PHP ssh2_exec no feof on empty stdout

Question

This PHP snippet should execute a simple command via SSH (stripped down to minimal working example):

$sshconn = ssh2_connect($HostAddr, 22);
ssh2_auth_pubkey_file($sshconn, $user, $sshkey . '.pub', $sshkey);
$stdout = ssh2_exec($sshconn, 'echo hello');
if ($stdout !== false)
{
    stream_set_blocking($stdout, true);
    while (!feof($stdout))
    {
        $proc_stdout = fgets($stdout, 3E6);
        if ($proc_stdout !== false) echo $proc_stdout;
    }
    fclose($stdout);
}

Works great as long as there is any output to stdout. However, if stdout remains empty, the loop turns into an endless loop.

$stdout = ssh2_exec($sshconn, 'echo hello >&2');

How do I read the stdout properly if

• stdout may be empty
• but stdout may also very large (several Gigabytes, impossible to slurp into a variable by a single call to stream_get_contents or the like).

---

Addendum: My real world code that hangs executes mysqldump with an unknown parameter. Obviously, echo -n >&2 works as expected, although its stdout is also empty.

Answer 1

If anyone stumbles upon this too: stderr has to be read too.

$sshconn = ssh2_connect($HostAddr, 22);
ssh2_auth_pubkey_file($sshconn, $user, $sshkey . '.pub', $sshkey);

$stdout = ssh2_exec($sshconn, 'echo Hello >&2');
if ($stdout !== false)
{
    $stderr = ssh2_fetch_stream($stdout, SSH2_STREAM_STDERR);
    stream_set_blocking($stdout, false);
    stream_set_blocking($stderr, false);
    while (!feof($stdout))
    {
            $proc_stdout = fgets($stdout, 3E6);
            if ($proc_stdout !== false) echo $proc_stdout;
            $proc_stderr = fgets($stderr, 3E6);
            if ($proc_stderr !== false) fwrite(STDERR, $stderr);
    }
    fclose($stdout); fclose($stderr);
}

The disadvantage of this is that the SSH connection is no longer usable afterwards (a further ssh2_exec cannot be executed).

A possible solution is to keep both streams blocking and read stdout and stderr non-interleaved:

$stdout = ssh2_exec($sshconn, 'echo Hello >&2');
$stderr = ssh2_fetch_stream($stdout, SSH2_STREAM_STDERR);
stream_set_blocking($stdout, true);
stream_set_blocking($stderr, true);
while ($content = fread($stdout, 3E6)) echo $content;
while ($content = fread($stderr, 3E6)) fwrite(STDERR, $content);

I invite anyone who knows how to have the streams interleaved and keep the SSH connection usable to post another answer.