agent: list supported backends in libssh2_build_options()

Internally: make the agent backend guards unique for each backend, make
them contain the backend name. Use them in `agent.c`, move them to a
separate header and also reuse them from `version.c`.

Ref: https://github.com/libssh2/libssh2/pull/2285#discussion_r3572085858
Follow-up to 2673970d2a #2178

Closes #2312

docs/libssh2_build_options.md

@@ -29,10 +29,15 @@ enabled/disabled statuses.
 # RETURN VALUE
 
 A read-only, space-separated list of key:value pairs describing build options.
-Options use `on` or `off`, except `crypto` which may report these values:
+Options use `on` or `off`, except:
+
+`crypto`, which reports one of these values:
 `AWS-LC`, `BoringSSL`, `Libgcrypt`, `LibreSSL`, `mbedTLS`, `OpenSSL`,
 `OpenSSL/1.1.1`, `OS400QC3`, `WinCNG`, `wolfSSL`.
 
+`agent`, which is listed for each supported agent backend:
+`Pageant` (Windows-specific), `OpenSSH` (Windows-specific), `Unix`.
+
 # EXAMPLE
 
 ~~~c

src/Makefile.inc

@@ -36,6 +36,7 @@ CSOURCES =              \
   wincng.c
 
 HHEADERS =              \
+  agent.h               \
   chacha.h              \
   channel.h             \
   cipher-chachapoly.h   \

src/agent.c

@@ -36,18 +36,7 @@
 
 #include <stdlib.h>  /* for getenv() */
 
-#ifdef HAVE_SYS_UN_H
-/* Use the existence of sys/un.h as a test if Unix domain socket is
-   supported. Windows also supports it via winsock*.h, but not used
-   here at this time. */
-#include <sys/un.h>
-#define HAVE_AF_UNIX
-#endif
-
-#if defined(_WIN32) && !defined(LIBSSH2_WINDOWS_UWP)
-#define HAVE_WIN32_AGENTS
-#endif
-
+#include "agent.h"
 #include "userauth.h"
 #include "session.h"
 
@@ -141,14 +130,14 @@ struct _LIBSSH2_AGENT {
 
     char *identity_agent_path; /* Path to a custom identity agent socket */
 
-#ifdef HAVE_WIN32_AGENTS
+#ifdef SSH2_AGENT_BACKEND_WIN32_OPENSSH
     OVERLAPPED overlapped;
     HANDLE pipe;
     BOOL pending_io;
 #endif
 };
 
-#ifdef HAVE_WIN32_AGENTS
+#ifdef SSH2_AGENT_BACKEND_WIN32_PAGEANT
 /* Code to talk to Pageant was taken from PuTTY.
  *
  * Portions copyright Robert de Bath, Joris van Rantwijk, Delian
@@ -253,9 +242,9 @@ static struct agent_ops agent_ops_pageant = {
     agent_transact_pageant,
     agent_disconnect_pageant
 };
-#endif /* HAVE_WIN32_AGENTS */
+#endif /* SSH2_AGENT_BACKEND_WIN32_PAGEANT */
 
-#ifdef HAVE_WIN32_AGENTS
+#ifdef SSH2_AGENT_BACKEND_WIN32_OPENSSH
 
 /* Code to talk to OpenSSH was taken and modified from the Win32 port of
  * Portable OpenSSH by the PowerShell team. Commit
@@ -537,9 +526,9 @@ static struct agent_ops agent_ops_openssh = {
     agent_disconnect_openssh
 };
 
-#endif /* HAVE_WIN32_AGENTS */
+#endif /* SSH2_AGENT_BACKEND_WIN32_OPENSSH */
 
-#ifdef HAVE_AF_UNIX
+#ifdef SSH2_AGENT_BACKEND_UNIX
