Issue ERROR if FREEZE mode can't be honored by COPY

Previously non-honored FREEZE mode was ignored.  This also issues an
appropriate error message based on the cause of the failure, per
suggestion from Tom.  Additional regression test case added.

doc/src/sgml/ref/copy.sgml

@@ -190,18 +190,14 @@ COPY { <replaceable class="parameter">table_name</replaceable> [ ( <replaceable
       would be after running the <command>VACUUM FREEZE</> command.
       This is intended as a performance option for initial data loading.
       Rows will be frozen only if the table being loaded has been created
-      in the current subtransaction, there are no cursors open and there
-      are no older snapshots held by this transaction. If those conditions
-      are not met the command will continue without error though will not
-      freeze rows. It is also possible in rare cases that the request
-      cannot be honoured for internal reasons, hence <literal>FREEZE</literal>
-      is more of a guideline than a hard rule.
+      or truncated in the current subtransaction, there are no cursors
+      open and there are no older snapshots held by this transaction.
      </para>
      <para>
       Note that all other sessions will immediately be able to see the data
       once it has been successfully loaded. This violates the normal rules
-      of MVCC visibility and by specifying this option the user acknowledges
-      explicitly that this is understood.
+      of MVCC visibility and users specifying should be aware of the
+      potential problems this might cause.
      </para>
     </listitem>
    </varlistentry>

src/backend/commands/copy.c

@@ -1978,8 +1978,6 @@ CopyFrom(CopyState cstate)
 	 * ROLLBACK TO save;
 	 * COPY ...
 	 *
-	 * However this is OK since at worst we will fail to make the optimization.
-	 *
 	 * Also, if the target file is new-in-transaction, we assume that checking
 	 * FSM for free space is a waste of time, even if we must use WAL because
 	 * of archiving.  This could possibly be wrong, but it's unlikely.
@@ -1991,6 +1989,7 @@ CopyFrom(CopyState cstate)
 	 * no additional work to enforce that.
 	 *----------
 	 */
+	/* createSubid is creation check, newRelfilenodeSubid is truncation check */
 	if (cstate->rel->rd_createSubid != InvalidSubTransactionId ||
 		cstate->rel->rd_newRelfilenodeSubid != InvalidSubTransactionId)
 	{
@@ -2006,18 +2005,27 @@ CopyFrom(CopyState cstate)
 		 * after xact cleanup. Note that the stronger test of exactly
 		 * which subtransaction created it is crucial for correctness
 		 * of this optimisation.
-		 *
-		 * As noted above rd_newRelfilenodeSubid is not set in all cases
-		 * where we can apply the optimization, so in those rare cases
-		 * where we cannot honour the request we do so silently.
 		 */
-		if (cstate->freeze &&
-			ThereAreNoPriorRegisteredSnapshots() &&
-			ThereAreNoReadyPortals() &&
-			(cstate->rel->rd_newRelfilenodeSubid == GetCurrentSubTransactionId() ||
-			 cstate->rel->rd_createSubid == GetCurrentSubTransactionId()))
-			hi_options |= HEAP_INSERT_FROZEN;
+		if (cstate->freeze)
+		{
+			if (!ThereAreNoPriorRegisteredSnapshots() || !ThereAreNoReadyPortals())
+				ereport(ERROR,
+						(ERRCODE_INVALID_TRANSACTION_STATE,
+						errmsg("cannot perform FREEZE because of prior transaction activity")));
+
+			if (cstate->rel->rd_createSubid == GetCurrentSubTransactionId() ||
+				cstate->rel->rd_newRelfilenodeSubid == GetCurrentSubTransactionId())
+				hi_options |= HEAP_INSERT_FROZEN;
+			else
+				ereport(ERROR,
+						(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE,
+						errmsg("cannot perform FREEZE because of transaction activity after table creation or truncation")));
+		}
 	}
+	else if (cstate->freeze)
+		ereport(ERROR,
+				(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE,
+				 errmsg("cannot perform FREEZE because the table was not created or truncated in the current transaction")));
 
 	/*
 	 * We need a ResultRelInfo so we can use the regular executor's

src/test/regress/expected/copy2.out

@@ -334,22 +334,20 @@ SELECT * FROM vistest;
 COMMIT;
 TRUNCATE vistest;
 COPY vistest FROM stdin CSV FREEZE;
+ERROR:  cannot perform FREEZE because the table was not created or truncated in the current transaction
+BEGIN;
+TRUNCATE vistest;
+SAVEPOINT s1;
+COPY vistest FROM stdin CSV FREEZE;
+ERROR:  cannot perform FREEZE because of transaction activity after table creation or truncation
+COMMIT;
 BEGIN;
 INSERT INTO vistest VALUES ('z');
 SAVEPOINT s1;
 TRUNCATE vistest;
 ROLLBACK TO SAVEPOINT s1;
 COPY vistest FROM stdin CSV FREEZE;
-SELECT * FROM vistest;
- a  
-----
- p
- g
- z
- d3
- e
-(5 rows)
-
+ERROR:  cannot perform FREEZE because the table was not created or truncated in the current transaction
 COMMIT;
 CREATE FUNCTION truncate_in_subxact() RETURNS VOID AS
 $$

src/test/regress/sql/copy2.sql

@@ -234,6 +234,14 @@ p
 g
 \.
 BEGIN;
+TRUNCATE vistest;
+SAVEPOINT s1;
+COPY vistest FROM stdin CSV FREEZE;
+m
+k
+\.
+COMMIT;
+BEGIN;
 INSERT INTO vistest VALUES ('z');
 SAVEPOINT s1;
 TRUNCATE vistest;
@@ -242,7 +250,6 @@ COPY vistest FROM stdin CSV FREEZE;
 d3
 e
 \.
-SELECT * FROM vistest;
 COMMIT;
 CREATE FUNCTION truncate_in_subxact() RETURNS VOID AS
 $$