Revert to 9.6 treatment of ALTER TYPE enumtype ADD VALUE.

This reverts commit 15bc038, along with the followon commits 1635e80
and 984c920 that tried to clean up the problems exposed by bug #14825.
The result was incomplete because it failed to address parallel-query
requirements.  With 10.0 release so close upon us, now does not seem like
the time to be adding more code to fix that.  I hope we can un-revert this
code and add the missing parallel query support during the v11 cycle.

Back-patch to v10.

Discussion: https://postgr.es/m/20170922185904.1448.16585@wrigleys.postgresql.org

Author: tglsfdc

doc/src/sgml/ref/alter_type.sgml

@@ -290,9 +290,8 @@ ALTER TYPE <replaceable class="PARAMETER">name</replaceable> RENAME VALUE <repla
   <title>Notes</title>
 
   <para>
-   If <command>ALTER TYPE ... ADD VALUE</> (the form that adds a new value to
-   an enum type) is executed inside a transaction block, the new value cannot
-   be used until after the transaction has been committed.
+   <command>ALTER TYPE ... ADD VALUE</> (the form that adds a new value to an
+   enum type) cannot be executed inside a transaction block.
   </para>
 
   <para>

src/backend/access/transam/xact.c

@@ -32,7 +32,6 @@
 #include "access/xlogutils.h"
 #include "catalog/catalog.h"
 #include "catalog/namespace.h"
-#include "catalog/pg_enum.h"
 #include "catalog/storage.h"
 #include "commands/async.h"
 #include "commands/tablecmds.h"
@@ -2129,7 +2128,6 @@ CommitTransaction(void)
 	AtCommit_Notify();
 	AtEOXact_GUC(true, 1);
 	AtEOXact_SPI(true);
-	AtEOXact_Enum();
 	AtEOXact_on_commit_actions(true);
 	AtEOXact_Namespace(true, is_parallel_worker);
 	AtEOXact_SMgr();
@@ -2408,7 +2406,6 @@ PrepareTransaction(void)
 	/* PREPARE acts the same as COMMIT as far as GUC is concerned */
 	AtEOXact_GUC(true, 1);
 	AtEOXact_SPI(true);
-	AtEOXact_Enum();
 	AtEOXact_on_commit_actions(true);
 	AtEOXact_Namespace(true, false);
 	AtEOXact_SMgr();
@@ -2611,7 +2608,6 @@ AbortTransaction(void)
 
 		AtEOXact_GUC(false, 1);
 		AtEOXact_SPI(false);
-		AtEOXact_Enum();
 		AtEOXact_on_commit_actions(false);
 		AtEOXact_Namespace(false, is_parallel_worker);
 		AtEOXact_SMgr();

src/backend/catalog/pg_enum.c

@@ -28,26 +28,13 @@
 #include "utils/builtins.h"
 #include "utils/catcache.h"
 #include "utils/fmgroids.h"
-#include "utils/hsearch.h"
-#include "utils/memutils.h"
 #include "utils/syscache.h"
 #include "utils/tqual.h"
 
 
 /* Potentially set by pg_upgrade_support functions */
 Oid			binary_upgrade_next_pg_enum_oid = InvalidOid;
 
-/*
- * Hash table of enum value OIDs created during the current transaction by
- * AddEnumLabel.  We disallow using these values 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* blacklisted; we assume those are created during
- * CREATE TYPE, so they can't go away unless the enum type itself does.
- */
-static HTAB *enum_blacklist = NULL;
-
 static void RenumberEnumType(Relation pg_enum, HeapTuple *existing, int nelems);
 static int	sort_order_cmp(const void *p1, const void *p2);
 
