blobfs: add result for set_cache_size

Since g_fs_cache_size only takes effect when creating
g_cache_pool, so spdk_fs_set_cache_size is only
permitted when cache pool is already freed or hasn't
been initialized.
Add a return value to indicate the result
of spdk_fs_set_cache_size.

Change-Id: I3828b136976d6f03f0751b2f20f68cd47c36ec04
Signed-off-by: Xiaodong Liu <xiaodong.liu@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/471869
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>

CHANGELOG.md

@@ -12,6 +12,10 @@ optimization for aarch64.
 `spdk_thread_send_msg` now returns int indicating if the message was successfully
 sent.
 
+### blobfs
+
+Added boolean return value for function spdk_fs_set_cache_size to indicate its operation result.
+
 ## v19.10:
 
 ### rpc

include/spdk/blobfs.h

@@ -364,8 +364,10 @@ int64_t spdk_file_read(struct spdk_file *file, struct spdk_fs_thread_ctx *ctx,
  * Set cache size for the blobstore filesystem.
  *
  * \param size_in_mb Cache size in megabytes.
+ *
+ * \return 0 on success, negative errno on failure.
  */
-void spdk_fs_set_cache_size(uint64_t size_in_mb);
+int spdk_fs_set_cache_size(uint64_t size_in_mb);
 
 /**
  * Obtain the cache size.

lib/blobfs/blobfs.c

@@ -1971,10 +1971,19 @@ spdk_fs_free_thread_ctx(struct spdk_fs_thread_ctx *ctx)
 	free(ctx);
 }
 
-void
+int
 spdk_fs_set_cache_size(uint64_t size_in_mb)
 {
+	/* setting g_fs_cache_size is only permitted if cache pool
+	 * is already freed or hasn't been initialized
+	 */
+	if (g_cache_pool != NULL) {
+		return -EPERM;
+	}
+
 	g_fs_cache_size = size_in_mb * 1024 * 1024;
+
+	return 0;
 }
 
 uint64_t