postgresql/src/backend/catalog/pg_enum.c

/*-------------------------------------------------------------------------
 *
 * pg_enum.c
 *	  routines to support manipulation of the pg_enum relation
 *
 * Copyright (c) 2006-2024, PostgreSQL Global Development Group
 *
 *
 * IDENTIFICATION
 *	  src/backend/catalog/pg_enum.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/genam.h"
#include "access/htup_details.h"
#include "access/table.h"
#include "access/xact.h"
#include "catalog/binary_upgrade.h"
#include "catalog/catalog.h"
#include "catalog/indexing.h"
#include "catalog/pg_enum.h"
#include "catalog/pg_type.h"
#include "miscadmin.h"
#include "nodes/value.h"
#include "storage/lmgr.h"
#include "utils/builtins.h"
#include "utils/catcache.h"
#include "utils/fmgroids.h"
#include "utils/hsearch.h"
#include "utils/memutils.h"
#include "utils/syscache.h"

/* Potentially set by pg_upgrade_support functions */
Oid			binary_upgrade_next_pg_enum_oid = InvalidOid;

/*
 * We keep two transaction-lifespan hash tables, one containing the OIDs
 * of enum types made in the current transaction, and one containing the
 * OIDs of enum values created during the current transaction by
 * AddEnumLabel (but only if their enum type is not in the first hash).
 *
 * We disallow using enum values in the second hash until the transaction is
 * committed; otherwise, they might get into indexes where we can't clean
 * them up, and then if the transaction rolls back we have a broken index.
 * (See comments for check_safe_enum_use() in enum.c.)  Values created by
 * EnumValuesCreate are *not* entered into the table; we assume those are
 * created during CREATE TYPE, so they can't go away unless the enum type
 * itself does.
 *
 * The motivation for treating enum values as safe if their type OID is
 * in the first hash is to allow CREATE TYPE AS ENUM; ALTER TYPE ADD VALUE;
 * followed by a use of the value in the same transaction.  This pattern
 * is really just as safe as creating the value during CREATE TYPE.
 * We need to support this because pg_dump in binary upgrade mode produces
 * commands like that.  But currently we only support it when the commands
 * are at the outermost transaction level, which is as much as we need for
 * pg_dump.  We could track subtransaction nesting of the commands to
 * analyze things more precisely, but for now we don't bother.
 */
static HTAB *uncommitted_enum_types = NULL;
static HTAB *uncommitted_enum_values = NULL;

static void init_uncommitted_enum_types(void);
static void init_uncommitted_enum_values(void);
static bool EnumTypeUncommitted(Oid typ_id);
static void RenumberEnumType(Relation pg_enum, HeapTuple *existing, int nelems);
static int	sort_order_cmp(const void *p1, const void *p2);


/*
 * EnumValuesCreate
 *		Create an entry in pg_enum for each of the supplied enum values.
 *
 * vals is a list of String values.
 *
 * We assume that this is called only by CREATE TYPE AS ENUM, and that it
 * will be called even if the vals list is empty.  So we can enter the
 * enum type's OID into uncommitted_enum_types here, rather than needing
 * another entry point to do it.
 */
void
EnumValuesCreate(Oid enumTypeOid, List *vals)
{
	Relation	pg_enum;
	Oid		   *oids;
	int			elemno,
				num_elems;
	ListCell   *lc;
	int			slotCount = 0;
	int			nslots;
	CatalogIndexState indstate;
	TupleTableSlot **slot;

	/*
	 * Remember the type OID as being made in the current transaction, but not
	 * if we're in a subtransaction.  (We could remember the OID anyway, in
	 * case a subsequent ALTER ADD VALUE occurs at outer level.  But that
	 * usage pattern seems unlikely enough that we'd probably just be wasting
	 * hashtable maintenance effort.)
	 */
	if (GetCurrentTransactionNestLevel() == 1)
	{
		if (uncommitted_enum_types == NULL)
			init_uncommitted_enum_types();
		(void) hash_search(uncommitted_enum_types, &enumTypeOid,
						   HASH_ENTER, NULL);
	}

	num_elems = list_length(vals);

	/*
	 * We do not bother to check the list of values for duplicates --- if you
	 * have any, you'll get a less-than-friendly unique-index violation. It is
	 * probably not worth trying harder.
	 */

	pg_enum = table_open(EnumRelationId, RowExclusiveLock);

