Add ExecCopySlotMinimalTupleExtra().

Allows an "extra" argument that allocates extra memory at the end of
the MinimalTuple. This is important for callers that need to store
additional data, but do not want to perform an additional allocation.

Suggested-by: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/CAApHDvppeqw2pNM-+ahBOJwq2QmC0hOAGsmCpC89QVmEoOvsdg@mail.gmail.com

src/backend/access/common/heaptuple.c

@@ -1452,9 +1452,11 @@ heap_freetuple(HeapTuple htup)
 MinimalTuple
 heap_form_minimal_tuple(TupleDesc tupleDescriptor,
 						const Datum *values,
-						const bool *isnull)
+						const bool *isnull,
+						Size extra)
 {
 	MinimalTuple tuple;			/* return tuple */
+	char	   *mem;
 	Size		len,
 				data_len;
 	int			hoff;
@@ -1462,6 +1464,8 @@ heap_form_minimal_tuple(TupleDesc tupleDescriptor,
 	int			numberOfAttributes = tupleDescriptor->natts;
 	int			i;
 
+	Assert(extra == MAXALIGN(extra));
+
 	if (numberOfAttributes > MaxTupleAttributeNumber)
 		ereport(ERROR,
 				(errcode(ERRCODE_TOO_MANY_COLUMNS),
@@ -1497,7 +1501,9 @@ heap_form_minimal_tuple(TupleDesc tupleDescriptor,
 	/*
 	 * Allocate and zero the space needed.
 	 */
-	tuple = (MinimalTuple) palloc0(len);
+	mem = palloc0(len + extra);
+	memset(mem, 0, extra);
+	tuple = (MinimalTuple) (mem + extra);
 
 	/*
 	 * And fill in the information.
@@ -1533,11 +1539,15 @@ heap_free_minimal_tuple(MinimalTuple mtup)
  * The result is allocated in the current memory context.
  */
 MinimalTuple
-heap_copy_minimal_tuple(MinimalTuple mtup)
+heap_copy_minimal_tuple(MinimalTuple mtup, Size extra)
 {
 	MinimalTuple result;
+	char	   *mem;
 
-	result = (MinimalTuple) palloc(mtup->t_len);
+	Assert(extra == MAXALIGN(extra));
+	mem = palloc(mtup->t_len + extra);
+	memset(mem, 0, extra);
+	result = (MinimalTuple) (mem + extra);
 	memcpy(result, mtup, mtup->t_len);
 	return result;
 }
@@ -1574,15 +1584,20 @@ heap_tuple_from_minimal_tuple(MinimalTuple mtup)
  * The result is allocated in the current memory context.
  */
 MinimalTuple
-minimal_tuple_from_heap_tuple(HeapTuple htup)
+minimal_tuple_from_heap_tuple(HeapTuple htup, Size extra)
 {
 	MinimalTuple result;
+	char	   *mem;
 	uint32		len;
 
+	Assert(extra == MAXALIGN(extra));
 	Assert(htup->t_len > MINIMAL_TUPLE_OFFSET);
 	len = htup->t_len - MINIMAL_TUPLE_OFFSET;
-	result = (MinimalTuple) palloc(len);
+	mem = palloc(len + extra);
+	memset(mem, 0, extra);
+	result = (MinimalTuple) (mem + extra);
 	memcpy(result, (char *) htup->t_data + MINIMAL_TUPLE_OFFSET, len);
+
 	result->t_len = len;
 	return result;
 }

src/backend/executor/execTuples.c

@@ -298,13 +298,14 @@ tts_virtual_copy_heap_tuple(TupleTableSlot *slot)
 }
 
 static MinimalTuple
-tts_virtual_copy_minimal_tuple(TupleTableSlot *slot)
+tts_virtual_copy_minimal_tuple(TupleTableSlot *slot, Size extra)
 {
 	Assert(!TTS_EMPTY(slot));
 
 	return heap_form_minimal_tuple(slot->tts_tupleDescriptor,
 								   slot->tts_values,
-								   slot->tts_isnull);
+								   slot->tts_isnull,
+								   extra);
 }
 