@@ -473,24 +460,6 @@ AddEnumLabel(Oid enumTypeOid,
 	heap_freetuple(enum_tup);
 
 	heap_close(pg_enum, RowExclusiveLock);
-
-	/* Set up the blacklist hash if not already done in this transaction */
-	if (enum_blacklist == NULL)
-	{
-		HASHCTL		hash_ctl;
-
-		memset(&hash_ctl, 0, sizeof(hash_ctl));
-		hash_ctl.keysize = sizeof(Oid);
-		hash_ctl.entrysize = sizeof(Oid);
-		hash_ctl.hcxt = TopTransactionContext;
-		enum_blacklist = hash_create("Enum value blacklist",
-									 32,
-									 &hash_ctl,
-									 HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
-	}
-
-	/* Add the new value to the blacklist */
-	(void) hash_search(enum_blacklist, &newOid, HASH_ENTER, NULL);
 }
 
 
@@ -578,39 +547,6 @@ RenameEnumLabel(Oid enumTypeOid,
 }
 
 
-/*
- * Test if the given enum value is on the blacklist
- */
-bool
-EnumBlacklisted(Oid enum_id)
-{
-	bool		found;
-
-	/* If we've made no blacklist table, all values are safe */
-	if (enum_blacklist == NULL)
-		return false;
-
-	/* Else, is it in the table? */
-	(void) hash_search(enum_blacklist, &enum_id, HASH_FIND, &found);
-	return found;
-}
-
-
-/*
- * Clean up enum stuff after end of top-level transaction.
- */
-void
-AtEOXact_Enum(void)
-{
-	/*
-	 * Reset the blacklist table, as all our enum values are now committed.
-	 * The memory will go away automatically when TopTransactionContext is
-	 * freed; it's sufficient to clear our pointer.
-	 */
-	enum_blacklist = NULL;
-}
-
-
 /*
  * RenumberEnumType
  *		Renumber existing enum elements to have sort positions 1..n.

src/backend/commands/typecmds.c

@@ -1222,10 +1222,10 @@ DefineEnum(CreateEnumStmt *stmt)
 
 /*
  * AlterEnum
- *		Adds a new label to an existing enum.
+ *		ALTER TYPE on an enum.
  */
 ObjectAddress
-AlterEnum(AlterEnumStmt *stmt)
+AlterEnum(AlterEnumStmt *stmt, bool isTopLevel)
 {
 	Oid			enum_type_oid;
 	TypeName   *typename;
@@ -1243,8 +1243,6 @@ AlterEnum(AlterEnumStmt *stmt)
 	/* Check it's an enum and check user has permission to ALTER the enum */
 	checkEnumOwner(tup);
 
-	ReleaseSysCache(tup);
-
 	if (stmt->oldVal)
 	{
 		/* Rename an existing label */
@@ -1253,6 +1251,27 @@ AlterEnum(AlterEnumStmt *stmt)
 	else
 	{
 		/* Add a new label */
+
+		/*
+		 * Ordinarily we disallow adding values within transaction blocks,
+		 * because we can't cope with enum OID values getting into indexes and
+		 * then having their defining pg_enum entries go away.  However, it's
+		 * okay if the enum type was created in the current transaction, since
+		 * then there can be no such indexes that wouldn't themselves go away
+		 * on rollback.  (We support this case because pg_dump
+		 * --binary-upgrade needs it.)  We test this by seeing if the pg_type
+		 * row has xmin == current XID and is not HEAP_UPDATED.  If it is
+		 * HEAP_UPDATED, we can't be sure whether the type was created or only
+		 * modified in this xact.  So we are disallowing some cases that could
+		 * theoretically be safe; but fortunately pg_dump only needs the
+		 * simplest case.
+		 */
+		if (HeapTupleHeaderGetXmin(tup->t_data) == GetCurrentTransactionId() &&
+			!(tup->t_data->t_infomask & HEAP_UPDATED))
+			 /* safe to do inside transaction block */ ;
+		else
+			PreventTransactionChain(isTopLevel, "ALTER TYPE ... ADD");
+
 		AddEnumLabel(enum_type_oid, stmt->newVal,
 					 stmt->newValNeighbor, stmt->newValIsAfter,
 					 stmt->skipIfNewValExists);
@@ -1262,6 +1281,8 @@ AlterEnum(AlterEnumStmt *stmt)
 