	/*
	 * Allocate OIDs for the enum's members.
	 *
	 * While this method does not absolutely guarantee that we generate no
	 * duplicate OIDs (since we haven't entered each oid into the table before
	 * allocating the next), trouble could only occur if the OID counter wraps
	 * all the way around before we finish. Which seems unlikely.
	 */
	oids = (Oid *) palloc(num_elems * sizeof(Oid));

	for (elemno = 0; elemno < num_elems; elemno++)
	{
		/*
		 * We assign even-numbered OIDs to all the new enum labels.  This
		 * tells the comparison functions the OIDs are in the correct sort
		 * order and can be compared directly.
		 */
		Oid			new_oid;

		do
		{
			new_oid = GetNewOidWithIndex(pg_enum, EnumOidIndexId,
										 Anum_pg_enum_oid);
		} while (new_oid & 1);
		oids[elemno] = new_oid;
	}

	/* sort them, just in case OID counter wrapped from high to low */
	qsort(oids, num_elems, sizeof(Oid), oid_cmp);

	/* and make the entries */
	indstate = CatalogOpenIndexes(pg_enum);

	/* allocate the slots to use and initialize them */
	nslots = Min(num_elems,
				 MAX_CATALOG_MULTI_INSERT_BYTES / sizeof(FormData_pg_enum));
	slot = palloc(sizeof(TupleTableSlot *) * nslots);
	for (int i = 0; i < nslots; i++)
		slot[i] = MakeSingleTupleTableSlot(RelationGetDescr(pg_enum),
										   &TTSOpsHeapTuple);

	elemno = 0;
	foreach(lc, vals)
	{
		char	   *lab = strVal(lfirst(lc));
		Name		enumlabel = palloc0(NAMEDATALEN);

		/*
		 * labels are stored in a name field, for easier syscache lookup, so
		 * check the length to make sure it's within range.
		 */
		if (strlen(lab) > (NAMEDATALEN - 1))
			ereport(ERROR,
					(errcode(ERRCODE_INVALID_NAME),
					 errmsg("invalid enum label \"%s\"", lab),
					 errdetail("Labels must be %d bytes or less.",
							   NAMEDATALEN - 1)));

		ExecClearTuple(slot[slotCount]);

		memset(slot[slotCount]->tts_isnull, false,
			   slot[slotCount]->tts_tupleDescriptor->natts * sizeof(bool));

		slot[slotCount]->tts_values[Anum_pg_enum_oid - 1] = ObjectIdGetDatum(oids[elemno]);
		slot[slotCount]->tts_values[Anum_pg_enum_enumtypid - 1] = ObjectIdGetDatum(enumTypeOid);
		slot[slotCount]->tts_values[Anum_pg_enum_enumsortorder - 1] = Float4GetDatum(elemno + 1);

		namestrcpy(enumlabel, lab);
		slot[slotCount]->tts_values[Anum_pg_enum_enumlabel - 1] = NameGetDatum(enumlabel);

		ExecStoreVirtualTuple(slot[slotCount]);
		slotCount++;

		/* if slots are full, insert a batch of tuples */
		if (slotCount == nslots)
		{
			CatalogTuplesMultiInsertWithInfo(pg_enum, slot, slotCount,
											 indstate);
			slotCount = 0;
		}

		elemno++;
	}

	/* Insert any tuples left in the buffer */
	if (slotCount > 0)
		CatalogTuplesMultiInsertWithInfo(pg_enum, slot, slotCount,
										 indstate);

	/* clean up */
	pfree(oids);
	for (int i = 0; i < nslots; i++)
		ExecDropSingleTupleTableSlot(slot[i]);
	CatalogCloseIndexes(indstate);
	table_close(pg_enum, RowExclusiveLock);
}


/*
 * EnumValuesDelete
 *		Remove all the pg_enum entries for the specified enum type.
 */
void
EnumValuesDelete(Oid enumTypeOid)
{
	Relation	pg_enum;
	ScanKeyData key[1];
	SysScanDesc scan;
	HeapTuple	tup;

	pg_enum = table_open(EnumRelationId, RowExclusiveLock);