 
@@ -472,14 +473,14 @@ tts_heap_copy_heap_tuple(TupleTableSlot *slot)
 }
 
 static MinimalTuple
-tts_heap_copy_minimal_tuple(TupleTableSlot *slot)
+tts_heap_copy_minimal_tuple(TupleTableSlot *slot, Size extra)
 {
 	HeapTupleTableSlot *hslot = (HeapTupleTableSlot *) slot;
 
 	if (!hslot->tuple)
 		tts_heap_materialize(slot);
 
-	return minimal_tuple_from_heap_tuple(hslot->tuple);
+	return minimal_tuple_from_heap_tuple(hslot->tuple, extra);
 }
 
 static void
@@ -607,7 +608,8 @@ tts_minimal_materialize(TupleTableSlot *slot)
 	{
 		mslot->mintuple = heap_form_minimal_tuple(slot->tts_tupleDescriptor,
 												  slot->tts_values,
-												  slot->tts_isnull);
+												  slot->tts_isnull,
+												  0);
 	}
 	else
 	{
@@ -617,7 +619,7 @@ tts_minimal_materialize(TupleTableSlot *slot)
 		 * TTS_FLAG_SHOULDFREE set).  Copy the minimal tuple into the given
 		 * slot's memory context.
 		 */
-		mslot->mintuple = heap_copy_minimal_tuple(mslot->mintuple);
+		mslot->mintuple = heap_copy_minimal_tuple(mslot->mintuple, 0);
 	}
 
 	slot->tts_flags |= TTS_FLAG_SHOULDFREE;
@@ -666,14 +668,14 @@ tts_minimal_copy_heap_tuple(TupleTableSlot *slot)
 }
 
 static MinimalTuple
-tts_minimal_copy_minimal_tuple(TupleTableSlot *slot)
+tts_minimal_copy_minimal_tuple(TupleTableSlot *slot, Size extra)
 {
 	MinimalTupleTableSlot *mslot = (MinimalTupleTableSlot *) slot;
 
 	if (!mslot->mintuple)
 		tts_minimal_materialize(slot);
 
-	return heap_copy_minimal_tuple(mslot->mintuple);
+	return heap_copy_minimal_tuple(mslot->mintuple, extra);
 }
 
 static void
@@ -926,7 +928,7 @@ tts_buffer_heap_copy_heap_tuple(TupleTableSlot *slot)
 }
 
 static MinimalTuple
-tts_buffer_heap_copy_minimal_tuple(TupleTableSlot *slot)
+tts_buffer_heap_copy_minimal_tuple(TupleTableSlot *slot, Size extra)
 {
 	BufferHeapTupleTableSlot *bslot = (BufferHeapTupleTableSlot *) slot;
 
@@ -935,7 +937,7 @@ tts_buffer_heap_copy_minimal_tuple(TupleTableSlot *slot)
 	if (!bslot->base.tuple)
 		tts_buffer_heap_materialize(slot);
 
-	return minimal_tuple_from_heap_tuple(bslot->base.tuple);
+	return minimal_tuple_from_heap_tuple(bslot->base.tuple, extra);
 }
 
 static inline void
@@ -1895,7 +1897,7 @@ ExecFetchSlotMinimalTuple(TupleTableSlot *slot,
 	{
 		if (shouldFree)
 			*shouldFree = true;
-		return slot->tts_ops->copy_minimal_tuple(slot);
+		return slot->tts_ops->copy_minimal_tuple(slot, 0);
 	}
 }
 

src/backend/executor/nodeGatherMerge.c

@@ -735,7 +735,7 @@ gm_readnext_tuple(GatherMergeState *gm_state, int nreader, bool nowait,
 	 * Since we'll be buffering these across multiple calls, we need to make a
 	 * copy.
 	 */
-	return tup ? heap_copy_minimal_tuple(tup) : NULL;
+	return tup ? heap_copy_minimal_tuple(tup, 0) : NULL;
 }
 
 /*

src/backend/utils/sort/tuplesortvariants.c

@@ -1002,7 +1002,7 @@ tuplesort_gettupleslot(Tuplesortstate *state, bool forward, bool copy,
 			*abbrev = stup.datum1;
 
 		if (copy)
-			stup.tuple = heap_copy_minimal_tuple((MinimalTuple) stup.tuple);
+			stup.tuple = heap_copy_minimal_tuple((MinimalTuple) stup.tuple, 0);
 