 	ObjectAddressSet(address, TypeRelationId, enum_type_oid);
 
+	ReleaseSysCache(tup);
+
 	return address;
 }
 

src/backend/tcop/utility.c

@@ -1412,7 +1412,7 @@ ProcessUtilitySlow(ParseState *pstate,
 				break;
 
 			case T_AlterEnumStmt:	/* ALTER TYPE (enum) */
-				address = AlterEnum((AlterEnumStmt *) parsetree);
+				address = AlterEnum((AlterEnumStmt *) parsetree, isTopLevel);
 				break;
 
 			case T_ViewStmt:	/* CREATE VIEW */

src/backend/utils/adt/enum.c

@@ -19,7 +19,6 @@
 #include "catalog/indexing.h"
 #include "catalog/pg_enum.h"
 #include "libpq/pqformat.h"
-#include "storage/procarray.h"
 #include "utils/array.h"
 #include "utils/builtins.h"
 #include "utils/fmgroids.h"
@@ -32,79 +31,6 @@ static Oid	enum_endpoint(Oid enumtypoid, ScanDirection direction);
 static ArrayType *enum_range_internal(Oid enumtypoid, Oid lower, Oid upper);
 
 
-/*
- * Disallow use of an uncommitted pg_enum tuple.
- *
- * We need to make sure that uncommitted enum values don't get into indexes.
- * If they did, and if we then rolled back the pg_enum addition, we'd have
- * broken the index because value comparisons will not work reliably without
- * an underlying pg_enum entry.  (Note that removal of the heap entry
- * containing an enum value is not sufficient to ensure that it doesn't appear
- * in upper levels of indexes.)  To do this we prevent an uncommitted row from
- * being used for any SQL-level purpose.  This is stronger than necessary,
- * since the value might not be getting inserted into a table or there might
- * be no index on its column, but it's easy to enforce centrally.
- *
- * However, it's okay to allow use of uncommitted values belonging to enum
- * types that were themselves created in the same transaction, because then
- * any such index would also be new and would go away altogether on rollback.
- * We don't implement that fully right now, but we do allow free use of enum
- * values created during CREATE TYPE AS ENUM, which are surely of the same
- * lifespan as the enum type.  (This case is required by "pg_restore -1".)
- * Values added by ALTER TYPE ADD VALUE are currently restricted, but could
- * be allowed if the enum type could be proven to have been created earlier
- * in the same transaction.  (Note that comparing tuple xmins would not work
- * for that, because the type tuple might have been updated in the current
- * transaction.  Subtransactions also create hazards to be accounted for.)
- *
- * This function needs to be called (directly or indirectly) in any of the
- * functions below that could return an enum value to SQL operations.
- */
-static void
-check_safe_enum_use(HeapTuple enumval_tup)
-{
-	TransactionId xmin;
-	Form_pg_enum en;
-
-	/*
-	 * If the row is hinted as committed, it's surely safe.  This provides a
-	 * fast path for all normal use-cases.
-	 */
-	if (HeapTupleHeaderXminCommitted(enumval_tup->t_data))
-		return;
-
-	/*
-	 * Usually, a row would get hinted as committed when it's read or loaded
-	 * into syscache; but just in case not, let's check the xmin directly.
-	 */
-	xmin = HeapTupleHeaderGetXmin(enumval_tup->t_data);
-	if (!TransactionIdIsInProgress(xmin) &&
-		TransactionIdDidCommit(xmin))
-		return;
-
-	/*
-	 * Check if the enum value is blacklisted.  If not, it's safe, because it
-	 * was made during CREATE TYPE AS ENUM and can't be shorter-lived than its
-	 * owning type.  (This'd also be false for values made by other
-	 * transactions; but the previous tests should have handled all of those.)
-	 */
-	if (!EnumBlacklisted(HeapTupleGetOid(enumval_tup)))
-		return;
-
-	/*
-	 * There might well be other tests we could do here to narrow down the
-	 * unsafe conditions, but for now just raise an exception.
-	 */
-	en = (Form_pg_enum) GETSTRUCT(enumval_tup);
-	ereport(ERROR,
-			(errcode(ERRCODE_UNSAFE_NEW_ENUM_VALUE_USAGE),
-			 errmsg("unsafe use of new value \"%s\" of enum type %s",
-					NameStr(en->enumlabel),
-					format_type_be(en->enumtypid)),
-			 errhint("New enum values must be committed before they can be used.")));
-}
-
-
 /* Basic I/O support */
 