	ScanKeyInit(&key[0],
				Anum_pg_enum_enumtypid,
				BTEqualStrategyNumber, F_OIDEQ,
				ObjectIdGetDatum(enumTypeOid));

	scan = systable_beginscan(pg_enum, EnumTypIdLabelIndexId, true,
							  NULL, 1, key);

	while (HeapTupleIsValid(tup = systable_getnext(scan)))
	{
		CatalogTupleDelete(pg_enum, &tup->t_self);
	}

	systable_endscan(scan);

	table_close(pg_enum, RowExclusiveLock);
}

/*
 * Initialize the uncommitted enum types table for this transaction.
 */
static void
init_uncommitted_enum_types(void)
{
	HASHCTL		hash_ctl;

	hash_ctl.keysize = sizeof(Oid);
	hash_ctl.entrysize = sizeof(Oid);
	hash_ctl.hcxt = TopTransactionContext;
	uncommitted_enum_types = hash_create("Uncommitted enum types",
										 32,
										 &hash_ctl,
										 HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
}

/*
 * Initialize the uncommitted enum values table for this transaction.
 */
static void
init_uncommitted_enum_values(void)
{
	HASHCTL		hash_ctl;

	hash_ctl.keysize = sizeof(Oid);
	hash_ctl.entrysize = sizeof(Oid);
	hash_ctl.hcxt = TopTransactionContext;
	uncommitted_enum_values = hash_create("Uncommitted enum values",
										  32,
										  &hash_ctl,
										  HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
}

/*
 * AddEnumLabel
 *		Add a new label to the enum set. By default it goes at
 *		the end, but the user can choose to place it before or
 *		after any existing set member.
 */
void
AddEnumLabel(Oid enumTypeOid,
			 const char *newVal,
			 const char *neighbor,
			 bool newValIsAfter,
			 bool skipIfExists)
{
	Relation	pg_enum;
	Oid			newOid;
	Datum		values[Natts_pg_enum];
	bool		nulls[Natts_pg_enum];
	NameData	enumlabel;
	HeapTuple	enum_tup;
	float4		newelemorder;
	HeapTuple  *existing;
	CatCList   *list;
	int			nelems;
	int			i;

	/* check length of new label is ok */
	if (strlen(newVal) > (NAMEDATALEN - 1))
		ereport(ERROR,
				(errcode(ERRCODE_INVALID_NAME),
				 errmsg("invalid enum label \"%s\"", newVal),
				 errdetail("Labels must be %d bytes or less.",
						   NAMEDATALEN - 1)));

	/*
	 * Acquire a lock on the enum type, which we won't release until commit.
	 * This ensures that two backends aren't concurrently modifying the same
	 * enum type.  Without that, we couldn't be sure to get a consistent view
	 * of the enum members via the syscache.  Note that this does not block
	 * other backends from inspecting the type; see comments for
	 * RenumberEnumType.
	 */
	LockDatabaseObject(TypeRelationId, enumTypeOid, 0, ExclusiveLock);

	/*
	 * Check if label is already in use.  The unique index on pg_enum would
	 * catch this anyway, but we prefer a friendlier error message, and
	 * besides we need a check to support IF NOT EXISTS.
	 */
	enum_tup = SearchSysCache2(ENUMTYPOIDNAME,
							   ObjectIdGetDatum(enumTypeOid),
							   CStringGetDatum(newVal));
	if (HeapTupleIsValid(enum_tup))
	{
		ReleaseSysCache(enum_tup);
		if (skipIfExists)
		{
			ereport(NOTICE,
					(errcode(ERRCODE_DUPLICATE_OBJECT),
					 errmsg("enum label \"%s\" already exists, skipping",
							newVal)));
			return;
		}
		else
			ereport(ERROR,
					(errcode(ERRCODE_DUPLICATE_OBJECT),
					 errmsg("enum label \"%s\" already exists",
							newVal)));
	}

	pg_enum = table_open(EnumRelationId, RowExclusiveLock);

	/* If we have to renumber the existing members, we restart from here */
restart:

	/* Get the list of existing members of the enum */
	list = SearchSysCacheList1(ENUMTYPOIDNAME,
							   ObjectIdGetDatum(enumTypeOid));
	nelems = list->n_members;