 		ExecStoreMinimalTuple((MinimalTuple) stup.tuple, slot, copy);
 		return true;

src/backend/utils/sort/tuplestore.c

@@ -787,7 +787,7 @@ tuplestore_putvalues(Tuplestorestate *state, TupleDesc tdesc,
 	MinimalTuple tuple;
 	MemoryContext oldcxt = MemoryContextSwitchTo(state->context);
 
-	tuple = heap_form_minimal_tuple(tdesc, values, isnull);
+	tuple = heap_form_minimal_tuple(tdesc, values, isnull, 0);
 	USEMEM(state, GetMemoryChunkSpace(tuple));
 
 	tuplestore_puttuple_common(state, tuple);
@@ -1139,7 +1139,7 @@ tuplestore_gettupleslot(Tuplestorestate *state, bool forward,
 	{
 		if (copy && !should_free)
 		{
-			tuple = heap_copy_minimal_tuple(tuple);
+			tuple = heap_copy_minimal_tuple(tuple, 0);
 			should_free = true;
 		}
 		ExecStoreMinimalTuple(tuple, slot, should_free);
@@ -1590,7 +1590,7 @@ copytup_heap(Tuplestorestate *state, void *tup)
 {
 	MinimalTuple tuple;
 
-	tuple = minimal_tuple_from_heap_tuple((HeapTuple) tup);
+	tuple = minimal_tuple_from_heap_tuple((HeapTuple) tup, 0);
 	USEMEM(state, GetMemoryChunkSpace(tuple));
 	return tuple;
 }

src/include/access/htup_details.h

@@ -839,11 +839,12 @@ extern void heap_deform_tuple(HeapTuple tuple, TupleDesc tupleDesc,
 							  Datum *values, bool *isnull);
 extern void heap_freetuple(HeapTuple htup);
 extern MinimalTuple heap_form_minimal_tuple(TupleDesc tupleDescriptor,
-											const Datum *values, const bool *isnull);
+											const Datum *values, const bool *isnull,
+											Size extra);
 extern void heap_free_minimal_tuple(MinimalTuple mtup);
-extern MinimalTuple heap_copy_minimal_tuple(MinimalTuple mtup);
+extern MinimalTuple heap_copy_minimal_tuple(MinimalTuple mtup, Size extra);
 extern HeapTuple heap_tuple_from_minimal_tuple(MinimalTuple mtup);
-extern MinimalTuple minimal_tuple_from_heap_tuple(HeapTuple htup);
+extern MinimalTuple minimal_tuple_from_heap_tuple(HeapTuple htup, Size extra);
 extern size_t varsize_any(void *p);
 extern HeapTuple heap_expand_tuple(HeapTuple sourceTuple, TupleDesc tupleDesc);
 extern MinimalTuple minimal_expand_tuple(HeapTuple sourceTuple, TupleDesc tupleDesc);

src/include/executor/tuptable.h

@@ -218,8 +218,12 @@ struct TupleTableSlotOps
 	 * meaningful "system columns" in the copy. The copy is not be "owned" by
 	 * the slot i.e. the caller has to take responsibility to free memory
 	 * consumed by the slot.
+	 *
+	 * The copy has "extra" bytes (maxaligned and zeroed) available before the
+	 * tuple, which is useful so that some callers may store extra data along
+	 * with the minimal tuple without the need for an additional allocation.
 	 */
-	MinimalTuple (*copy_minimal_tuple) (TupleTableSlot *slot);
+	MinimalTuple (*copy_minimal_tuple) (TupleTableSlot *slot, Size extra);
 };
 
 /*
@@ -491,7 +495,19 @@ ExecCopySlotHeapTuple(TupleTableSlot *slot)
 static inline MinimalTuple
 ExecCopySlotMinimalTuple(TupleTableSlot *slot)
 {
-	return slot->tts_ops->copy_minimal_tuple(slot);
+	return slot->tts_ops->copy_minimal_tuple(slot, 0);
+}
+
+/*
+ * ExecCopySlotMinimalTupleExtra - return MinimalTuple allocated in caller's
+ * context, with extra bytes (maxaligned and zeroed) before the tuple for data
+ * the caller wishes to store along with the tuple (without requiring the
+ * caller to make an additional allocation).
+ */
+static inline MinimalTuple
+ExecCopySlotMinimalTupleExtra(TupleTableSlot *slot, Size extra)
+{
+	return slot->tts_ops->copy_minimal_tuple(slot, extra);
 }
 
 /*