 Datum
@@ -133,9 +59,6 @@ enum_in(PG_FUNCTION_ARGS)
 						format_type_be(enumtypoid),
 						name)));
 
-	/* check it's safe to use in SQL */
-	check_safe_enum_use(tup);
-
 	/*
 	 * This comes from pg_enum.oid and stores system oids in user tables. This
 	 * oid must be preserved by binary upgrades.
@@ -201,9 +124,6 @@ enum_recv(PG_FUNCTION_ARGS)
 						format_type_be(enumtypoid),
 						name)));
 
-	/* check it's safe to use in SQL */
-	check_safe_enum_use(tup);
-
 	enumoid = HeapTupleGetOid(tup);
 
 	ReleaseSysCache(tup);
@@ -411,16 +331,9 @@ enum_endpoint(Oid enumtypoid, ScanDirection direction)
 
 	enum_tuple = systable_getnext_ordered(enum_scan, direction);
 	if (HeapTupleIsValid(enum_tuple))
-	{
-		/* check it's safe to use in SQL */
-		check_safe_enum_use(enum_tuple);
 		minmax = HeapTupleGetOid(enum_tuple);
-	}
 	else
-	{
-		/* should only happen with an empty enum */
 		minmax = InvalidOid;
-	}
 
 	systable_endscan_ordered(enum_scan);
 	index_close(enum_idx, AccessShareLock);
@@ -581,9 +494,6 @@ enum_range_internal(Oid enumtypoid, Oid lower, Oid upper)
 
 		if (left_found)
 		{
-			/* check it's safe to use in SQL */
-			check_safe_enum_use(enum_tuple);
-
 			if (cnt >= max)
 			{
 				max *= 2;

src/backend/utils/errcodes.txt

@@ -400,7 +400,6 @@ Section: Class 55 - Object Not In Prerequisite State
 55006    E    ERRCODE_OBJECT_IN_USE                                          object_in_use
 55P02    E    ERRCODE_CANT_CHANGE_RUNTIME_PARAM                              cant_change_runtime_param
 55P03    E    ERRCODE_LOCK_NOT_AVAILABLE                                     lock_not_available
-55P04    E    ERRCODE_UNSAFE_NEW_ENUM_VALUE_USAGE                            unsafe_new_enum_value_usage
 
 Section: Class 57 - Operator Intervention
 

src/include/catalog/pg_enum.h

@@ -69,7 +69,5 @@ extern void AddEnumLabel(Oid enumTypeOid, const char *newVal,
 			 bool skipIfExists);
 extern void RenameEnumLabel(Oid enumTypeOid,
 				const char *oldVal, const char *newVal);
-extern bool EnumBlacklisted(Oid enum_id);
-extern void AtEOXact_Enum(void);
 
 #endif							/* PG_ENUM_H */

src/include/commands/typecmds.h

@@ -26,7 +26,7 @@ extern void RemoveTypeById(Oid typeOid);
 extern ObjectAddress DefineDomain(CreateDomainStmt *stmt);
 extern ObjectAddress DefineEnum(CreateEnumStmt *stmt);
 extern ObjectAddress DefineRange(CreateRangeStmt *stmt);
-extern ObjectAddress AlterEnum(AlterEnumStmt *stmt);
+extern ObjectAddress AlterEnum(AlterEnumStmt *stmt, bool isTopLevel);
 extern ObjectAddress DefineCompositeType(RangeVar *typevar, List *coldeflist);
 extern Oid	AssignTypeArrayOid(void);