	/* Sort the existing members by enumsortorder */
	existing = (HeapTuple *) palloc(nelems * sizeof(HeapTuple));
	for (i = 0; i < nelems; i++)
		existing[i] = &(list->members[i]->tuple);

	qsort(existing, nelems, sizeof(HeapTuple), sort_order_cmp);

	if (neighbor == NULL)
	{
		/*
		 * Put the new label at the end of the list. No change to existing
		 * tuples is required.
		 */
		if (nelems > 0)
		{
			Form_pg_enum en = (Form_pg_enum) GETSTRUCT(existing[nelems - 1]);

			newelemorder = en->enumsortorder + 1;
		}
		else
			newelemorder = 1;
	}
	else
	{
		/* BEFORE or AFTER was specified */
		int			nbr_index;
		int			other_nbr_index;
		Form_pg_enum nbr_en;
		Form_pg_enum other_nbr_en;

		/* Locate the neighbor element */
		for (nbr_index = 0; nbr_index < nelems; nbr_index++)
		{
			Form_pg_enum en = (Form_pg_enum) GETSTRUCT(existing[nbr_index]);

			if (strcmp(NameStr(en->enumlabel), neighbor) == 0)
				break;
		}
		if (nbr_index >= nelems)
			ereport(ERROR,
					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
					 errmsg("\"%s\" is not an existing enum label",
							neighbor)));
		nbr_en = (Form_pg_enum) GETSTRUCT(existing[nbr_index]);

		/*
		 * Attempt to assign an appropriate enumsortorder value: one less than
		 * the smallest member, one more than the largest member, or halfway
		 * between two existing members.
		 *
		 * In the "halfway" case, because of the finite precision of float4,
		 * we might compute a value that's actually equal to one or the other
		 * of its neighbors.  In that case we renumber the existing members
		 * and try again.
		 */
		if (newValIsAfter)
			other_nbr_index = nbr_index + 1;
		else
			other_nbr_index = nbr_index - 1;

		if (other_nbr_index < 0)
			newelemorder = nbr_en->enumsortorder - 1;
		else if (other_nbr_index >= nelems)
			newelemorder = nbr_en->enumsortorder + 1;
		else
		{
			/*
			 * The midpoint value computed here has to be rounded to float4
			 * precision, else our equality comparisons against the adjacent
			 * values are meaningless.  The most portable way of forcing that
			 * to happen with non-C-standard-compliant compilers is to store
			 * it into a volatile variable.
			 */
			volatile float4 midpoint;

			other_nbr_en = (Form_pg_enum) GETSTRUCT(existing[other_nbr_index]);
			midpoint = (nbr_en->enumsortorder +
						other_nbr_en->enumsortorder) / 2;

			if (midpoint == nbr_en->enumsortorder ||
				midpoint == other_nbr_en->enumsortorder)
			{
				RenumberEnumType(pg_enum, existing, nelems);
				/* Clean up and start over */
				pfree(existing);
				ReleaseCatCacheList(list);
				goto restart;
			}

			newelemorder = midpoint;
		}
	}

	/* Get a new OID for the new label */
	if (IsBinaryUpgrade)
	{
		if (!OidIsValid(binary_upgrade_next_pg_enum_oid))
			ereport(ERROR,
					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
					 errmsg("pg_enum OID value not set when in binary upgrade mode")));

		/*
		 * Use binary-upgrade override for pg_enum.oid, if supplied. During
		 * binary upgrade, all pg_enum.oid's are set this way so they are
		 * guaranteed to be consistent.
		 */
		if (neighbor != NULL)
			ereport(ERROR,
					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
					 errmsg("ALTER TYPE ADD BEFORE/AFTER is incompatible with binary upgrade")));

		newOid = binary_upgrade_next_pg_enum_oid;
		binary_upgrade_next_pg_enum_oid = InvalidOid;
	}
	else
	{
		/*
		 * Normal case: we need to allocate a new Oid for the value.
		 *
		 * We want to give the new element an even-numbered Oid if it's safe,
		 * which is to say it compares correctly to all pre-existing even
		 * numbered Oids in the enum.  Otherwise, we must give it an odd Oid.
		 */
		for (;;)
		{
			bool		sorts_ok;

			/* Get a new OID (different from all existing pg_enum tuples) */
			newOid = GetNewOidWithIndex(pg_enum, EnumOidIndexId,
										Anum_pg_enum_oid);

