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Re: [PATCH 09/14] block/qcow2: qcow2_get_specific_info(): drop error pro

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [PATCH 09/14] block/qcow2: qcow2_get_specific_info(): drop error propagation
Date: Thu, 17 Sep 2020 21:52:27 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.2.2

17.09.2020 19:32, Alberto Garcia wrote:
On Wed 09 Sep 2020 08:59:25 PM CEST, Vladimir Sementsov-Ogievskiy 
<vsementsov@virtuozzo.com> wrote:

+ * On success return true with bm_list set (probably to NULL, if no bitmaps),

" probably " ? :-)

I note this as "set to NULL" is not obvious thing (is it "unset" ? :).. And by "probably" I mean 
"may be", i.e. NULL is just one of possible cases. Probably I use "probably" in a wrong way?

+ * on failure return false with errp set.
-Qcow2BitmapInfoList *qcow2_get_bitmap_info_list(BlockDriverState *bs,
-                                                Error **errp)
+bool qcow2_get_bitmap_info_list(BlockDriverState *bs,
+                                Qcow2BitmapInfoList **info_list, Error **errp)
      BDRVQcow2State *s = bs->opaque;
      Qcow2BitmapList *bm_list;
      Qcow2Bitmap *bm;
-    Qcow2BitmapInfoList *list = NULL;
-    Qcow2BitmapInfoList **plist = &list;

So here 'list' points at NULL and 'plist' at &list.

Hmm, to be precise, list _is_ NULL (and points nowhere), and plist points to 

-        *plist = obj;
-        plist = &obj->next;

In the original code 'plist' is updated when you add a new element, so
it always points at the end of the list. But 'list' is unchanged and it
still points at the first element.

So the caller receives a pointer to the first element.

+        *info_list = obj;
+        info_list = &obj->next;

But in the new code there is only one variable (passed by the caller),
which always points at the end of the list.

No: at first "*info_list = obj", we set the result which user will get, users 
pointer now points to the first object in the list.
Then, at "info_list = &obj->next", we reassign info_list to another pointer: to "next" 
field of first list item. So, all further "*info_list = obj" are note visible to the caller.

Actually, the logic is not changed, just instead of plist we use info_list, and instead of list - a variable which 
should be defined in the caller. Look: in old code, first "*plist = obj" sets "list" variable, but 
all further "*plist = obj" don't change "list" variable.

Best regards,

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