lib/nvmf: add helper functions for future RPCs.

Functions added in this patch:
spdk_nvmf_tgt_get_name - get human readable name from target.
spdk_nvmf_get_first_tgt - start iterating over global list of targets
spdk_nvmf_get_next_tgt - get next target in iteration

These functions will facilitate the following RPC

nvmf_get_targets - get the names of all active NVMe-oF targets.

In this series, I will also add two more RPCs, nvmf_create_target, and
nvmf_destroy_target, as wrappers around the create and destroy
functions. Since all of these changes are pretty minor and closely
related, I will just do one big changelog entry at the end.

Change-Id: Ia9f1248fbf9726fa3889998a169211fb25e724f2
Signed-off-by: Seth Howell <seth.howell@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/468386
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Alexey Marchuk <alexeymar@mellanox.com>
Reviewed-by: Paul Luse <paul.e.luse@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>

include/spdk/nvmf.h

@@ -136,6 +136,15 @@ void spdk_nvmf_tgt_destroy(struct spdk_nvmf_tgt *tgt,
 			   spdk_nvmf_tgt_destroy_done_fn cb_fn,
 			   void *cb_arg);
 
+/**
+ * Get the name of an NVMe-oF target.
+ *
+ * \param tgt The target from which to get the name.
+ *
+ * \return The name of the target as a null terminated string.
+ */
+const char *spdk_nvmf_tgt_get_name(struct spdk_nvmf_tgt *tgt);
+
 /**
  * Get a pointer to an NVMe-oF target.
  *
@@ -150,6 +159,26 @@ void spdk_nvmf_tgt_destroy(struct spdk_nvmf_tgt *tgt,
  */
 struct spdk_nvmf_tgt *spdk_nvmf_get_tgt(const char *name);
 
+/**
+ * Get the pointer to the first NVMe-oF target.
+ *
+ * Combined with spdk_nvmf_get_next_tgt to iterate over all available targets.
+ *
+ * \return The first NVMe-oF target.
+ */
+struct spdk_nvmf_tgt *spdk_nvmf_get_first_tgt(void);
+
+/**
+ * Get the pointer to the first NVMe-oF target.
+ *
+ * Combined with spdk_nvmf_get_first_tgt to iterate over all available targets.
+ *
+ * \param prev A pointer to the last NVMe-oF target.
+ *
+ * \return The first NVMe-oF target.
+ */
+struct spdk_nvmf_tgt *spdk_nvmf_get_next_tgt(struct spdk_nvmf_tgt *prev);
+
 /**
  * Write NVMe-oF target configuration into provided JSON context.
  * \param w JSON write context

lib/nvmf/nvmf.c

@@ -314,6 +314,12 @@ spdk_nvmf_tgt_destroy(struct spdk_nvmf_tgt *tgt,
 	spdk_io_device_unregister(tgt, spdk_nvmf_tgt_destroy_cb);
 }
 
+const char *
+spdk_nvmf_tgt_get_name(struct spdk_nvmf_tgt *tgt)
+{
+	return tgt->name;
+}
+
 struct spdk_nvmf_tgt *
 spdk_nvmf_get_tgt(const char *name)
 {
@@ -342,6 +348,18 @@ spdk_nvmf_get_tgt(const char *name)
 	return NULL;
 }
 
+struct spdk_nvmf_tgt *
+spdk_nvmf_get_first_tgt(void)
+{
+	return TAILQ_FIRST(&g_nvmf_tgts);
+}
+
+struct spdk_nvmf_tgt *
+spdk_nvmf_get_next_tgt(struct spdk_nvmf_tgt *prev)
+{
+	return TAILQ_NEXT(prev, link);
+}
+
 static void
 spdk_nvmf_write_subsystem_config_json(struct spdk_json_write_ctx *w,
 				      struct spdk_nvmf_subsystem *subsystem)