 static int agent_connect_unix(LIBSSH2_AGENT *agent)
 {
     const char *path;
@@ -702,18 +691,20 @@ static struct agent_ops agent_ops_unix = {
     agent_transact_unix,
     agent_disconnect_unix
 };
-#endif /* HAVE_AF_UNIX */
+#endif /* SSH2_AGENT_BACKEND_UNIX */
 
 static struct {
     const char *name;
     struct agent_ops *ops;
 } agent_supported_backends[] = {
-#ifdef HAVE_WIN32_AGENTS
-    { "Pageant", &agent_ops_pageant },
-    { "OpenSSH", &agent_ops_openssh },
+#ifdef SSH2_AGENT_BACKEND_WIN32_PAGEANT
+    { SSH2_AGENT_BACKEND_WIN32_PAGEANT, &agent_ops_pageant },
 #endif
-#ifdef HAVE_AF_UNIX
-    { "Unix", &agent_ops_unix },
+#ifdef SSH2_AGENT_BACKEND_WIN32_OPENSSH
+    { SSH2_AGENT_BACKEND_WIN32_OPENSSH, &agent_ops_openssh },
+#endif
+#ifdef SSH2_AGENT_BACKEND_UNIX
+    { SSH2_AGENT_BACKEND_UNIX, &agent_ops_unix },
 #endif
     { NULL, NULL }
 };
@@ -1075,7 +1066,7 @@ LIBSSH2_AGENT *libssh2_agent_init(LIBSSH2_SESSION *session)
     agent->identity_agent_path = NULL;
     ssh2_list_init(&agent->head);
 
-#ifdef HAVE_WIN32_AGENTS
+#ifdef SSH2_AGENT_BACKEND_WIN32_OPENSSH
     agent->pipe = INVALID_HANDLE_VALUE;
     memset(&agent->overlapped, 0, sizeof(OVERLAPPED));
     agent->pending_io = FALSE;

src/agent.h

@@ -0,0 +1,24 @@
+#ifndef LIBSSH2_AGENT_H
+#define LIBSSH2_AGENT_H
+/* Copyright (C) Daiki Ueno
+ * Copyright (C) Daniel Stenberg
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include "libssh2_priv.h"
+
+#if defined(_WIN32) && !defined(LIBSSH2_WINDOWS_UWP)
+#define SSH2_AGENT_BACKEND_WIN32_PAGEANT "Pageant"
+#define SSH2_AGENT_BACKEND_WIN32_OPENSSH "OpenSSH"
+#endif
+
+#ifdef HAVE_SYS_UN_H
+/* Use the existence of sys/un.h as a test if Unix domain socket (AF_UNIX)
+   is supported. Windows also supports it via winsock*.h, but not used here
+   at this time. */
+#include <sys/un.h>
+#define SSH2_AGENT_BACKEND_UNIX "Unix"
+#endif
+
+#endif /* LIBSSH2_AGENT_H */

src/version.c

@@ -32,6 +32,8 @@
 
 #include "libssh2_priv.h"
 
+#include "agent.h"
+
 const char *libssh2_version(int req_version_num)
 {
     if(req_version_num <= LIBSSH2_VERSION_NUM)
@@ -45,8 +47,7 @@ libssh2_crypto_engine_t libssh2_crypto_engine(void)
 }
 
 static const char *ssh2_build_options =
-    "crypto:"
-    SSH2_CRYPTO_ENGINE_NAME
+    "crypto:" SSH2_CRYPTO_ENGINE_NAME
     " "
     "MD5:"
 #if LIBSSH2_MD5
@@ -167,6 +168,18 @@ static const char *ssh2_build_options =
     "off"
 #endif
     " "
+#ifdef SSH2_AGENT_BACKEND_WIN32_PAGEANT
+    "agent:" SSH2_AGENT_BACKEND_WIN32_PAGEANT
+    " "
+#endif
+#ifdef SSH2_AGENT_BACKEND_WIN32_OPENSSH
+    "agent:" SSH2_AGENT_BACKEND_WIN32_OPENSSH
+    " "
+#endif
+#ifdef SSH2_AGENT_BACKEND_UNIX
+    "agent:" SSH2_AGENT_BACKEND_UNIX
+    " "
+#endif
     "clear-memory:"
 #ifndef LIBSSH2_NO_CLEAR_MEMORY
     "on"