			/*
			 * Detect whether it sorts correctly relative to existing
			 * even-numbered labels of the enum.  We can ignore existing
			 * labels with odd Oids, since a comparison involving one of those
			 * will not take the fast path anyway.
			 */
			sorts_ok = true;
			for (i = 0; i < nelems; i++)
			{
				HeapTuple	exists_tup = existing[i];
				Form_pg_enum exists_en = (Form_pg_enum) GETSTRUCT(exists_tup);
				Oid			exists_oid = exists_en->oid;

				if (exists_oid & 1)
					continue;	/* ignore odd Oids */

				if (exists_en->enumsortorder < newelemorder)
				{
					/* should sort before */
					if (exists_oid >= newOid)
					{
						sorts_ok = false;
						break;
					}
				}
				else
				{
					/* should sort after */
					if (exists_oid <= newOid)
					{
						sorts_ok = false;
						break;
					}
				}
			}

			if (sorts_ok)
			{
				/* If it's even and sorts OK, we're done. */
				if ((newOid & 1) == 0)
					break;

				/*
				 * If it's odd, and sorts OK, loop back to get another OID and
				 * try again.  Probably, the next available even OID will sort
				 * correctly too, so it's worth trying.
				 */
			}
			else
			{
				/*
				 * If it's odd, and does not sort correctly, we're done.
				 * (Probably, the next available even OID would sort
				 * incorrectly too, so no point in trying again.)
				 */
				if (newOid & 1)
					break;

				/*
				 * If it's even, and does not sort correctly, loop back to get
				 * another OID and try again.  (We *must* reject this case.)
				 */
			}
		}
	}

	/* Done with info about existing members */
	pfree(existing);
	ReleaseCatCacheList(list);

	/* Create the new pg_enum entry */
	memset(nulls, false, sizeof(nulls));
	values[Anum_pg_enum_oid - 1] = ObjectIdGetDatum(newOid);
	values[Anum_pg_enum_enumtypid - 1] = ObjectIdGetDatum(enumTypeOid);
	values[Anum_pg_enum_enumsortorder - 1] = Float4GetDatum(newelemorder);
	namestrcpy(&enumlabel, newVal);
	values[Anum_pg_enum_enumlabel - 1] = NameGetDatum(&enumlabel);
	enum_tup = heap_form_tuple(RelationGetDescr(pg_enum), values, nulls);
	CatalogTupleInsert(pg_enum, enum_tup);
	heap_freetuple(enum_tup);

	table_close(pg_enum, RowExclusiveLock);

	/*
	 * If the enum type itself is uncommitted, we need not enter the new enum
	 * value into uncommitted_enum_values, because the type won't survive if
	 * the value doesn't.  (This is basically the same reasoning as for values
	 * made directly by CREATE TYPE AS ENUM.)  However, apply this rule only
	 * when we are not inside a subtransaction; if we're more deeply nested
	 * than the CREATE TYPE then the conclusion doesn't hold.  We could expend
	 * more effort to track the subtransaction level of CREATE TYPE, but for
	 * now we're only concerned about making the world safe for pg_dump in
	 * binary upgrade mode, and that won't use subtransactions.
	 */
	if (GetCurrentTransactionNestLevel() == 1 &&
		EnumTypeUncommitted(enumTypeOid))
		return;

	/* Set up the uncommitted values table if not already done in this tx */
	if (uncommitted_enum_values == NULL)
		init_uncommitted_enum_values();

	/* Add the new value to the table */
	(void) hash_search(uncommitted_enum_values, &newOid, HASH_ENTER, NULL);
}


/*
 * RenameEnumLabel
 *		Rename a label in an enum set.
 */
void
RenameEnumLabel(Oid enumTypeOid,
				const char *oldVal,
				const char *newVal)
{
	Relation	pg_enum;
	HeapTuple	enum_tup;
	Form_pg_enum en;
	CatCList   *list;
	int			nelems;
	HeapTuple	old_tup;
	bool		found_new;
	int			i;

	/* check length of new label is ok */
	if (strlen(newVal) > (NAMEDATALEN - 1))
		ereport(ERROR,
				(errcode(ERRCODE_INVALID_NAME),
				 errmsg("invalid enum label \"%s\"", newVal),
				 errdetail("Labels must be %d bytes or less.",
						   NAMEDATALEN - 1)));

