Implement "set cwd" command on GDB

This commit adds new "set/show cwd" commands, which are used to
set/show the current working directory of the inferior that will be
started.

The idea here is that "set cwd" will become the de facto way of
setting the inferior's cwd.  Currently, the user can use "cd" for
that, but there are side effects: with "cd", GDB also switches to
another directory, and that can impact the loading of scripts and
other files.  With "set cwd", we separate the logic into a new
command.

To maintain backward compatibility, if the user issues a "cd" command
but doesn't use "set cwd", then the inferior's cwd will still be
changed according to what the user specified.  However, "set cwd" has
precedence over "cd", so it can always be used to override it.

"set cwd" works in the following way:

- If the user sets the inferior's cwd by using "set cwd", then this
  directory is saved into current_inferior ()->cwd and is used when
  the inferior is started (see below).

- If the user doesn't set the inferior's cwd by using "set cwd", but
  rather use the "cd" command as before, then this directory is
  inherited by the inferior because GDB will have chdir'd into it.

On Unix-like hosts, the way the directory is changed before the
inferior execution is by expanding the user set directory before the
fork, and then "chdir" after the call to fork/vfork on
"fork_inferior", but before the actual execution.  On Windows, the
inferior cwd set by the user is passed directly to the CreateProcess
call, which takes care of the actual chdir for us.

This way, we'll make sure that GDB's cwd is not affected by the user
set cwd.

gdb/ChangeLog:
2017-10-04  Sergio Durigan Junior  <sergiodj@redhat.com>

	* NEWS (New commands): Mention "set/show cwd".
	* cli/cli-cmds.c (_initialize_cli_cmds): Mention "set cwd" on
	"cd" command's help text.
	* common/common-inferior.h (get_inferior_cwd): New prototype.
	* infcmd.c (inferior_cwd_scratch): New global variable.
	(set_inferior_cwd): New function.
	(get_inferior_cwd): Likewise.
	(set_cwd_command): Likewise.
	(show_cwd_command): Likewise.
	(_initialize_infcmd): Add "set/show cwd" commands.
	* inferior.h (class inferior) <cwd>: New field.
	* nat/fork-inferior.c: Include "gdb_tilde_expand.h".
	(fork_inferior): Change inferior's cwd before its execution.
	* windows-nat.c (windows_create_inferior): Pass inferior's cwd
	to CreateProcess.

gdb/gdbserver/ChangeLog:
2017-10-04  Sergio Durigan Junior  <sergiodj@redhat.com>

	* inferiors.c (current_inferior_cwd): New global variable.
	(get_inferior_cwd): New function.
	* inferiors.h (struct process_info) <cwd>: New field.

gdb/doc/ChangeLog:
2017-10-04  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.texinfo (Starting your Program) <The working directory.>:
	Mention new "set cwd" command.
	(Working Directory) <Your Program's Working Directory>:
	Rephrase to explain that "set cwd" exists and is the default
	way to change the inferior's cwd.

gdb/testsuite/ChangeLog:
2017-10-04  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.base/set-cwd.c: New file.
	* gdb.base/set-cwd.exp: Likewise.

1 files changed, 77 insertions, 0 deletions

diff --git a/gdb/infcmd.c b/gdb/infcmd.c
index da16f5e..dbdf273 100644
--- a/gdb/infcmd.c
+++ b/gdb/infcmd.c
@@ -103,6 +103,10 @@ static void run_command (char *, int);
 
 static char *inferior_args_scratch;
 
+/* Scratch area where the new cwd will be stored by 'set cwd'.  */
+
+static char *inferior_cwd_scratch;
+
 /* Scratch area where 'set inferior-tty' will store user-provided value.
    We'll immediate copy it into per-inferior storage.  */
 
@@ -238,6 +242,60 @@ show_args_command (struct ui_file *file, int from_tty,
   deprecated_show_value_hack (file, from_tty, c, get_inferior_args ());
 }
 
+/* Set the inferior current working directory.  If CWD is NULL, unset
+   the directory.  */
+
+static void
+set_inferior_cwd (const char *cwd)
+{
+  struct inferior *inf = current_inferior ();
+
+  gdb_assert (inf != NULL);
+
+  if (cwd == NULL)
+    inf->cwd.reset ();
+  else
+    inf->cwd.reset (xstrdup (cwd));
+}
+
+/* See common/common-inferior.h.  */
+
+const char *
+get_inferior_cwd ()
+{
+  return current_inferior ()->cwd.get ();
+}
+
+/* Handle the 'set cwd' command.  */
+
+static void
+set_cwd_command (char *args, int from_tty, struct cmd_list_element *c)
+{
+  if (*inferior_cwd_scratch == '\0')
+    set_inferior_cwd (NULL);
+  else
+    set_inferior_cwd (inferior_cwd_scratch);
+}
+
+/* Handle the 'show cwd' command.  */
+
+static void
+show_cwd_command (struct ui_file *file, int from_tty,
+		  struct cmd_list_element *c, const char *value)
+{
+  const char *cwd = get_inferior_cwd ();
+
+  if (cwd == NULL)
+    fprintf_filtered (gdb_stdout,
+		      _("\
+You have not set the inferior's current working directory.\n\
+The inferior will inherit GDB's cwd.\n"));
+  else
+    fprintf_filtered (gdb_stdout,
+		      _("Current working directory that will be used "
+			"when starting the inferior is \"%s\".\n"), cwd);
+}
+
 
 /* Compute command-line string given argument vector.  This does the
    same shell processing as fork_inferior.  */
@@ -3253,6 +3311,25 @@ Follow this command with any number of args, to be passed to the program."),
   gdb_assert (c != NULL);
   set_cmd_completer (c, filename_completer);
 
+  cmd_name = "cwd";
+  add_setshow_string_noescape_cmd (cmd_name, class_run,
+				   &inferior_cwd_scratch, _("\
+Set the current working directory to be used when the inferior is started.\n\
+Changing this setting does not have any effect on inferiors that are\n\
+already running."),
+				   _("\
+Show the current working directory that is used when the inferior is started."),
+				   _("\
+Use this command to change the current working directory that will be used\n\
+when the inferior is started.  This setting does not affect GDB's current\n\
+working directory."),
+				   set_cwd_command,
+				   show_cwd_command,
+				   &setlist, &showlist);
+  c = lookup_cmd (&cmd_name, setlist, "", -1, 1);
+  gdb_assert (c != NULL);
+  set_cmd_completer (c, filename_completer);
+
   c = add_cmd ("environment", no_class, environment_info, _("\
 The environment to give the program, or one variable's value.\n\
 With an argument VAR, prints the value of environment variable VAR to\n\