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List:       jakarta-commons-dev
Subject:    [jira] [Created] (VFS-589) SFTP moveTo operation hangs if the server does not support SSH channelExe
From:       "L (JIRA)" <jira () apache ! org>
Date:       2015-11-30 10:36:10
Message-ID: JIRA.12916925.1448879730000.220108.1448879770923 () Atlassian ! JIRA
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L created VFS-589:
---------------------

             Summary: SFTP moveTo operation hangs if the server does not support SSH \
channelExec.  Key: VFS-589
                 URL: https://issues.apache.org/jira/browse/VFS-589
             Project: Commons VFS
          Issue Type: Bug
    Affects Versions: Nightly Builds
            Reporter: L


In our case the server was explicitly configured to disable SSH channelExec.
Our code was hanging trying to execute moveTo(). Stacktrace:

{code}
   java.lang.Thread.State: TIMED_WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    at java.io.PipedInputStream.read(PipedInputStream.java:326)
    - locked <0x00000006a7a184d0> (a com.jcraft.jsch.Channel$MyPipedInputStream)
    at java.io.PipedInputStream.read(PipedInputStream.java:377)
    - locked <0x00000006a7a184d0> (a com.jcraft.jsch.Channel$MyPipedInputStream)
    at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:284)
    at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:326)
    at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:178)
    - locked <0x00000006a7a184b8> (a java.io.InputStreamReader)
    at java.io.InputStreamReader.read(InputStreamReader.java:184)
    at org.apache.commons.vfs2.provider.sftp.SftpFileSystem.executeCommand(SftpFileSystem.java:328)
  at org.apache.commons.vfs2.provider.sftp.SftpFileSystem.getGroupsIds(SftpFileSystem.java:260)
  at org.apache.commons.vfs2.provider.sftp.SftpFileObject.getPermissions(SftpFileObject.java:317)
  at org.apache.commons.vfs2.provider.sftp.SftpFileObject.doIsWriteable(SftpFileObject.java:357)
  at org.apache.commons.vfs2.provider.AbstractFileObject.isWriteable(AbstractFileObject.java:1791)
  at org.apache.commons.vfs2.impl.DecoratedFileObject.isWriteable(DecoratedFileObject.java:229)
  at org.apache.commons.vfs2.cache.OnCallRefreshFileObject.isWriteable(OnCallRefreshFileObject.java:156)
  at org.apache.commons.vfs2.provider.AbstractFileObject.moveTo(AbstractFileObject.java:1857)
  at org.apache.commons.vfs2.impl.DecoratedFileObject.moveTo(DecoratedFileObject.java:241)
  at org.apache.commons.vfs2.cache.OnCallRefreshFileObject.moveTo(OnCallRefreshFileObject.java:184)
                
...
{code}

Technically the connection was alive because the session had a configured timeout and \
the jcraft code kept sending keepalive SSH_MSG_GLOBAL_REQUEST messages, but the \
thread performing FileObject.moveTo() did not return from moveTo(). I have changed \
SftpProviderTestCase to reproduce the problem: testRenameFile() hangs. The patch \
(patch_sftp_tests_hang_no_exec.diff) is attached.


I traced the problem to the fact that VFS invokes method \
com.jcraft.jsch.Channel.connect(). This method uses timeout value 0, in which case \
class com.jcraft.jsch.ChannelExec creates an instance of class \
com.jcraft.jsch.RequestExec that sends an SSH packet SSH_MSG_CHANNEL_REQUEST with \
"want reply" set to 0. Correspondingly, if the server supports SSH channelExec, it \
executes the specified command and returns some data. But if the server *does not* \
support SSH channelExec it sends nothing back while jcraft code tries to read \
something. This is the hang I am observing.


The fix would be to invoke com.jcraft.jsch.Channel.connect(int connectTimeout).
As a result jcraft sends an SSH packet SSH_MSG_CHANNEL_REQUEST with "want reply" set \
to 1 *and* it waits for an answer *and* it reacts to the answer.

Correspondingly, if the server supports SSH channelExec, it sends an SSH packet \
SSH_MSG_CHANNEL_SUCCESS and the executes the specified command and returns some data. \
If the server *does not* support SSH channelExec it sends an SSH packet \
SSH_MSG_CHANNEL_FAILURE. jcraft reacts on either of this messages because if waits \
for one of them. If it receives SSH_MSG_CHANNEL_SUCCESS it goes further and reads the \
response of the executed command. If it receives SSH_MSG_CHANNEL_FAILURE it \
immediately reports this by throwing JSchException with message "failed to send \
channel request". There is no hang whatsoever. Instead all tests from \
ProviderRenameTests fail with errors like

{code}
Could not determine if file "sftp://testtest@localhost:50036/write-tests" is \
writeable. {code}


The test suite actually hangs at the end, but this is caused by \
https://issues.apache.org/jira/browse/VFS-588


I have patched VFS classes to always open jcraft's channels with timeouts. In \
addition the patch always sets some default timeout value on jcraft's session if none \
was configured via SftpFileSystemConfigBuilder. Patch is also attached:  \
patch_sftp_timeouts.diff




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