Use access() to check file existence in GetNewRelFileNode()

Previous code used BasicOpenFile() and close() just to check for a file
collision, while there is no need to hold open a file descriptor but
that's an overkill here.

Author: Paul Guo
Reviewed-by: Peter Eisentraut, Michael Paquier
Discussion: https://postgr.es/m/CABQrizcUtiHaquxK=d4etBX8GF9kbZB50Nt1gO9_aN-e9SptyQ@mail.gmail.com

src/backend/catalog/catalog.c

@@ -397,7 +397,6 @@ GetNewRelFileNode(Oid reltablespace, Relation pg_class, char relpersistence)
 {
 	RelFileNodeBackend rnode;
 	char	   *rpath;
-	int			fd;
 	bool		collides;
 	BackendId	backend;
 
@@ -445,12 +444,10 @@ GetNewRelFileNode(Oid reltablespace, Relation pg_class, char relpersistence)
 
 		/* Check for existing file of same name */
 		rpath = relpath(rnode, MAIN_FORKNUM);
-		fd = BasicOpenFile(rpath, O_RDONLY | PG_BINARY);
 
-		if (fd >= 0)
+		if (access(rpath, F_OK) == 0)
 		{
 			/* definite collision */
-			close(fd);
 			collides = true;
 		}
 		else
@@ -458,13 +455,9 @@ GetNewRelFileNode(Oid reltablespace, Relation pg_class, char relpersistence)
 			/*
 			 * Here we have a little bit of a dilemma: if errno is something
 			 * other than ENOENT, should we declare a collision and loop? In
-			 * particular one might think this advisable for, say, EPERM.
-			 * However there really shouldn't be any unreadable files in a
-			 * tablespace directory, and if the EPERM is actually complaining
-			 * that we can't read the directory itself, we'd be in an infinite
-			 * loop.  In practice it seems best to go ahead regardless of the
-			 * errno.  If there is a colliding file we will get an smgr
-			 * failure when we attempt to create the new relation file.
+			 * practice it seems best to go ahead regardless of the errno.  If
+			 * there is a colliding file we will get an smgr failure when we
+			 * attempt to create the new relation file.
 			 */
 			collides = false;
 		}