Verify that expected slot types match returned slot types.

This is important so JIT compilation knows what kind of tuple slot the
deforming routine can expect. There's also optimization potential for
expression initialization without JIT compilation. It e.g. seems
plausible to elide EEOP_*_FETCHSOME ops entirely when dealing with
virtual slots.

Author: Andres Freund
Discussion: https://postgr.es/m/20181105210039.hh4vvi4vwoq5ba2q@alap3.anarazel.de

src/backend/executor/execExprInterp.c

@@ -143,6 +143,7 @@ static void ExecInitInterpreter(void);
 
 /* support functions */
 static void CheckVarSlotCompatibility(TupleTableSlot *slot, int attnum, Oid vartype);
+static void CheckOpSlotCompatibility(ExprEvalStep *op, TupleTableSlot *slot);
 static TupleDesc get_cached_rowtype(Oid type_id, int32 typmod,
 				   TupleDesc *cache_field, ExprContext *econtext);
 static void ShutdownTupleDescRef(Datum arg);
@@ -425,6 +426,8 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
 
 		EEO_CASE(EEOP_INNER_FETCHSOME)
 		{
+			CheckOpSlotCompatibility(op, innerslot);
+
 			/* XXX: worthwhile to check tts_nvalid inline first? */
 			slot_getsomeattrs(innerslot, op->d.fetch.last_var);
 
@@ -433,6 +436,8 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
 
 		EEO_CASE(EEOP_OUTER_FETCHSOME)
 		{
+			CheckOpSlotCompatibility(op, outerslot);
+
 			slot_getsomeattrs(outerslot, op->d.fetch.last_var);
 
 			EEO_NEXT();
@@ -440,6 +445,8 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
 
 		EEO_CASE(EEOP_SCAN_FETCHSOME)
 		{
+			CheckOpSlotCompatibility(op, scanslot);
+
 			slot_getsomeattrs(scanslot, op->d.fetch.last_var);
 
 			EEO_NEXT();
@@ -1854,6 +1861,39 @@ CheckVarSlotCompatibility(TupleTableSlot *slot, int attnum, Oid vartype)
 	}
 }
 
+/*
+ * Verify that the slot is compatible with a EEOP_*_FETCHSOME operation.
+ */
+static void
+CheckOpSlotCompatibility(ExprEvalStep *op, TupleTableSlot *slot)
+{
+#ifdef USE_ASSERT_CHECKING
+	/* there's nothing to check */
+	if (!op->d.fetch.fixed)
+		return;
+
+	/*
+	 * Should probably fixed at some point, but for now it's easier to allow
+	 * buffer and heap tuples to be used interchangably.
+	 */
+	if (slot->tts_ops == &TTSOpsBufferTuple &&
+		op->d.fetch.kind == &TTSOpsHeapTuple)
+		return;
+	if (slot->tts_ops == &TTSOpsHeapTuple &&
+		op->d.fetch.kind == &TTSOpsBufferTuple)
+		return;
+
+	/*
+	 * At the moment we consider it OK if a virtual slot is used instead of a
+	 * specific type of slot, as a virtual slot never needs to be deformed.
+	 */
+	if (slot->tts_ops == &TTSOpsVirtual)
+		return;
+
+	Assert(op->d.fetch.kind == slot->tts_ops);
+#endif
+}
+
 /*
  * get_cached_rowtype: utility function to lookup a rowtype tupdesc
  *
@@ -1921,6 +1961,8 @@ ExecJustInnerVar(ExprState *state, ExprContext *econtext, bool *isnull)
 	int			attnum = op->d.var.attnum + 1;
 	TupleTableSlot *slot = econtext->ecxt_innertuple;
 
+	CheckOpSlotCompatibility(&state->steps[0], slot);
+
 	/*
 	 * Since we use slot_getattr(), we don't need to implement the FETCHSOME
 	 * step explicitly, and we also needn't Assert that the attnum is in range
@@ -1937,6 +1979,8 @@ ExecJustOuterVar(ExprState *state, ExprContext *econtext, bool *isnull)
 	int			attnum = op->d.var.attnum + 1;
 	TupleTableSlot *slot = econtext->ecxt_outertuple;
 
+	CheckOpSlotCompatibility(&state->steps[0], slot);
+
 	/* See comments in ExecJustInnerVar */
 	return slot_getattr(slot, attnum, isnull);
 }
@@ -1949,6 +1993,8 @@ ExecJustScanVar(ExprState *state, ExprContext *econtext, bool *isnull)
 	int			attnum = op->d.var.attnum + 1;
 	TupleTableSlot *slot = econtext->ecxt_scantuple;
 
+	CheckOpSlotCompatibility(&state->steps[0], slot);
+
 	/* See comments in ExecJustInnerVar */
 	return slot_getattr(slot, attnum, isnull);
 }
@@ -1973,6 +2019,8 @@ ExecJustAssignInnerVar(ExprState *state, ExprContext *econtext, bool *isnull)
 	TupleTableSlot *inslot = econtext->ecxt_innertuple;
 	TupleTableSlot *outslot = state->resultslot;
 
+	CheckOpSlotCompatibility(&state->steps[0], inslot);
+
 	/*
 	 * We do not need CheckVarSlotCompatibility here; that was taken care of
 	 * at compilation time.
@@ -1996,6 +2044,8 @@ ExecJustAssignOuterVar(ExprState *state, ExprContext *econtext, bool *isnull)
 	TupleTableSlot *inslot = econtext->ecxt_outertuple;
 	TupleTableSlot *outslot = state->resultslot;
 
+	CheckOpSlotCompatibility(&state->steps[0], inslot);
+
 	/* See comments in ExecJustAssignInnerVar */
 	outslot->tts_values[resultnum] =
 		slot_getattr(inslot, attnum, &outslot->tts_isnull[resultnum]);
@@ -2012,6 +2062,8 @@ ExecJustAssignScanVar(ExprState *state, ExprContext *econtext, bool *isnull)
 	TupleTableSlot *inslot = econtext->ecxt_scantuple;
 	TupleTableSlot *outslot = state->resultslot;
 
+	CheckOpSlotCompatibility(&state->steps[0], inslot);
+
 	/* See comments in ExecJustAssignInnerVar */
 	outslot->tts_values[resultnum] =
 		slot_getattr(inslot, attnum, &outslot->tts_isnull[resultnum]);