Fix initdb to reject a relative path for -X (--xlogdir) argument. This

doesn't work, and the real reason why not is it's unclear where the path
is relative to (initdb's CWD, or the data directory?).  We could make an
arbitrary decision, but it seems best to make the user be unambiguous.
Per gripe from Devrim.

src/bin/initdb/initdb.c

@@ -42,7 +42,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  * Portions taken from FreeBSD.
  *
- * $PostgreSQL: pgsql/src/bin/initdb/initdb.c,v 1.155 2008/02/29 23:31:20 adunstan Exp $
+ * $PostgreSQL: pgsql/src/bin/initdb/initdb.c,v 1.156 2008/06/02 03:48:00 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -3002,8 +3002,13 @@ main(int argc, char *argv[])
 	{
 		char	   *linkloc;
 
-		linkloc = (char *) pg_malloc(strlen(pg_data) + 8 + 2);
-		sprintf(linkloc, "%s/pg_xlog", pg_data);
+		/* clean up xlog directory name, check it's absolute */
+		canonicalize_path(xlog_dir);
+		if (!is_absolute_path(xlog_dir))
+		{
+			fprintf(stderr, _("%s: xlog directory location must be an absolute path\n"), progname);
+			exit_nicely();
+		}
 
 		/* check if the specified xlog directory is empty */
 		switch (check_data_dir(xlog_dir))
@@ -3021,9 +3026,7 @@ main(int argc, char *argv[])
 					exit_nicely();
 				}
 				else
-				{
 					check_ok();
-				}
 
 				made_new_xlogdir = true;
 				break;
@@ -3053,7 +3056,7 @@ main(int argc, char *argv[])
 						_("If you want to store the transaction log there, either\n"
 						  "remove or empty the directory \"%s\".\n"),
 						xlog_dir);
-				exit(1);		/* no further message needed */
+				exit_nicely();
 
 			default:
 				/* Trouble accessing directory */
@@ -3062,6 +3065,10 @@ main(int argc, char *argv[])
 				exit_nicely();
 		}
 
+		/* form name of the place where the symlink must go */
+		linkloc = (char *) pg_malloc(strlen(pg_data) + 8 + 1);
+		sprintf(linkloc, "%s/pg_xlog", pg_data);
+
 #ifdef HAVE_SYMLINK
 		if (symlink(xlog_dir, linkloc) != 0)
 		{