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, 51 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.python/py-parameter.exp b/gdb/testsuite/gdb.python/py-parameter.exp
index ef24dbe..5543b21 100644
--- a/gdb/testsuite/gdb.python/py-parameter.exp
+++ b/gdb/testsuite/gdb.python/py-parameter.exp
@@ -37,6 +37,57 @@ if { [is_remote host] } {
 }
 gdb_test "python print (gdb.parameter ('directories'))" $directories
 
+# Check we can correctly read the data-directory parameter.  First,
+# grab the value as read directly from the GDB CLI.
+set dd ""
+gdb_test_multiple "show data-directory" \
+    "find the initial data-directory value" {
+	-re -wrap "GDB's data directory is \"(\[^\r\n\]+)\"\\." {
+	    set dd $expect_out(1,string)
+	    pass $gdb_test_name
+	}
+    }
+
+# Now print the data-directory from Python.
+gdb_test "python print (gdb.parameter ('data-directory'))" $dd
+
+# Next change the data-directory to a relative path.  Internally GDB
+# will resolve this to an absolute path, which Python should then see.
+#
+# GDB is currently running in '...../build/gdb/testsuite/' and the
+# test output is being written to:
+#   ...../build/gdb/testsuite/outputs/gdb.python/py-parameter/
+#
+# So create the relative path './outputs/gdb.python/py-parameter/' and
+# set the data-directory to that, we should then see the absolute path.
+
+set abs_path_to_output_dir [standard_output_file ""]
+set abs_path_to_cwd $objdir
+set rel_path_to_output_dir \
+    [file join "." [string replace ${abs_path_to_output_dir} 0 \
+			[string length ${abs_path_to_cwd}] ""]]
+gdb_test_no_output "set data-directory ${rel_path_to_output_dir}"
+
+gdb_test "python print (gdb.parameter ('data-directory'))" \
+    ${abs_path_to_output_dir} \
+    "python sees absolute version of data-directory path"
+
+# While we're here, check we see the correct path at GDB's CLI.
+gdb_test "show data-directory" \
+    "GDB's data directory is \"${abs_path_to_output_dir}\"\\." \
+    "check modified data-directory at the CLI"
+
+# Now lets set the data-directory back to what it was initially.
+gdb_test_no_output "set data-directory ${dd}"
+
+# And check we see the restored value at CLI and from Python.
+gdb_test "show data-directory" \
+    "GDB's data directory is \"${dd}\"\\." \
+    "check original data-directory was restored at the CLI"
+
+gdb_test "python print (gdb.parameter ('data-directory'))" ${dd} \
+    "python sees restored data-directory value"
+
 # Test a simple boolean parameter.
 with_test_prefix "boolean parameter" {
     gdb_test_multiline "Simple gdb booleanparameter" \