Allow the FW_CFG_DATA_GENERATOR interface get_data() handler to
return NULL when there is nothing to generate. In that case
fw_cfg_add_file_from_generator() will not add any item and
return %true.

Reported-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20241213133352.10915-4-philmd@linaro.org>
---
 include/hw/nvram/fw_cfg.h | 13 ++++++++-----
 hw/nvram/fw_cfg.c         |  3 ++-
 2 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/include/hw/nvram/fw_cfg.h b/include/hw/nvram/fw_cfg.h
index fcb06f18cc3..6089681f421 100644
--- a/include/hw/nvram/fw_cfg.h
+++ b/include/hw/nvram/fw_cfg.h
@@ -30,8 +30,9 @@ struct FWCfgDataGeneratorClass {
      * @obj: the object implementing this interface
      * @errp: pointer to a NULL-initialized error object
      *
-     * Returns: reference to a byte array containing the data on success,
-     *          or NULL on error.
+     * Returns: A byte array containing data to add, or NULL without
+     *          @errp set if no data is required, or NULL with @errp
+     *          set on failure.
      *
      * The caller should release the reference when no longer
      * required.
@@ -298,14 +299,16 @@ void *fw_cfg_modify_file(FWCfgState *s, const char *filename, void *data,
  * @parent: the object in which to resolve the @part
  * @errp: pointer to a NULL initialized error object
  *
- * Add a new NAMED fw_cfg item with the content generated from the
- * @part object. The data generated by the @part object is copied
- * into the data structure of the fw_cfg device.
+ * If the @part object generates content, add a new NAMED fw_cfg item with it.
+ * The data generated by the @part object is copied into the data structure of
+ * the fw_cfg device.
  * The next available (unused) selector key starting at FW_CFG_FILE_FIRST
  * will be used; also, a new entry will be added to the file directory
  * structure residing at key value FW_CFG_FILE_DIR, containing the item name,
  * data size, and assigned selector key value.
  *
+ * If the @part object does not generate content, no fw_cfg item is added.
+ *
  * Returns: %true on success, %false on error.
  */
 bool fw_cfg_add_file_from_generator(FWCfgState *s,
diff --git a/hw/nvram/fw_cfg.c b/hw/nvram/fw_cfg.c
index b94cd27bd85..46c62c8f09e 100644
--- a/hw/nvram/fw_cfg.c
+++ b/hw/nvram/fw_cfg.c
@@ -1031,6 +1031,7 @@ bool fw_cfg_add_file_from_generator(FWCfgState *s,
                                     Object *parent, const char *part,
                                     const char *filename, Error **errp)
 {
+    ERRP_GUARD();
     FWCfgDataGeneratorClass *klass;
     GByteArray *array;
     Object *obj;
@@ -1048,7 +1049,7 @@ bool fw_cfg_add_file_from_generator(FWCfgState *s,
     }
     klass = FW_CFG_DATA_GENERATOR_GET_CLASS(obj);
     array = klass->get_data(obj, errp);
-    if (!array) {
+    if (*errp || !array) {
         return false;
     }
     size = array->len;
-- 
2.45.2