bug in libssh2 0.18, where if I have a channel in non-blocking
mode and I alternate reading from the standard and error streams of the
channel, then no data is ever read; that is channel_read_ex always returns
LIBSSH2_ERROR_EAGAIN, even though the remote process is sending data.
Basically, if you have a channel in non-blocking mode, then if you have code
like this:
while (stillSomeDataToRead) {
// read from stderr stream
nRead = libssh2_channel_read_stderr(
channel, buf, bufsize);
// check for errors, handle input
// read from std
nRead = libssh2_channel_read(
channel, buf, bufsize);
// check for errors, handle input
}
then it often is the case that the channel_reads just keep returning
LIBSSH2_ERROR_EAGAIN (-37) forever. If I comment out the
libssh2_channel_read_stderr statement, then it does eventually read data from
the channel's stdout stream. So it seems that somehow, calling
libssh2_channel_read_stderr prevents libssh2_channel_read from ever receiving
any data, even though channel_read_stderr always just returns EAGAIN.
I have attached a program which demonstrates the bug. I have tried this on an
i386 Linux machine, and an Intel Mac running Leopard, and the bug occurs in
both cases. Occasionally the program works correctly so you may need to run it
several times to see the problem. If the program works correctly, then it will
output
...
channel_read err returned -37
channel_read returned -37
channel_read err returned -37
channel_read returned -37
channel_read err returned -37
channel_read returned 4096
If the failure occurs, then the "returned 4096" line never comes, and the reads
just keep returning -37 forever.
Run the program as
./libssh2hang2 YOUPASSWORD
The program will try to establish a connection to the localhost and log in
using password auth. Pass your password as the first cmdline argument. It
will then exec the "yes" command, which just outputs an endless stream of
characters, put the channel into non-blocking mode, and then keep reading until
it's read at least 4096 bytes. If you disable the libssh2_channel_read_stderr
statement, then it always works.
Please let me know if you need any more information.
----------------------------------------------------------------------
>Comment By: Daniel Stenberg (bagder)
Date: 2009-04-28 14:49
Message:
Thanks for telling, case is closed!
----------------------------------------------------------------------
Comment By: jls (apjenkins)
Date: 2009-04-15 17:01
Message:
I just tried my test program with version libssh2 1.1 on OS X 10.5.6, and
the bug seems to be fixed. Thank you.
----------------------------------------------------------------------
Comment By: Daniel Stenberg (bagder)
Date: 2009-04-15 09:10
Message:
Does this still happen with version 1.1?
----------------------------------------------------------------------
Comment By: jls (apjenkins)
Date: 2007-12-30 19:44
Message:
Logged In: YES
user_id=996635
Originator: YES
A workaround for this problem seems to be to use libssh2_poll to wait for
there to be std or extended data on a channel, and only call
libssh2_channel_read_ex on a channel stream if libssh2_poll indicates there
is data to be read.
----------------------------------------------------------------------
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https://sourceforge.net/tracker/?func=detail&atid=703942&aid=1860776&group_id=125852
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