Thread: Segment fault when excuting SPI function On PG with commit 41c6a5be

Hi, all

When I used SPI_execute_plan function on PG12 which commit 41c6a5be is used,
Segment fault occurred.

PS: If commit 41c6a5be is not used, this phenomenon will not happen. 

Reproduce:
In a background process, the following steps are executed.
--------------------------
StartTransactionCommand();
SPI_connect();　　　　　　　
plan = SPI_prepare(query,0,NULL); ★the query is a SELECT SQL. 
SPI_keepplan(plan);　　　　　　　　 
SPI_finish();　　　　　　　　　　　
CommitTransactionCommand();　　　
StartTransactionCommand();　　　　
SPI_connect();
SPI_execute_plan(plan, NULL, NULL, true, 0); ★Segment fault
--------------------------

Core stack:
Stack trace of thread 43926:
                #0  0x0000000000772f19 EnsurePortalSnapshotExists (postgres)
                #1  0x000000000064f85c _SPI_execute_plan (postgres)
                #2  0x000000000064fbd1 SPI_execute_plan (postgres)
                #3  0x00007fbee784402e xxx (xxx.so)
                #4  0x00007fbee78424ae xxxx (xxxx.so)
                #5  0x00000000006e91f5 StartBackgroundWorker (postgres)
                #6  0x00000000006f5e25 maybe_start_bgworkers (postgres)
                #7  0x00000000006f6121 reaper (postgres)
                #8  0x00007fbeedf48dc0 __restore_rt (libpthread.so.0)
                #9  0x00007fbeebadf75b __select (libc.so.6)
                #10 0x00000000006f6ea6 ServerLoop (postgres)
                #11 0x00000000006f8c30 PostmasterMain (postgres)
                #12 0x0000000000485d99 main (postgres)
                #13 0x00007fbeeba0f873 __libc_start_main (libc.so.6)
                #14 0x0000000000485e3e _start (postgres)

Is there a bug in the commit 41c6a5be?  Please confirm it.

Regards, Liu Huailing

On Fri, Jul 30, 2021 at 6:57 PM liuhuailing@fujitsu.com
<liuhuailing@fujitsu.com> wrote:
>
> When I used SPI_execute_plan function on PG12 which commit 41c6a5be is used,
> Segment fault occurred.

I'm not seeing any such commit.
Can you provide a link to the commit in GitHub?

Regards,
Greg Nancarrow
Fujitsu Australia

On Fri, Jul 30, 2021 at 08:57:35AM +0000, liuhuailing@fujitsu.com wrote:
> When I used SPI_execute_plan function on PG12 which commit 41c6a5be is used,
> Segment fault occurred.
>
> PS: If commit 41c6a5be is not used, this phenomenon will not happen.

I see nothing wrong in this commit, FWIW.

> Reproduce:
> In a background process, the following steps are executed.
> --------------------------
> StartTransactionCommand();
> SPI_connect();　　　　　　　
> plan = SPI_prepare(query,0,NULL); ★the query is a SELECT SQL.
> SPI_keepplan(plan);　　　　　　　　
> SPI_finish();　　　　　　　　　　　
> CommitTransactionCommand();　　　
> StartTransactionCommand();　　　　
> SPI_connect();
> SPI_execute_plan(plan, NULL, NULL, true, 0); ★Segment fault
> --------------------------

But I see an issue with your code.  It seems to me that you should
push a snapshot before doing SPI_prepare() and SPI_execute_plan(),
as of:
PushActiveSnapshot(GetTransactionSnapshot());
--
Michael

Michael Paquier <michael@paquier.xyz> writes:
> On Fri, Jul 30, 2021 at 08:57:35AM +0000, liuhuailing@fujitsu.com wrote:
>> When I used SPI_execute_plan function on PG12 which commit 41c6a5be is used,
>> Segment fault occurred.

> I see nothing wrong in this commit, FWIW.
> But I see an issue with your code.  It seems to me that you should
> push a snapshot before doing SPI_prepare() and SPI_execute_plan(),
> as of:
> PushActiveSnapshot(GetTransactionSnapshot());

Yes.  What that commit did was to formalize the requirement that
a snapshot exist *before* entering SPI.  Before that, you might
have gotten away without one, depending on what you were trying
to do (in particular, detoasting a toasted output Datum would
fail if you lack an external snapshot).

This isn't the first complaint we've seen from somebody whose
code used to work and now fails there, however.  I wonder if
we should convert the Assert into an actual test-and-elog, say

    /* Otherwise, we'd better have an active Portal */
    portal = ActivePortal;
-    Assert(portal != NULL);
+    if (unlikely(portal == NULL))
+        elog(ERROR, "must have an outer snapshot or portal");
    Assert(portal->portalSnapshot == NULL);

Perhaps that would help people to realize that the bug is theirs
not EnsurePortalSnapshotExists's.

I gather BTW that the OP is trying to test C code in a non-assert
build.  Not a great approach.

            regards, tom lane

> On 30 Jul 2021, at 17:06, Tom Lane <tgl@sss.pgh.pa.us> wrote:

> I wonder if we should convert the Assert into an actual test-and-elog, say
> 
>     /* Otherwise, we'd better have an active Portal */
>     portal = ActivePortal;
> -    Assert(portal != NULL);
> +    if (unlikely(portal == NULL))
> +        elog(ERROR, "must have an outer snapshot or portal");
>     Assert(portal->portalSnapshot == NULL);
> 
> Perhaps that would help people to realize that the bug is theirs
> not EnsurePortalSnapshotExists's.

+1, that would probably be quite helpful.

--
Daniel Gustafsson        https://vmware.com/

Daniel Gustafsson <daniel@yesql.se> writes:
>> On 30 Jul 2021, at 17:06, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> I wonder if we should convert the Assert into an actual test-and-elog, say
>> 
>>     /* Otherwise, we'd better have an active Portal */
>>     portal = ActivePortal;
>> -    Assert(portal != NULL);
>> +    if (unlikely(portal == NULL))
>> +        elog(ERROR, "must have an outer snapshot or portal");
>>     Assert(portal->portalSnapshot == NULL);
>> 
>> Perhaps that would help people to realize that the bug is theirs
>> not EnsurePortalSnapshotExists's.

> +1, that would probably be quite helpful.

Happy to make it so.  Anyone have suggestions about the wording of
the message?

            regards, tom lane

On Fri, Jul 30, 2021 at 11:22:43AM -0400, Tom Lane wrote:
> Happy to make it so.  Anyone have suggestions about the wording of
> the message?

For the archives, this has been applied as of ef12f32, and the new
message seems explicit enough:
+   if (unlikely(portal == NULL))
+       elog(ERROR, "cannot execute SQL without an outer snapshot or portal");
--
Michael