BUG #18986: SIGSEGV in nodeModifyTable.c during Parallel Execution

The following bug has been logged on the website:

Bug reference:      18986
Logged by:          Yaroslav Syrytsia
Email address:      y@lll.gd
PostgreSQL version: 17.5
Operating system:   Ubuntu 22.04.5
Description:

Hello,

During testing a flow in an app, I encountered a segfault that only happens
during parallel execution:
https://gist.github.com/joy4eg/708458e204f52129a8e54a13534586b7

Looking into the coredump, it looks like a NULL pointer dereference is
happening at

https://github.com/postgres/postgres/blob/5e2f3df49d4298c6097789364a5a53be172f6e85/src/backend/executor/nodeModifyTable.c#L752

After further investigation, that part of the code looks suspicious as well:
static void
ExecInitInsertProjection(ModifyTableState *mtstate,
                                                 ResultRelInfo
*resultRelInfo)
{
    ...
        /* Build ProjectionInfo if needed (it probably isn't). */
        if (need_projection)
        {
                TupleDesc       relDesc =
RelationGetDescr(resultRelInfo->ri_RelationDesc);

                /* need an expression context to do the projection */
                if (mtstate->ps.ps_ExprContext == NULL)
                        ExecAssignExprContext(estate, &mtstate->ps);

                resultRelInfo->ri_projectNew =
                        ExecBuildProjectionInfo(insertTargetList,
mtstate->ps.ps_ExprContext,
resultRelInfo->ri_newTupleSlot,
&mtstate->ps,
relDesc);
        }

        resultRelInfo->ri_projectNewInfoValid = true; // <--- it's set to
true even if need_projection may be NULL.
}

(gdb) frame 0
#0  ExecGetUpdateNewTuple (oldSlot=0x55defb35ae20, planSlot=0x55defb41d9d8,
relinfo=0x55defb312108) at
executor/./build/../src/backend/executor/nodeModifyTable.c:752
752             econtext = newProj->pi_exprContext;
(gdb) p relinfo
$1 = (ResultRelInfo *) 0x55defb312108
(gdb) p *relinfo
$2 = {type = T_ResultRelInfo, ri_RangeTableIndex = 15, ri_RelationDesc =
0x7f0c0eb80500, ri_NumIndices = 21, ri_IndexRelationDescs = 0x55defb358248,
ri_IndexRelationInfo = 0x55defb358350, ri_RowIdAttNo = 1,
  ri_extraUpdatedCols = 0x55defb357300, ri_projectNew = 0x0, ri_newTupleSlot
= 0x55defb323cf0, ri_oldTupleSlot = 0x55defb323ae8, ri_projectNewInfoValid =
true, ri_needLockTagTuple = false, ri_TrigDesc = 0x55defb312310,
  ri_TrigFunctions = 0x55defb31a318, ri_TrigWhenExprs = 0x55defb2d8cf8,
ri_TrigInstrument = 0x0, ri_ReturningSlot = 0x0, ri_TrigOldSlot =
0x55defb35ae20, ri_TrigNewSlot = 0x0, ri_FdwRoutine = 0x0, ri_FdwState =
0x0,
  ri_usesFdwDirectModify = false, ri_NumSlots = 0, ri_NumSlotsInitialized =
0, ri_BatchSize = 0, ri_Slots = 0x0, ri_PlanSlots = 0x0, ri_WithCheckOptions
= 0x0, ri_WithCheckOptionExprs = 0x0, ri_ConstraintExprs = 0x0,
  ri_GeneratedExprsI = 0x0, ri_GeneratedExprsU = 0x55defb358140,
ri_NumGeneratedNeededI = 0, ri_NumGeneratedNeededU = 2, ri_returningList =
0x55defb39c630, ri_projectReturning = 0x55defb323590,
ri_onConflictArbiterIndexes = 0x0,
  ri_onConflict = 0x0, ri_MergeActions = {0x55defb324518, 0x0,
0x55defb328e50}, ri_MergeJoinCondition = 0x0, ri_PartitionCheckExpr = 0x0,
ri_ChildToRootMap = 0x0, ri_ChildToRootMapValid = false, ri_RootToChildMap =
0x0,
  ri_RootToChildMapValid = false, ri_RootResultRelInfo = 0x0,
ri_PartitionTupleSlot = 0x0, ri_CopyMultiInsertBuffer = 0x0,
ri_ancestorResultRels = 0x0}

Where:
- ri_projectNewInfoValid = true
- ri_projectNew = 0x0

Version tested:
- 17.5 (ubuntu, apt, with and without pgroonga)
- postgres:18beta1-bookworm (docker, without pgroonga)
- postgres:17.5-bookworm (docker, without pgroonga)

Yaroslav.