Fix alignment in BRIN minmax-multi deserialization

The deserialization failed to ensure correct alignment, as it assumed it
can simply point into the serialized value. The serialization however
ignores alignment and copies just the significant bytes in order to make
the result as small as possible. This caused failures on systems that
are sensitive to mialigned addresses, like sparc, or with address
sanitizer enabled.

Fixed by copying the serialized data to ensure proper alignment. While
at it, fix an issue with serialization on big endian machines, using the
same store_att_byval/fetch_att trick as extended statistics.

Discussion: https://postgr.es/0c8c3304-d3dd-5e29-d5ac-b50589a23c8c%40enterprisedb.com

Author: tvondra

src/backend/access/brin/brin_minmax_multi.c

@@ -665,7 +665,20 @@ range_serialize(Ranges *range)
 	{
 		if (typbyval)			/* simple by-value data types */
 		{
-			memcpy(ptr, &range->values[i], typlen);
+			Datum		tmp;
+
+			/*
+			 * For values passed by value, we need to copy just the
+			 * significant bytes - we can't use memcpy directly, as that
+			 * assumes little endian behavior.  store_att_byval does
+			 * almost what we need, but it requires properly aligned
+			 * buffer - the output buffer does not guarantee that. So we
+			 * simply use a local Datum variable (which guarantees proper
+			 * alignment), and then copy the value from it.
+			 */
+			store_att_byval(&tmp, range->values[i], typlen);
+
+			memcpy(ptr, &tmp, typlen);
 			ptr += typlen;
 		}
 		else if (typlen > 0)	/* fixed-length by-ref types */
@@ -710,9 +723,11 @@ range_deserialize(int maxvalues, SerializedRanges *serialized)
 {
 	int			i,
 				nvalues;
-	char	   *ptr;
+	char	   *ptr,
+			   *dataptr;
 	bool		typbyval;
 	int			typlen;
+	Size		datalen;
 
 	Ranges	   *range;
 
@@ -740,34 +755,81 @@ range_deserialize(int maxvalues, SerializedRanges *serialized)
 	typlen = get_typlen(serialized->typid);
 
 	/*
-	 * And now deconstruct the values into Datum array. We don't need to copy
-	 * the values and will instead just point the values to the serialized
-	 * varlena value (assuming it will be kept around).
+	 * And now deconstruct the values into Datum array. We have to copy the
+	 * data because the serialized representation ignores alignment, and we
+	 * don't want to rely it will be kept around anyway.
+	 */
+	ptr = serialized->data;
+
+	/*
+	 * We don't want to allocate many pieces, so we just allocate everything
+	 * in one chunk. How much space will we need?
+	 *
+	 * XXX We don't need to copy simple by-value data types.
+	 */
+	datalen = 0;
+	dataptr = NULL;
+	for (i = 0; (i < nvalues) && (!typbyval); i++)
+	{
+		if (typlen > 0)	/* fixed-length by-ref types */
+			datalen += MAXALIGN(typlen);
+		else if (typlen == -1)	/* varlena */
+		{
+			datalen += MAXALIGN(VARSIZE_ANY(DatumGetPointer(ptr)));
+			ptr += VARSIZE_ANY(DatumGetPointer(ptr));
+		}
+		else if (typlen == -2)	/* cstring */
+		{
+			datalen += MAXALIGN(strlen(DatumGetPointer(ptr)) + 1);
+			ptr += strlen(DatumGetPointer(ptr)) + 1;
+		}
+	}
+
+	if (datalen > 0)
+		dataptr = palloc(datalen);
+
+	/*
+	 * Restore the source pointer (might have been modified when calculating
+	 * the space we need to allocate).
 	 */
 	ptr = serialized->data;
 
 	for (i = 0; i < nvalues; i++)
 	{
 		if (typbyval)			/* simple by-value data types */
 		{
-			memcpy(&range->values[i], ptr, typlen);
+			Datum		v = 0;
+
+			memcpy(&v, ptr, typlen);
+
+			range->values[i] = fetch_att(&v, true, typlen);
 			ptr += typlen;
 		}
 		else if (typlen > 0)	/* fixed-length by-ref types */
 		{
-			/* no copy, just set the value to the pointer */
-			range->values[i] = PointerGetDatum(ptr);
+			range->values[i] = PointerGetDatum(dataptr);
+
+			memcpy(dataptr, ptr, typlen);
+			dataptr += MAXALIGN(typlen);
+
 			ptr += typlen;
 		}
 		else if (typlen == -1)	/* varlena */
 		{
-			range->values[i] = PointerGetDatum(ptr);
-			ptr += VARSIZE_ANY(DatumGetPointer(range->values[i]));
+			range->values[i] = PointerGetDatum(dataptr);
+
+			memcpy(dataptr, ptr, VARSIZE_ANY(ptr));
+			dataptr += MAXALIGN(VARSIZE_ANY(ptr));
+			ptr += VARSIZE_ANY(ptr);
 		}
 		else if (typlen == -2)	/* cstring */
 		{
-			range->values[i] = PointerGetDatum(ptr);
-			ptr += strlen(DatumGetPointer(range->values[i])) + 1;
+			Size	slen = strlen(ptr) + 1;
+			range->values[i] = PointerGetDatum(dataptr);
+
+			memcpy(dataptr, ptr, slen);
+			dataptr += MAXALIGN(slen);
+			ptr += (slen);
 		}
 
 		/* make sure we haven't overflown the buffer end */
@@ -2823,7 +2885,6 @@ minmax_multi_get_strategy_procinfo(BrinDesc *bdesc, uint16 attno, Oid subtype,
 								ObjectIdGetDatum(attr->atttypid),
 								ObjectIdGetDatum(subtype),
 								Int16GetDatum(strategynum));
-
 		if (!HeapTupleIsValid(tuple))
 			elog(ERROR, "missing operator %d(%u,%u) in opfamily %u",
 				 strategynum, attr->atttypid, subtype, opfamily);