Constify command_line_input

This changes command_line_input to return a "const char *", which is
appropriate because the memory is owned by command_line_input.  Then
it fixes up the users.

I looked at making command_line_input transfer ownership to its caller
instead, but this is complicated due to the way read_next_line is
called, so I decided against it.

Tested by rebuilding.

gdb/ChangeLog
2019-11-08  Tom Tromey  <tromey@adacore.com>

	* top.c (read_command_file): Update.
	(command_line_input): Make return type const.
	* python/py-gdb-readline.c: Update.
	* linespec.c (decode_line_2): Update.
	* defs.h (command_line_input): Make return type const.
	* cli/cli-script.c (read_next_line): Make return type const.
	* ada-lang.c (get_selections): Update.

Change-Id: I27e6c9477fd1005ab5b16e0d337e4c015b6e6248

gdb/ChangeLog

@@ -1,3 +1,13 @@
+2019-11-08  Tom Tromey  <tromey@adacore.com>
+
+	* top.c (read_command_file): Update.
+	(command_line_input): Make return type const.
+	* python/py-gdb-readline.c: Update.
+	* linespec.c (decode_line_2): Update.
+	* defs.h (command_line_input): Make return type const.
+	* cli/cli-script.c (read_next_line): Make return type const.
+	* ada-lang.c (get_selections): Update.
+
 2019-11-06  Christian Biesinger  <cbiesinger@google.com>
 
 	* linux-tdep.c (linux_info_proc): Use strtok_r instead of strtok.

gdb/ada-lang.c

@@ -3273,7 +3273,7 @@ static int
 get_selections (int *choices, int n_choices, int max_results,
                 int is_all_choice, const char *annotation_suffix)
 {
-  char *args;
+  const char *args;
   const char *prompt;
   int n_chosen;
   int first_choice = is_all_choice ? 2 : 1;

gdb/cli/cli-script.c

@@ -50,7 +50,7 @@ recurse_read_control_structure
 static void do_define_command (const char *comname, int from_tty,
 			       const counted_command_line *commands);
 
-static char *read_next_line (void);
+static const char *read_next_line ();
 
 /* Level of control structure when reading.  */
 static int control_level;
@@ -890,8 +890,8 @@ user_args::insert_args (const char *line) const
    recurse_read_control_structure whenever we need to read commands
    from stdin.  */
 
-static char *
-read_next_line (void)
+static const char *
+read_next_line ()
 {
   struct ui *ui = current_ui;
   char *prompt_ptr, control_prompt[256];

gdb/defs.h

@@ -306,7 +306,7 @@ typedef void initialize_file_ftype (void);
 
 extern char *gdb_readline_wrapper (const char *);
 
-extern char *command_line_input (const char *, const char *);
+extern const char *command_line_input (const char *, const char *);
 
 extern void print_prompt (void);
 

gdb/linespec.c

@@ -1489,7 +1489,7 @@ decode_line_2 (struct linespec_state *self,
 	       std::vector<symtab_and_line> *result,
 	       const char *select_mode)
 {
-  char *args;
+  const char *args;
   const char *prompt;
   int i;
   std::vector<const char *> filters;

gdb/python/py-gdb-readline.c

@@ -37,7 +37,8 @@ gdbpy_readline_wrapper (FILE *sys_stdin, FILE *sys_stdout,
 #endif
 {
   int n;
-  char *p = NULL, *q;
+  const char *p = NULL;
+  char *q;
 
   try
     {

gdb/top.c

@@ -420,7 +420,7 @@ read_command_file (FILE *stream)
 
   while (ui->instream != NULL && !feof (ui->instream))
     {
-      char *command;
+      const char *command;
 
       /* Get a command-line.  This calls the readline package.  */
       command = command_line_input (NULL, NULL);
@@ -1210,7 +1210,7 @@ gdb_safe_append_history (void)
    This routine either uses fancy command line editing or simple input
    as the user has requested.  */
 
-char *
+const char *
 command_line_input (const char *prompt_arg, const char *annotation_suffix)
 {
   static struct buffer cmd_line_buffer;