agent.c: Add support for Windows OpenSSH agent (#517)

Files: agent.c, agent.h, agent_win.c

Notes:
* agent: Add support for Windows OpenSSH agent

The implementation was partially taken and modified from that found in
the Portable OpenSSH port to Win32 by the PowerShell team, but mostly
based on the existing Unix OpenSSH agent support.

https://github.com/PowerShell/openssh-portable

Regarding the partial transfer support implementation: partial transfers
are easy to deal with, but you need to track additional state when
non-blocking IO enters the picture. A tracker of how many bytes have
been transfered has been placed in the transfer context struct as that's
where it makes most sense. This tracker isn't placed behind a WIN32
 #ifdef as it will probably be useful for other agent implementations.

* agent: win32 openssh: Disable overlapped IO

Non-blocking IO is not currently supported by the surrounding agent
code, despite a lot of the code having everything set up to handle it.

Credit:
Co-authored-by: Gabriel Smith <gabriel.smith@precisionot.com>

src/agent.c

@@ -38,6 +38,7 @@
  */
 
 #include "libssh2_priv.h"
+#include "agent.h"
 #include "misc.h"
 #include <errno.h>
 #ifdef HAVE_SYS_UN_H
@@ -50,6 +51,9 @@
 #endif
 #include "userauth.h"
 #include "session.h"
+#ifdef WIN32
+#include <stdlib.h>
+#endif
 
 /* Requests from client to agent for protocol 1 key operations */
 #define SSH_AGENTC_REQUEST_RSA_IDENTITIES 1
@@ -90,58 +94,6 @@
 #define SSH_AGENT_CONSTRAIN_LIFETIME 1
 #define SSH_AGENT_CONSTRAIN_CONFIRM 2
 
-/* non-blocking mode on agent connection is not yet implemented, but
-   for future use. */
-typedef enum {
-    agent_NB_state_init = 0,
-    agent_NB_state_request_created,
-    agent_NB_state_request_length_sent,
-    agent_NB_state_request_sent,
-    agent_NB_state_response_length_received,
-    agent_NB_state_response_received
-} agent_nonblocking_states;
-
-typedef struct agent_transaction_ctx {
-    unsigned char *request;
-    size_t request_len;
-    unsigned char *response;
-    size_t response_len;
-    agent_nonblocking_states state;
-} *agent_transaction_ctx_t;
-
-typedef int (*agent_connect_func)(LIBSSH2_AGENT *agent);
-typedef int (*agent_transact_func)(LIBSSH2_AGENT *agent,
-                                   agent_transaction_ctx_t transctx);
-typedef int (*agent_disconnect_func)(LIBSSH2_AGENT *agent);
-
-struct agent_publickey {
-    struct list_node node;
-
-    /* this is the struct we expose externally */
-    struct libssh2_agent_publickey external;
-};
-
-struct agent_ops {
-    agent_connect_func connect;
-    agent_transact_func transact;
-    agent_disconnect_func disconnect;
-};
-
-struct _LIBSSH2_AGENT
-{
-    LIBSSH2_SESSION *session;  /* the session this "belongs to" */
-
-    libssh2_socket_t fd;
-
-    struct agent_ops *ops;
-
-    struct agent_transaction_ctx transctx;
-    struct agent_publickey *identity;
-    struct list_head head;              /* list of public keys */
-
-    char *identity_agent_path; /* Path to a custom identity agent socket */
-};
-
 #ifdef PF_UNIX
 static int
 agent_connect_unix(LIBSSH2_AGENT *agent)
@@ -404,6 +356,7 @@ static struct {
 } supported_backends[] = {
 #ifdef WIN32
     {"Pageant", &agent_ops_pageant},
+    {"OpenSSH", &agent_ops_openssh},
 #endif  /* WIN32 */
 #ifdef PF_UNIX
     {"Unix", &agent_ops_unix},
@@ -441,6 +394,7 @@ agent_sign(LIBSSH2_SESSION *session, unsigned char **sig, size_t *sig_len,
         _libssh2_store_u32(&s, 0);
 
         transctx->request_len = s - transctx->request;
+        transctx->send_recv_total = 0;
         transctx->state = agent_NB_state_request_created;
     }
 
@@ -541,6 +495,7 @@ agent_list_identities(LIBSSH2_AGENT *agent)
     if(transctx->state == agent_NB_state_init) {
         transctx->request = &c;
         transctx->request_len = 1;
+        transctx->send_recv_total = 0;
         transctx->state = agent_NB_state_request_created;
     }
 
@@ -717,6 +672,12 @@ libssh2_agent_init(LIBSSH2_SESSION *session)
     agent->identity_agent_path = NULL;
     _libssh2_list_init(&agent->head);
 
+#ifdef WIN32
+    agent->pipe = INVALID_HANDLE_VALUE;
+    memset(&agent->overlapped, 0, sizeof(OVERLAPPED));
+    agent->pending_io = FALSE;
+#endif
+
     return agent;
 }