Use FLEXIBLE_ARRAY_MEMBER in struct varlena.

This forces some minor coding adjustments in tuptoaster.c and inv_api.c,
but the new coding there is cleaner anyway.

Michael Paquier

src/backend/access/heap/tuptoaster.c

@@ -1365,11 +1365,13 @@ toast_save_datum(Relation rel, Datum value,
 	CommandId	mycid = GetCurrentCommandId(true);
 	struct varlena *result;
 	struct varatt_external toast_pointer;
-	struct
+	union
 	{
 		struct varlena hdr;
-		char		data[TOAST_MAX_CHUNK_SIZE]; /* make struct big enough */
-		int32		align_it;	/* ensure struct is aligned well enough */
+		/* this is to make the union big enough for a chunk: */
+		char		data[TOAST_MAX_CHUNK_SIZE + VARHDRSZ];
+		/* ensure union is aligned well enough: */
+		int32		align_it;
 	}			chunk_data;
 	int32		chunk_size;
 	int32		chunk_seq = 0;

src/backend/storage/large_object/inv_api.c

@@ -562,11 +562,13 @@ inv_write(LargeObjectDesc *obj_desc, const char *buf, int nbytes)
 	bool		neednextpage;
 	bytea	   *datafield;
 	bool		pfreeit;
-	struct
+	union
 	{
 		bytea		hdr;
-		char		data[LOBLKSIZE];	/* make struct big enough */
-		int32		align_it;	/* ensure struct is aligned well enough */
+		/* this is to make the union big enough for a LO data chunk: */
+		char		data[LOBLKSIZE + VARHDRSZ];
+		/* ensure union is aligned well enough: */
+		int32		align_it;
 	}			workbuf;
 	char	   *workb = VARDATA(&workbuf.hdr);
 	HeapTuple	newtup;
@@ -748,11 +750,13 @@ inv_truncate(LargeObjectDesc *obj_desc, int64 len)
 	SysScanDesc sd;
 	HeapTuple	oldtuple;
 	Form_pg_largeobject olddata;
-	struct
+	union
 	{
 		bytea		hdr;
-		char		data[LOBLKSIZE];	/* make struct big enough */
-		int32		align_it;	/* ensure struct is aligned well enough */
+		/* this is to make the union big enough for a LO data chunk: */
+		char		data[LOBLKSIZE + VARHDRSZ];
+		/* ensure union is aligned well enough: */
+		int32		align_it;
 	}			workbuf;
 	char	   *workb = VARDATA(&workbuf.hdr);
 	HeapTuple	newtup;

src/include/c.h

@@ -391,7 +391,7 @@ typedef struct
 struct varlena
 {
 	char		vl_len_[4];		/* Do not touch this field directly! */
-	char		vl_dat[1];
+	char		vl_dat[FLEXIBLE_ARRAY_MEMBER];	/* Data content is here */
 };
 
 #define VARHDRSZ		((int32) sizeof(int32))