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Re: [PATCH v7 21/21] nbd: assert that Error** is not NULL in nbd_iter_ch

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [PATCH v7 21/21] nbd: assert that Error** is not NULL in nbd_iter_channel_error
Date: Thu, 5 Dec 2019 17:39:21 +0000

05.12.2019 20:14, Eric Blake wrote:
> On 12/5/19 9:20 AM, Vladimir Sementsov-Ogievskiy wrote:
>> The local_err parameter is not here to return information about
>> nbd_iter_channel_error failure. Instead it's assumed to be filled when
>> passed to the function. This is already stressed by its name
>> (local_err, instead of classic errp). Stress it additionally by
>> assertion.
>> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
>> ---
>>   block/nbd.c | 1 +
>>   1 file changed, 1 insertion(+)
>> diff --git a/block/nbd.c b/block/nbd.c
>> index 5f18f78a94..d085554f21 100644
>> --- a/block/nbd.c
>> +++ b/block/nbd.c
>> @@ -866,6 +866,7 @@ typedef struct NBDReplyChunkIter {
>>   static void nbd_iter_channel_error(NBDReplyChunkIter *iter,
>>                                      int ret, Error **local_err)
>>   {
>> +    assert(local_err && *local_err);
> Why are we forbidding grandparent callers from passing NULL when they want to 
> ignore an error?  We are called by several parent functions that get an errp 
> from the grandparent, and use this function to do some common grunt work.  
> Let's look at the possibilities:
> 1. grandparent passes address of a local error: we want to append to the 
> error message, parent will then deal with the error how it wants (report it, 
> ignore it, propagate it, whatever)
> 2. grandparent passes &error_fatal: we want to append a hint, but before 
> ERRP_AUTO_PROPAGATE, the parent has already exited.  After 
> ERRP_AUTO_PROPAGATE, this looks like case 1.
> 3. grandparent passes &error_abort: we should never be reached (failure 
> earlier in the parent should have already aborted) - true whether or not 
> 4. grandparent passes NULL to ignore the error. Does not currently happen in 
> any of the grandparent callers, because if it did, your assertion in this 
> patch would now fire.  After ERRP_AUTO_PROPAGATE, this would look like case 1.
> Would it be better to assert(!local_err || *local_err)?  The assertion as 
> written is too strict without ERRP_AUTO_PROPAGATE, but you get away with it 
> because none of the grandparents pass NULL; but is appropriate as written for 
> after after the macro conversion so then we wonder if churn on the macro is 
> worth it.

We don't have any grandparents, this function is always called on local_err. 
And it's argument named local_err to stress it.
The function is an API to report error, and it wants filled local_err object.

It will crash anyway if local_err is NULL, as it dereferences it.

I just want to place an assertion at start of functions like this,
which will be easily recognizable by coccinelle.


We can improve the API, to support local_err==NULL, for the case when original 
request was called with
errp==NULL, but for this we'll need more changes, like, pass errp to 
it into iter object...

But how to detect it in code? Something like

--- a/block/nbd.c
+++ b/block/nbd.c
@@ -1059,8 +1059,10 @@ static int nbd_co_receive_blockstatus_reply(BDRVNBDState 
              if (received) {
                  nbd_channel_error(s, -EINVAL);
-                error_setg(&local_err, "Several BLOCK_STATUS chunks in reply");
-                nbd_iter_channel_error(&iter, -EINVAL, &local_err);
+                if (errp) {
+                    error_setg(&local_err, "Several BLOCK_STATUS chunks in 
+                }
+                nbd_iter_channel_error(&iter, -EINVAL, errp ? &local_err : 
              received = true;

is ugly..

So, to support original errp=NULL the whole thing should be refactored.. Not 
worth it, I think.

>>       assert(ret < 0);
>>       if (!iter->ret) {

Best regards,

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