PHP ssh2_exec no feof on empty stdout - php

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.