Agent forwarding login stuck

ver: `2.7-prerelease`
system: `Windows 10`
Linux Server ssh version: `OpenSSH_7.4p1, OpenSSL 1.0.2k-fips 26 Jan 2017`

SecureCRT does not rely on the local `ssh-agent` service, Just check `Add key to agent` and `Enable OpenSSH agent forwarding`.
WindTerm operation is more complicated. You need to start the `ssh-agent` service and then manually execute `ssh-add private_key` to add the key.
The server login is always stuck. I have to use ctrl+c to terminate it. Then the terminal symbol becomes `-bash-4.2$`.

linux server log:
```
Jul 17 20:57:07 ****** sshd[941]: debug1: Forked child 9867.
Jul 17 20:57:07 ****** sshd[9867]: debug1: Set /proc/self/oom_score_adj to 0
Jul 17 20:57:07 ****** sshd[9867]: debug1: rexec start in 5 out 5 newsock 5 pipe 7 sock 8
Jul 17 20:57:07 ****** sshd[9867]: debug1: inetd sockets after dupping: 3, 3
Jul 17 20:57:07 ****** sshd[9867]: Connection from ***local IP*** port ***local port*** on ***remote IP*** port ***remote port***
Jul 17 20:57:07 ****** sshd[9867]: debug1: Client protocol version 2.0; client software version libssh_0.9.6
Jul 17 20:57:07 ****** sshd[9867]: debug1: no match: libssh_0.9.6
Jul 17 20:57:07 ****** sshd[9867]: debug1: Local version string SSH-2.0-OpenSSH_7.4
Jul 17 20:57:07 ****** sshd[9867]: debug1: Enabling compatibility mode for protocol 2.0
Jul 17 20:57:07 ****** sshd[9867]: debug1: SELinux support disabled [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: permanently_set_uid: 74/74 [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: list_hostkey_types: ssh-rsa,rsa-sha2-512,rsa-sha2-256,ecdsa-sha2-nistp256,ssh-ed25519 [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: SSH2_MSG_KEXINIT sent [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: SSH2_MSG_KEXINIT received [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: kex: algorithm: curve25519-sha256 [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: kex: host key algorithm: ssh-ed25519 [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: kex: client->server cipher: aes256-gcm@openssh.com MAC: compression: none [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: kex: server->client cipher: aes256-gcm@openssh.com MAC: compression: none [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: kex: curve25519-sha256 need=32 dh_need=32 [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: kex: curve25519-sha256 need=32 dh_need=32 [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: expecting SSH2_MSG_KEX_ECDH_INIT [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: rekey after 4294967296 blocks [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: SSH2_MSG_NEWKEYS sent [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: expecting SSH2_MSG_NEWKEYS [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: SSH2_MSG_NEWKEYS received [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: rekey after 4294967296 blocks [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: KEX done [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: userauth-request for user admin service ssh-connection method none [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: attempt 0 failures 0 [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: PAM: initializing for "admin"
Jul 17 20:57:07 ****** sshd[9867]: debug1: PAM: setting PAM_RHOST to "***local IP***"
Jul 17 20:57:07 ****** sshd[9867]: debug1: PAM: setting PAM_TTY to "ssh"
Jul 17 20:57:07 ****** sshd[9867]: debug1: userauth-request for user admin service ssh-connection method publickey [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: attempt 1 failures 0 [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: userauth_pubkey: test whether pkalg/pkblob are acceptable for RSA SHA256:****** [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: temporarily_use_uid: 1001/1001 (e=0/0)
Jul 17 20:57:07 ****** sshd[9867]: debug1: trying public key file /home/admin/.ssh/authorized_keys
Jul 17 20:57:07 ****** sshd[9867]: debug1: fd 4 clearing O_NONBLOCK
Jul 17 20:57:07 ****** sshd[9867]: debug1: matching key found: file /home/admin/.ssh/authorized_keys, line 13 RSA SHA256:******
Jul 17 20:57:07 ****** sshd[9867]: debug1: restore_uid: 0/0
Jul 17 20:57:07 ****** sshd[9867]: Postponed publickey for admin from ***local IP*** port ***local port*** ssh2 [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: userauth-request for user admin service ssh-connection method publickey [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: attempt 2 failures 0 [preauth]
Jul 17 20:57:07 ****** sshd[9867]: debug1: temporarily_use_uid: 1001/1001 (e=0/0)
Jul 17 20:57:07 ****** sshd[9867]: debug1: trying public key file /home/admin/.ssh/authorized_keys