	/*
	 * Acquire a lock on the enum type, which we won't release until commit.
	 * This ensures that two backends aren't concurrently modifying the same
	 * enum type.  Since we are not changing the type's sort order, this is
	 * probably not really necessary, but there seems no reason not to take
	 * the lock to be sure.
	 */
	LockDatabaseObject(TypeRelationId, enumTypeOid, 0, ExclusiveLock);

	pg_enum = table_open(EnumRelationId, RowExclusiveLock);

	/* Get the list of existing members of the enum */
	list = SearchSysCacheList1(ENUMTYPOIDNAME,
							   ObjectIdGetDatum(enumTypeOid));
	nelems = list->n_members;

	/*
	 * Locate the element to rename and check if the new label is already in
	 * use.  (The unique index on pg_enum would catch that anyway, but we
	 * prefer a friendlier error message.)
	 */
	old_tup = NULL;
	found_new = false;
	for (i = 0; i < nelems; i++)
	{
		enum_tup = &(list->members[i]->tuple);
		en = (Form_pg_enum) GETSTRUCT(enum_tup);
		if (strcmp(NameStr(en->enumlabel), oldVal) == 0)
			old_tup = enum_tup;
		if (strcmp(NameStr(en->enumlabel), newVal) == 0)
			found_new = true;
	}
	if (!old_tup)
		ereport(ERROR,
				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
				 errmsg("\"%s\" is not an existing enum label",
						oldVal)));
	if (found_new)
		ereport(ERROR,
				(errcode(ERRCODE_DUPLICATE_OBJECT),
				 errmsg("enum label \"%s\" already exists",
						newVal)));

	/* OK, make a writable copy of old tuple */
	enum_tup = heap_copytuple(old_tup);
	en = (Form_pg_enum) GETSTRUCT(enum_tup);

	ReleaseCatCacheList(list);

	/* Update the pg_enum entry */
	namestrcpy(&en->enumlabel, newVal);
	CatalogTupleUpdate(pg_enum, &enum_tup->t_self, enum_tup);
	heap_freetuple(enum_tup);

	table_close(pg_enum, RowExclusiveLock);
}


/*
 * Test if the given type OID is in the table of uncommitted enum types.
 */
static bool
EnumTypeUncommitted(Oid typ_id)
{
	bool		found;

	/* If we've made no uncommitted types table, it's not in the table */
	if (uncommitted_enum_types == NULL)
		return false;

	/* Else, is it in the table? */
	(void) hash_search(uncommitted_enum_types, &typ_id, HASH_FIND, &found);
	return found;
}


/*
 * Test if the given enum value is in the table of uncommitted enum values.
 */
bool
EnumUncommitted(Oid enum_id)
{
	bool		found;

	/* If we've made no uncommitted values table, it's not in the table */
	if (uncommitted_enum_values == NULL)
		return false;

	/* Else, is it in the table? */
	(void) hash_search(uncommitted_enum_values, &enum_id, HASH_FIND, &found);
	return found;
}


/*
 * Clean up enum stuff after end of top-level transaction.
 */
void
AtEOXact_Enum(void)
{
	/*
	 * Reset the uncommitted tables, as all our tuples are now committed. The
	 * memory will go away automatically when TopTransactionContext is freed;
	 * it's sufficient to clear our pointers.
	 */
	uncommitted_enum_types = NULL;
	uncommitted_enum_values = NULL;
}


/*
 * RenumberEnumType
 *		Renumber existing enum elements to have sort positions 1..n.
 *
 * We avoid doing this unless absolutely necessary; in most installations
 * it will never happen.  The reason is that updating existing pg_enum
 * entries creates hazards for other backends that are concurrently reading
 * pg_enum.  Although system catalog scans now use MVCC semantics, the
 * syscache machinery might read different pg_enum entries under different
 * snapshots, so some other backend might get confused about the proper
 * ordering if a concurrent renumbering occurs.
 *
 * We therefore make the following choices:
 *
 * 1. Any code that is interested in the enumsortorder values MUST read
 * all the relevant pg_enum entries with a single MVCC snapshot, or else
 * acquire lock on the enum type to prevent concurrent execution of
 * AddEnumLabel().
 *
 * 2. Code that is not examining enumsortorder can use a syscache
 * (for example, enum_in and enum_out do so).
 */
static void
RenumberEnumType(Relation pg_enum, HeapTuple *existing, int nelems)
{
	int			i;

