hw/nvram/fw_cfg: Let fw_cfg_add_from_generator() return boolean value

Commits b6d7e9b66f..a43770df5d simplified the error propagation.
Similarly to commit 6fd5bef10b "qom: Make functions taking Error**
return bool, not void", let fw_cfg_add_from_generator() return a
boolean value, not void.
This allow to simplify parse_fw_cfg() and fixes the error handling
issue reported by Coverity (CID 1430396):

  In parse_fw_cfg():

    Variable assigned once to a constant guards dead code.

    Local variable local_err is assigned only once, to a constant
    value, making it effectively constant throughout its scope.
    If this is not the intent, examine the logic to see if there
    is a missing assignment that would make local_err not remain
    constant.

It's the call of fw_cfg_add_from_generator():

        Error *local_err = NULL;

        fw_cfg_add_from_generator(fw_cfg, name, gen_id, errp);
        if (local_err) {
            error_propagate(errp, local_err);
            return -1;
        }
        return 0;

If it fails, parse_fw_cfg() sets an error and returns 0, which is
wrong. Harmless, because the only caller passes &error_fatal.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Fixes: Coverity CID 1430396: 'Constant' variable guards dead code (DEADCODE)
Fixes: 6552d87c48 ("softmmu/vl: Let -fw_cfg option take a 'gen_id' argument")
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200721131911.27380-3-philmd@redhat.com>

hw/nvram/fw_cfg.c

@@ -1032,7 +1032,7 @@ void *fw_cfg_modify_file(FWCfgState *s, const char *filename,
     return NULL;
 }
 
-void fw_cfg_add_from_generator(FWCfgState *s, const char *filename,
+bool fw_cfg_add_from_generator(FWCfgState *s, const char *filename,
                                const char *gen_id, Error **errp)
 {
     FWCfgDataGeneratorClass *klass;
@@ -1043,20 +1043,22 @@ void fw_cfg_add_from_generator(FWCfgState *s, const char *filename,
     obj = object_resolve_path_component(object_get_objects_root(), gen_id);
     if (!obj) {
         error_setg(errp, "Cannot find object ID '%s'", gen_id);
-        return;
+        return false;
     }
     if (!object_dynamic_cast(obj, TYPE_FW_CFG_DATA_GENERATOR_INTERFACE)) {
         error_setg(errp, "Object ID '%s' is not a '%s' subclass",
                    gen_id, TYPE_FW_CFG_DATA_GENERATOR_INTERFACE);
-        return;
+        return false;
     }
     klass = FW_CFG_DATA_GENERATOR_GET_CLASS(obj);
     array = klass->get_data(obj, errp);
     if (!array) {
-        return;
+        return false;
     }
     size = array->len;
     fw_cfg_add_file(s, filename, g_byte_array_free(array, TRUE), size);
+
+    return true;
 }
 
 static void fw_cfg_machine_reset(void *opaque)

include/hw/nvram/fw_cfg.h

@@ -304,8 +304,10 @@ void *fw_cfg_modify_file(FWCfgState *s, const char *filename, void *data,
  * 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.
+ *
+ * Returns: %true on success, %false on error.
  */
-void fw_cfg_add_from_generator(FWCfgState *s, const char *filename,
+bool fw_cfg_add_from_generator(FWCfgState *s, const char *filename,
                                const char *gen_id, Error **errp);
 
 FWCfgState *fw_cfg_init_io_dma(uint32_t iobase, uint32_t dma_iobase,

softmmu/vl.c

@@ -2070,11 +2070,7 @@ static int parse_fw_cfg(void *opaque, QemuOpts *opts, Error **errp)
         size = strlen(str); /* NUL terminator NOT included in fw_cfg blob */
         buf = g_memdup(str, size);
     } else if (nonempty_str(gen_id)) {
-        Error *local_err = NULL;
-
-        fw_cfg_add_from_generator(fw_cfg, name, gen_id, errp);
-        if (local_err) {
-            error_propagate(errp, local_err);
+        if (!fw_cfg_add_from_generator(fw_cfg, name, gen_id, errp)) {
             return -1;
         }
         return 0;