using ssh2_exec, confused by streams

sneakyimp

I'm using [man]ssh2_exec[/man] so that a command server can control a slave (don't worry this is not a botnet). I've got good results when my commands are successful, but I'm wondering how to detect error conditions if I happen to call a bad command. If I specify a non-command ("blargo" in this case) then this script hangs while it's trying to read the main stream output:

	echo "preparing to connect\n";
	$connection = ssh2_connect("11.22.33.44", 22);
	if ($connection === FALSE) {
		throw new Exception("connection failed and returned FALSE.");
	}
	if (!is_resource($connection)) {
		throw new Exception("connection is not a resource");
	}

echo "connected, preparing to authenticate\n";

// this returns true on success, false on failure
if (!ssh2_auth_password($connection, "user", "password")) {
	throw new Exception("Authentication failed");
}

echo "authentication successful, preparing to exec command:\nblargo\n"; // not a valid command

$stream = ssh2_exec($connection, "blargo");
if ($stream === FALSE) {
	throw new Exception("ssh2_exec failed, returning FALSE");
}


echo "command complete, reading main stream\n";
// read up to 1MB from the stream
$output = stream_get_contents($stream, 1000000); // code execution blocks here waiting for the input that will never come
echo "stream read\n";
if ($output === FALSE) {
	fclose($stream);
	throw new Exception("Failed to read stream contents");
}

echo "=== OUTPUT ===\n" . $output . "\n======\n";

fclose($stream);
echo "stream closed\n";

the output looks something like this:

preparing to connect
connected, preparing to authenticate
authentication successful, preparing to exec command:
blargo
command complete, reading main stream
(execution hangs here...waiting for input that never comes)

How can I avoid indefinite blocking waiting for input that will never return? How can I detect ssh2_exec commands that fail because they are not valid commands?

dalecosp

What about using ssh_fetch_stream() and specifying which File Descriptor you're wanting to read?

sneakyimp

I've tried that and the problem I have is that this line blocks when there is an error:

$output = stream_get_contents($stream, 1000000);

the script hangs on that line and I never get around to reading the error stream contents. Here is my current script:

<?php
/**
 * Script for testing ssh login
 */

if (!extension_loaded("ssh2")) {
	die("SSH2 extension not found");
}

require_once "db_bootstrap.php";

require_once EREP_JOOMLA_PATH . "/erp_php/includes/classes/EREPImageDaemonServer.php";

define("SSH_COMMAND_BULL****", "blargo"); // not a command at all, should result in error
define("SSH_COMMAND_LS", "ls -al"); // a valid command

try {
	$ims = MyServer::fetch_by_id($GLOBALS["jdbo"], 20); // fetches an object representing a cloud server

echo "preparing to connect\n";
$connection = ssh2_connect($ims->public_ip, 22);
if ($connection === FALSE) {
	throw new Exception("connection failed and returned FALSE.");
}

if (!is_resource($connection)) {
	echo "NOT A RESOURCE\n";
	var_dump($connection);
	throw new Exception("connection is not a resource");
}

echo "connected, preparing to authenticate\n";

// this returns true on success, false on failure
if (!ssh2_auth_password($connection, "root", $ims->admin_password)) {
	throw new Exception("Authentication failed");
}

echo "authentication successful, preparing to exec command:\n" . SSH_COMMAND_BULL**** . "\n";

$stream = ssh2_exec($connection, SSH_COMMAND_BULL****);
$error_stream = ssh2_fetch_stream($stream, SSH2_STREAM_STDERR);

if ($stream === FALSE) {
	throw new Exception("ssh2_exec failed, returning FALSE");
}


echo "command complete, reading main stream\n";
// read up to 1MB from the stream
$output = stream_get_contents($stream, 1000000); // this command blocks when there is an error
echo "stream read\n";
fclose($stream);
echo "stream closed\n";
if ($output === FALSE) {
	throw new Exception("Failed to read stream contents");
}
echo "=== OUTPUT ===\n" . $output . "\n======\n";

echo "reading error stream\n";
$error_output = stream_get_contents($error_stream, 1000000);
echo "error stream read\n";
fclose($error_stream);
echo "error_stream_closed\n";
if ($error_output === FALSE) {
	throw new Exception("Failed to read error stream contents");
}
echo "=== ERROR OUTPUT ===\n" . $error_output . "\n======\n";

echo "script complete\n";
} catch (Exception $e) {
	die("An exception was caught:" . $e->getMessage() . "\n\n");
}

The output:

preparing to connect
connected, preparing to authenticate
authentication successful, preparing to exec command:
blargo
command complete, reading main stream
(script hangs here, refusing to go forward)

Interestingly, if I execute a valid command, the script will complete without any problems. The problem is that I can't seem to find any data value that will tell me if there has been an error or not. the ssh2_exec command does not return FALSE when the command is bad. It still returns a valid resource so there's nothing to indicate there was a problem.

sneakyimp

I believe this post on the php documentation comments describes the problem I'm having.

sneakyimp

The problem is basically that there does not seem to be any value I can check or function that I can call to tell me whether my command was successful before I must choose to either read the primary stream with the command output or the error stream containing STDERR messages. If I choose the wrong one, the stream_get_contents function blocks, halting my script. I've tried stream_set_blocking and stream_set_timeout to prevent this blocking and this doesn't seem to work. If I read the wrong stream first, my script gets stuck.

dalecosp

Ok, would this hackish approach work?

In your command, redirect STDOUT and STDERR to a file, say in /tmp (or a memory-based device like /dev/shm would be even better), and then issue a remote call to cat that file back to you in a subsequent request?