	/*
	 * We should only need to increase existing elements' enumsortorders,
	 * never decrease them.  Therefore, work from the end backwards, to avoid
	 * unwanted uniqueness violations.
	 */
	for (i = nelems - 1; i >= 0; i--)
	{
		HeapTuple	newtup;
		Form_pg_enum en;
		float4		newsortorder;

		newtup = heap_copytuple(existing[i]);
		en = (Form_pg_enum) GETSTRUCT(newtup);

		newsortorder = i + 1;
		if (en->enumsortorder != newsortorder)
		{
			en->enumsortorder = newsortorder;

			CatalogTupleUpdate(pg_enum, &newtup->t_self, newtup);
		}

		heap_freetuple(newtup);
	}

	/* Make the updates visible */
	CommandCounterIncrement();
}


/* qsort comparison function for tuples by sort order */
static int
sort_order_cmp(const void *p1, const void *p2)
{
	HeapTuple	v1 = *((const HeapTuple *) p1);
	HeapTuple	v2 = *((const HeapTuple *) p2);
	Form_pg_enum en1 = (Form_pg_enum) GETSTRUCT(v1);
	Form_pg_enum en2 = (Form_pg_enum) GETSTRUCT(v2);

	if (en1->enumsortorder < en2->enumsortorder)
		return -1;
	else if (en1->enumsortorder > en2->enumsortorder)
		return 1;
	else
		return 0;
}

Size
EstimateUncommittedEnumsSpace(void)
{
	size_t		entries = 0;

	if (uncommitted_enum_types)
		entries += hash_get_num_entries(uncommitted_enum_types);
	if (uncommitted_enum_values)
		entries += hash_get_num_entries(uncommitted_enum_values);

	/* Add two for the terminators. */
	return sizeof(Oid) * (entries + 2);
}

void
SerializeUncommittedEnums(void *space, Size size)
{
	Oid		   *serialized = (Oid *) space;

	/*
	 * Make sure the hash tables haven't changed in size since the caller
	 * reserved the space.
	 */
	Assert(size == EstimateUncommittedEnumsSpace());

	/* Write out all the OIDs from the types hash table, if there is one. */
	if (uncommitted_enum_types)
	{
		HASH_SEQ_STATUS status;
		Oid		   *value;

		hash_seq_init(&status, uncommitted_enum_types);
		while ((value = (Oid *) hash_seq_search(&status)))
			*serialized++ = *value;
	}

	/* Write out the terminator. */
	*serialized++ = InvalidOid;

	/* Write out all the OIDs from the values hash table, if there is one. */
	if (uncommitted_enum_values)
	{
		HASH_SEQ_STATUS status;
		Oid		   *value;

		hash_seq_init(&status, uncommitted_enum_values);
		while ((value = (Oid *) hash_seq_search(&status)))
			*serialized++ = *value;
	}

	/* Write out the terminator. */
	*serialized++ = InvalidOid;

	/*
	 * Make sure the amount of space we actually used matches what was
	 * estimated.
	 */
	Assert((char *) serialized == ((char *) space) + size);
}

void
RestoreUncommittedEnums(void *space)
{
	Oid		   *serialized = (Oid *) space;

	Assert(!uncommitted_enum_types);
	Assert(!uncommitted_enum_values);

	/*
	 * If either list is empty then don't even bother to create that hash
	 * table.  This is the common case, since most transactions don't create
	 * or alter enums.
	 */
	if (OidIsValid(*serialized))
	{
		/* Read all the types into a new hash table. */
		init_uncommitted_enum_types();
		do
		{
			(void) hash_search(uncommitted_enum_types, serialized++,
							   HASH_ENTER, NULL);
		} while (OidIsValid(*serialized));
	}
	serialized++;
	if (OidIsValid(*serialized))
	{
		/* Read all the values into a new hash table. */
		init_uncommitted_enum_values();
		do
		{
			(void) hash_search(uncommitted_enum_values, serialized++,
							   HASH_ENTER, NULL);
		} while (OidIsValid(*serialized));
	}
}