Allow LOCK TABLE .. ROW EXCLUSIVE MODE with INSERT

INSERT acquires RowExclusiveLock during normal operation and therefore
it makes sense to allow LOCK TABLE .. ROW EXCLUSIVE MODE to be executed
by users who have INSERT rights on a table (even if they don't have
UPDATE or DELETE).

Not back-patching this as it's a behavior change which, strictly
speaking, loosens security restrictions.

Per discussion with Tom and Robert (circa 2013).

doc/src/sgml/ref/lock.sgml

@@ -161,9 +161,11 @@ LOCK [ TABLE ] [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
 
    <para>
     <literal>LOCK TABLE ... IN ACCESS SHARE MODE</> requires <literal>SELECT</>
-    privileges on the target table.  All other forms of <command>LOCK</>
-    require table-level <literal>UPDATE</>, <literal>DELETE</>, or
-    <literal>TRUNCATE</> privileges.
+    privileges on the target table.  <literal>LOCK TABLE ... IN ROW EXCLUSIVE
+    MODE</> requires <literal>INSERT</>, <literal>UPDATE</>, <literal>DELETE</>,
+    or <literal>TRUNCATE</> privileges on the target table. All other forms of
+    <command>LOCK</> require table-level <literal>UPDATE</>, <literal>DELETE</>,
+    or <literal>TRUNCATE</> privileges.
    </para>
 
    <para>

src/backend/commands/lockcmds.c

@@ -169,13 +169,17 @@ static AclResult
 LockTableAclCheck(Oid reloid, LOCKMODE lockmode)
 {
 	AclResult	aclresult;
+	AclMode		aclmask;
 
 	/* Verify adequate privilege */
 	if (lockmode == AccessShareLock)
-		aclresult = pg_class_aclcheck(reloid, GetUserId(),
-									  ACL_SELECT);
+		aclmask = ACL_SELECT;
+	else if (lockmode == RowExclusiveLock)
+		aclmask = ACL_INSERT | ACL_UPDATE | ACL_DELETE | ACL_TRUNCATE;
 	else
-		aclresult = pg_class_aclcheck(reloid, GetUserId(),
-									  ACL_UPDATE | ACL_DELETE | ACL_TRUNCATE);
+		aclmask = ACL_UPDATE | ACL_DELETE | ACL_TRUNCATE;
+
+	aclresult = pg_class_aclcheck(reloid, GetUserId(), aclmask);
+
 	return aclresult;
 }