Rename "pg_xlog" directory to "pg_wal".

"xlog" is not a particularly clear abbreviation for "write-ahead log",
and it sometimes confuses users into believe that the contents of the
"pg_xlog" directory are not critical data, leading to unpleasant
consequences.  So, rename the directory to "pg_wal".

This patch modifies pg_upgrade and pg_basebackup to understand both
the old and new directory layouts; the former is necessary given the
purpose of the tool, while the latter merely avoids an unnecessary
backward-compatibility break.

We may wish to consider renaming other programs, switches, and
functions which still use the old "xlog" naming to also refer to
"wal".  However, that's still under discussion, so let's do just this
much for now.

Discussion: CAB7nPqTeC-8+zux8_-4ZD46V7YPwooeFxgndfsq5Rg8ibLVm1A@mail.gmail.com

Michael Paquier

src/backend/access/transam/xlogarchive.c

@@ -421,7 +421,7 @@ ExecuteRecoveryCommand(char *command, char *commandName, bool failOnSignal)
 /*
  * A file was restored from the archive under a temporary filename (path),
  * and now we want to keep it. Rename it under the permanent filename in
- * in pg_xlog (xlogfname), replacing any existing file with the same name.
+ * in pg_wal (xlogfname), replacing any existing file with the same name.
  */
 void
 KeepFileRestoredFromArchive(char *path, char *xlogfname)