Jul 17 20:57:07 ****** sshd[9867]: debug1: fd 4 clearing O_NONBLOCK
Jul 17 20:57:07 ****** sshd[9867]: debug1: matching key found: file /home/admin/.ssh/authorized_keys, line 13 RSA SHA256:******
Jul 17 20:57:07 ****** sshd[9867]: debug1: restore_uid: 0/0
Jul 17 20:57:07 ****** sshd[9867]: debug1: do_pam_account: called
Jul 17 20:57:07 ****** sshd[9867]: Accepted publickey for admin from ***local IP*** port ***local port*** ssh2: RSA SHA256:******
Jul 17 20:57:07 ****** sshd[9867]: debug1: monitor_child_preauth: admin has been authenticated by privileged process
Jul 17 20:57:07 ****** sshd[9867]: debug1: monitor_read_log: child log fd closed
Jul 17 20:57:07 ****** sshd[9867]: debug1: SELinux support disabled
Jul 17 20:57:07 ****** sshd[9867]: debug1: PAM: establishing credentials
Jul 17 20:57:07 ****** sshd[9867]: pam_unix(sshd:session): session opened for user admin by (uid=0)
Jul 17 20:57:07 ****** sshd[9867]: User child is on pid 10017
Jul 17 20:57:07 ****** sshd[10017]: debug1: PAM: establishing credentials
Jul 17 20:57:07 ****** sshd[10017]: debug1: permanently_set_uid: 1001/1001
Jul 17 20:57:07 ****** sshd[10017]: debug1: rekey after 4294967296 blocks
Jul 17 20:57:07 ****** sshd[10017]: debug1: rekey after 4294967296 blocks
Jul 17 20:57:07 ****** sshd[10017]: debug1: ssh_packet_set_postauth: called
Jul 17 20:57:07 ****** sshd[10017]: debug1: Entering interactive session for SSH2.
Jul 17 20:57:07 ****** sshd[10017]: debug1: server_init_dispatch
Jul 17 20:57:07 ****** sshd[10017]: debug1: server_input_channel_open: ctype session rchan 43 win 64000 max 261120
Jul 17 20:57:07 ****** sshd[10017]: debug1: input_session_request
Jul 17 20:57:07 ****** sshd[10017]: debug1: channel 0: new [server-session]
Jul 17 20:57:07 ****** sshd[10017]: debug1: session_new: session 0
Jul 17 20:57:07 ****** sshd[10017]: debug1: session_open: channel 0
Jul 17 20:57:07 ****** sshd[10017]: debug1: session_open: session 0: link with channel 0
Jul 17 20:57:07 ****** sshd[10017]: debug1: server_input_channel_open: confirm session
Jul 17 20:57:07 ****** sshd[10017]: debug1: server_input_channel_req: channel 0 request auth-agent-req@openssh.com reply 0
Jul 17 20:57:07 ****** sshd[10017]: debug1: session_by_channel: session 0 channel 0
Jul 17 20:57:07 ****** sshd[10017]: debug1: session_input_channel_req: session 0 req auth-agent-req@openssh.com
Jul 17 20:57:07 ****** sshd[10017]: debug1: temporarily_use_uid: 1001/1001 (e=1001/1001)
Jul 17 20:57:07 ****** sshd[10017]: debug1: restore_uid: (unprivileged)
Jul 17 20:57:07 ****** sshd[10017]: debug1: channel 1: new [auth socket]
Jul 17 20:57:07 ****** sshd[10017]: debug1: server_input_channel_req: channel 0 request pty-req reply 1
Jul 17 20:57:07 ****** sshd[10017]: debug1: session_by_channel: session 0 channel 0
Jul 17 20:57:07 ****** sshd[10017]: debug1: session_input_channel_req: session 0 req pty-req
Jul 17 20:57:07 ****** sshd[10017]: debug1: Allocating pty.
Jul 17 20:57:07 ****** sshd[9867]: debug1: session_new: session 0
Jul 17 20:57:07 ****** sshd[9867]: debug1: SELinux support disabled
Jul 17 20:57:07 ****** sshd[10017]: debug1: session_pty_req: session 0 alloc /dev/pts/4
Jul 17 20:57:08 ****** sshd[10017]: debug1: server_input_channel_req: channel 0 request shell reply 1
Jul 17 20:57:08 ****** sshd[10017]: debug1: session_by_channel: session 0 channel 0
Jul 17 20:57:08 ****** sshd[10017]: debug1: session_input_channel_req: session 0 req shell
Jul 17 20:57:08 ****** sshd[10017]: Starting session: shell on pts/4 for admin from ***local IP*** port ***local port*** id 0
Jul 17 20:57:08 ****** sshd[10175]: debug1: Setting controlling tty using TIOCSCTTY.
Jul 17 20:57:08 ****** sshd[10017]: debug1: server_input_channel_req: channel 0 request window-change reply 0
Jul 17 20:57:08 ****** sshd[10017]: debug1: session_by_channel: session 0 channel 0
Jul 17 20:57:08 ****** sshd[10017]: debug1: session_input_channel_req: session 0 req window-change
Jul 17 20:57:08 ****** sshd[10017]: debug1: server_input_channel_req: channel 0 request window-change reply 0
Jul 17 20:57:08 ****** sshd[10017]: debug1: session_by_channel: session 0 channel 0
Jul 17 20:57:08 ****** sshd[10017]: debug1: session_input_channel_req: session 0 req window-change
Jul 17 20:57:09 ****** sshd[10017]: debug1: channel 2: new [accepted auth socket]
```

Start by reproducing the stuck login on Windows 10 with agent forwarding enabled, using the reported OpenSSH server and libssh_0.9.6 details. Compare the client behavior with the server log around auth-agent-req@openssh.com and the accepted auth socket. Done means the forwarded-agent session reaches a usable shell without requiring Ctrl+C.

Written by the indexing model from the issue text.