gdb/py: fix gdb.parameter('data-directory')

It was reported on IRC that using gdb.parameter('data-directory')
doesn't work correctly.

The problem is that the data directory is stored in 'gdb_datadir',
however the set/show command is associated with a temporary
'staged_gdb_datadir'.

When the user does 'set data-directory VALUE', the VALUE is stored in
'staged_gdb_datadir' by GDB, then set_gdb_datadir is called.  This in
turn calls set_gdb_data_directory to copy the value from
staged_gdb_datadir into gdb_datadir.

However, set_gdb_data_directory will resolve relative paths, so the
value stored in gdb_datadir might not match the value in
staged_gdb_datadir.

The Python gdb.parameter API fetches the parameter values by accessing
the variable associated with the show command, so in this case
staged_gdb_datadir.  This causes two problems:

1. Initially staged_gdb_datadir is NULL, and remains as such until the
user does 'set data-directory VALUE' (which might never happen), but
gdb_datadir starts with GDB's default data-directory value.  So
initially from Python gdb.parameter('data-directory') will return the
empty string, even though at GDB's CLI 'show data-directory' prints a
real path.

2. If the user does 'set data-directory ./some/relative/path', GDB
will resolve the relative path, thus, 'show data-directory' at the CLI
will print an absolute path.  However, the value is staged_gdb_datadir
will still be the relative path, and gdb.parameter('data-directory')
from Python will return the relative path.

In this commit I fix both of these issues by:

1. Initialising the value in staged_gdb_datadir based on the initial
value in gdb_datadir, and

2. In set_gdb_datadir, after calling set_gdb_data_directory, I copy
the value in gdb_datadir back into staged_gdb_datadir.

With these two changes in place the value in staged_gdb_datadir should
always match the value in gdb_datadir, and accessing data-directory
from Python should now work correctly.

gdb/ChangeLog:

	* top.c (staged_gdb_datadir): Update comment.
	(set_gdb_datadir): Copy the value of gdb_datadir back into
	staged_datadir.
	(init_main): Initialise staged_gdb_datadir.

gdb/testsuite/ChangeLog:

	* gdb.python/py-parameter.exp: Add test for reading data-directory
	using gdb.parameter API.

1 files changed, 15 insertions, 2 deletions

diff --git a/gdb/top.c b/gdb/top.c
index 6dd2fe8..c374a8d 100644
--- a/gdb/top.c
+++ b/gdb/top.c
@@ -2111,7 +2111,10 @@ show_exec_done_display_p (struct ui_file *file, int from_tty,
 		    value);
 }
 
-/* New values of the "data-directory" parameter are staged here.  */
+/* New values of the "data-directory" parameter are staged here.
+   Extension languages, for example Python's gdb.parameter API, will read
+   the value directory from this variable, so we must ensure that this
+   always contains the correct value.  */
 static char *staged_gdb_datadir;
 
 /* "set" command for the gdb_datadir configuration variable.  */
@@ -2120,6 +2123,14 @@ static void
 set_gdb_datadir (const char *args, int from_tty, struct cmd_list_element *c)
 {
   set_gdb_data_directory (staged_gdb_datadir);
+
+  /* SET_GDB_DATA_DIRECTORY will resolve relative paths in
+     STAGED_GDB_DATADIR, so we now copy the value from GDB_DATADIR
+     back into STAGED_GDB_DATADIR so the extension languages can read the
+     correct value.  */
+  free (staged_gdb_datadir);
+  staged_gdb_datadir = strdup (gdb_datadir.c_str ());
+
   gdb::observers::gdb_datadir_changed.notify ();
 }
 
@@ -2290,7 +2301,9 @@ Use \"on\" to enable the notification, and \"off\" to disable it."),
 When set, GDB uses the specified path to search for data files."),
 			   set_gdb_datadir, show_gdb_datadir,
 			   &setlist,
-			   &showlist);
+			    &showlist);
+  /* Prime the initial value for data-directory.  */
+  staged_gdb_datadir = strdup (gdb_datadir.c_str ());
 
   add_setshow_auto_boolean_cmd ("interactive-mode", class_support,
 				&interactive_mode, _("\