Partially flatten struct tupleDesc so that it can be used in DSM.

TupleDesc's attributes were already stored in contiguous memory after the
struct.  Go one step further and get rid of the array of pointers to
attributes so that they can be stored in shared memory mapped at different
addresses in each backend.  This won't work for TupleDescs with contraints
and defaults, since those point to other objects, but for many purposes
only attributes are needed.

Author: Thomas Munro
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/CAEepm=0ZtQ-SpsgCyzzYpsXS6e=kZWqk3g5Ygn3MDV7A8dabUA@mail.gmail.com

src/backend/access/common/tupdesc.c

@@ -41,8 +41,6 @@ TupleDesc
 CreateTemplateTupleDesc(int natts, bool hasoid)
 {
 	TupleDesc	desc;
-	char	   *stg;
-	int			attroffset;
 
 	/*
 	 * sanity checks
@@ -51,38 +49,10 @@ CreateTemplateTupleDesc(int natts, bool hasoid)
 
 	/*
 	 * Allocate enough memory for the tuple descriptor, including the
-	 * attribute rows, and set up the attribute row pointers.
-	 *
-	 * Note: we assume that sizeof(struct tupleDesc) is a multiple of the
-	 * struct pointer alignment requirement, and hence we don't need to insert
-	 * alignment padding between the struct and the array of attribute row
-	 * pointers.
-	 *
-	 * Note: Only the fixed part of pg_attribute rows is included in tuple
-	 * descriptors, so we only need ATTRIBUTE_FIXED_PART_SIZE space per attr.
-	 * That might need alignment padding, however.
+	 * attribute rows.
 	 */
-	attroffset = sizeof(struct tupleDesc) + natts * sizeof(Form_pg_attribute);
-	attroffset = MAXALIGN(attroffset);
-	stg = palloc(attroffset + natts * MAXALIGN(ATTRIBUTE_FIXED_PART_SIZE));
-	desc = (TupleDesc) stg;
-
-	if (natts > 0)
-	{
-		Form_pg_attribute *attrs;
-		int			i;
-
-		attrs = (Form_pg_attribute *) (stg + sizeof(struct tupleDesc));
-		desc->attrs = attrs;
-		stg += attroffset;
-		for (i = 0; i < natts; i++)
-		{
-			attrs[i] = (Form_pg_attribute) stg;
-			stg += MAXALIGN(ATTRIBUTE_FIXED_PART_SIZE);
-		}
-	}
-	else
-		desc->attrs = NULL;
+	desc = (TupleDesc) palloc(offsetof(struct tupleDesc, attrs) +
+							  natts * sizeof(FormData_pg_attribute));
 
 	/*
 	 * Initialize other fields of the tupdesc.
@@ -99,12 +69,9 @@ CreateTemplateTupleDesc(int natts, bool hasoid)
 
 /*
  * CreateTupleDesc
- *		This function allocates a new TupleDesc pointing to a given
+ *		This function allocates a new TupleDesc by copying a given
  *		Form_pg_attribute array.
  *
- * Note: if the TupleDesc is ever freed, the Form_pg_attribute array
- * will not be freed thereby.
- *
  * Tuple type ID information is initially set for an anonymous record type;
  * caller can overwrite this if needed.
  */
@@ -112,20 +79,12 @@ TupleDesc
 CreateTupleDesc(int natts, bool hasoid, Form_pg_attribute *attrs)
 {
 	TupleDesc	desc;
+	int			i;
 
-	/*
-	 * sanity checks
-	 */
-	AssertArg(natts >= 0);
+	desc = CreateTemplateTupleDesc(natts, hasoid);
 
-	desc = (TupleDesc) palloc(sizeof(struct tupleDesc));
-	desc->attrs = attrs;
-	desc->natts = natts;
-	desc->constr = NULL;
-	desc->tdtypeid = RECORDOID;
-	desc->tdtypmod = -1;
-	desc->tdhasoid = hasoid;
-	desc->tdrefcount = -1;		/* assume not reference-counted */
+	for (i = 0; i < natts; ++i)
+		memcpy(TupleDescAttr(desc, i), attrs[i], ATTRIBUTE_FIXED_PART_SIZE);
 
 	return desc;
 }
@@ -147,10 +106,12 @@ CreateTupleDescCopy(TupleDesc tupdesc)
 
 	for (i = 0; i < desc->natts; i++)
 	{
-		memcpy(desc->attrs[i], tupdesc->attrs[i], ATTRIBUTE_FIXED_PART_SIZE);
-		desc->attrs[i]->attnotnull = false;
-		desc->attrs[i]->atthasdef = false;
-		desc->attrs[i]->attidentity = '\0';
+		Form_pg_attribute att = TupleDescAttr(desc, i);
+
+		memcpy(att, &tupdesc->attrs[i], ATTRIBUTE_FIXED_PART_SIZE);
+		att->attnotnull = false;
+		att->atthasdef = false;
+		att->attidentity = '\0';
 	}
 
 	desc->tdtypeid = tupdesc->tdtypeid;

src/include/access/tupdesc.h

@@ -71,17 +71,17 @@ typedef struct tupleConstr
 typedef struct tupleDesc
 {
 	int			natts;			/* number of attributes in the tuple */
-	Form_pg_attribute *attrs;
-	/* attrs[N] is a pointer to the description of Attribute Number N+1 */
-	TupleConstr *constr;		/* constraints, or NULL if none */
 	Oid			tdtypeid;		/* composite type ID for tuple type */
 	int32		tdtypmod;		/* typmod for tuple type */
 	bool		tdhasoid;		/* tuple has oid attribute in its header */
 	int			tdrefcount;		/* reference count, or -1 if not counting */
+	TupleConstr *constr;		/* constraints, or NULL if none */
+	/* attrs[N] is the description of Attribute Number N+1 */
+	FormData_pg_attribute attrs[FLEXIBLE_ARRAY_MEMBER];
 }		   *TupleDesc;
 
 /* Accessor for the i'th attribute of tupdesc. */
-#define TupleDescAttr(tupdesc, i) ((tupdesc)->attrs[(i)])
+#define TupleDescAttr(tupdesc, i) (&(tupdesc)->attrs[(i)])
 
 extern TupleDesc CreateTemplateTupleDesc(int natts, bool hasoid);