ike-sa-manager: Allow plugins to provide IKE SPIs via a callback

Plugins must depend on `libcharon-sa-managers` to ensure the manager
exists.

src/libcharon/sa/ike_sa_manager.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2008 Tobias Brunner
+ * Copyright (C) 2008-2015 Tobias Brunner
  * Copyright (C) 2005-2008 Martin Willi
  * Copyright (C) 2005 Jan Hutter
  * Hochschule fuer Technik Rapperswil
@@ -30,6 +30,16 @@ typedef struct ike_sa_manager_t ike_sa_manager_t;
 #include <encoding/message.h>
 #include <config/peer_cfg.h>
 
+/**
+ * Callback called to generate an IKE SPI.
+ *
+ * This may be called from multiple threads concurrently.
+ *
+ * @param data		data supplied during registration of the callback
+ * @return			allocated SPI, 0 on failure
+ */
+typedef u_int64_t (*spi_cb_t)(void *data);
+
 /**
  * Manages and synchronizes access to all IKE_SAs.
  *
@@ -226,6 +236,15 @@ struct ike_sa_manager_t {
 	u_int (*get_half_open_count)(ike_sa_manager_t *this, host_t *ip,
 								 bool responder_only);
 
+	/**
+	 * Set the callback to generate IKE SPIs
+	 *
+	 * @param callback		callback to register
+	 * @param data			data provided to callback
+	 */
+	void (*set_spi_cb)(ike_sa_manager_t *this, spi_cb_t callback,
+					   void *data);
+
 	/**
 	 * Delete all existing IKE_SAs and destroy them immediately.
 	 *