Re: subxcnt defined as signed integer in SnapshotData and SerializeSnapshotData

On Fri, May 8, 2015 at 3:55 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
> On 7 May 2015 at 21:40, Michael Paquier <michael.paquier@gmail.com> wrote:
>>
>> Hi all,
>>
>> Coverity is complaining about the following assertion introduced in
>> commit 924bcf4 (parallel stuff, SerializeSnapshot@snapmgr.c):
>> +       Assert(snapshot->xcnt >= 0);
>>
>> Now the thing is that this assertion does not make much sense, because
>> SnapshotData defines subxcnt as uint32 in snapshot.h. While we could
>> simply remove this assertion, I am wondering if we could not change
>> subxcnt to uint32 instead.
>>
>> SnapshotData has been introduced in 2008 by d43b085, with this comment:
>> +       int32           subxcnt;                /* # of xact ids in
>> subxip[], -1 if overflow */
>> Comment regarding negative values removed in efc16ea5.
>>
>> Now, by looking at the code on HEAD, I am seeing no code paths that
>> make use of negative values of subxcnt. Perhaps I am missing
>> something?
>
>
> So the comment is wrong? It does not set to -1 at overflow anymore?

SnapshotData.suboverflowed is used instead. Have a look at efc16ea5 in
procarray.c to convince yourself:

@@ -785,16 +1121,17 @@ GetSnapshotData(Snapshot snapshot)
                 *
                 * Again, our own XIDs are not included in the snapshot.
                 */
-               if (subcount >= 0 && proc != MyProc)
+               if (!suboverflowed && proc != MyProc)
                {
                        if (proc->subxids.overflowed)
-                               subcount = -1;  /* overflowed */
+                               suboverflowed = true;
                        else

I think that we should redefine subxcnt as uint32 for consistency with
xcnt, and remove the two assertions that 924bcf4 has introduced. I
could get a patch quickly done FWIW.