[gdb] Make execute_command_to_string return string on throw

The pattern for using execute_command_to_string is:
...
  std::string output;
  output = execute_fn_to_string (fn, term_out);
...

This results in a problem when using it in a try/catch:
...
  try
    {
      output = execute_fn_to_string (fn, term_out)
    }
  catch (const gdb_exception &e)
    {
      /* Use output.  */
    }
...

If an expection was thrown during execute_fn_to_string, then the output
remains unassigned, while it could be worthwhile to known what output was
generated by gdb before the expection was thrown.

Fix this by returning the string using a parameter instead:
...
  execute_fn_to_string (output, fn, term_out)
...

Also add a variant without string parameter, to support places where the
function is used while ignoring the result:
...
  execute_fn_to_string (fn, term_out)
...

Tested on x86_64-linux.

gdb/complaints.c

@@ -91,7 +91,7 @@ test_complaints ()
   do									\
     {									\
       std::string output;						\
-      output = execute_fn_to_string ([]() { complaint (STR); }, false);	\
+      execute_fn_to_string (output, []() { complaint (STR); }, false);	\
       std::string expected						\
 	= _("During symbol reading: ") + std::string (STR "\n");	\
       SELF_CHECK (output == expected);					\
@@ -102,7 +102,7 @@ test_complaints ()
   do									\
     {									\
       std::string output;						\
-      output = execute_fn_to_string ([]() { complaint (STR); }, false);	\
+      execute_fn_to_string (output, []() { complaint (STR); }, false);	\
       SELF_CHECK (output.empty ());					\
       SELF_CHECK (counters[STR] == CNT);				\
     } while (0)

gdb/gdbcmd.h

@@ -42,7 +42,8 @@ extern void execute_fn_to_ui_file (struct ui_file *file, std::function<void(void
    (e.g. with styling).  When TERM_OUT is false raw output will be collected
    (e.g. no styling).  */
 
-extern std::string execute_fn_to_string (std::function<void(void)> fn, bool term_out);
+extern void execute_fn_to_string (std::string &res,
+				  std::function<void(void)> fn, bool term_out);
 
 /* As execute_fn_to_ui_file, but run execute_command for P and FROM_TTY.  */
 
@@ -51,8 +52,13 @@ extern void execute_command_to_ui_file (struct ui_file *file,
 
 /* As execute_fn_to_string, but run execute_command for P and FROM_TTY.  */
 
-extern std::string execute_command_to_string (const char *p, int from_tty,
-					      bool term_out);
+extern void execute_command_to_string (std::string &res, const char *p,
+				       int from_tty, bool term_out);
+
+/* As execute_command_to_string, but ignore resulting string.  */
+
+extern void execute_command_to_string (const char *p,
+				       int from_tty, bool term_out);
 
 extern void print_command_line (struct command_line *, unsigned int,
 				struct ui_file *);

gdb/guile/guile.c

@@ -302,7 +302,7 @@ gdbscm_execute_gdb_command (SCM command_scm, SCM rest)
 
       scoped_restore preventer = prevent_dont_repeat ();
       if (to_string)
-	to_string_res = execute_command_to_string (command, from_tty, false);
+	execute_command_to_string (to_string_res, command, from_tty, false);
       else
 	execute_command (command, from_tty);
 

gdb/python/python.c

@@ -1924,11 +1924,11 @@ namespace selftests {
 static void
 test_python ()
 {
-#define CMD execute_command_to_string ("python print(5)", 0, true);
+#define CMD(S) execute_command_to_string (S, "python print(5)", 0, true)
 
   std::string output;
 
-  output = CMD;
+  CMD (output);
   SELF_CHECK (output == "5\n");
   output.clear ();
 
@@ -1937,7 +1937,7 @@ test_python ()
     = make_scoped_restore (&gdb_python_initialized, 0);
   try
     {
-      output = CMD;
+      CMD (output);
     }
   catch (const gdb_exception &e)
     {

gdb/stack.c

@@ -3028,8 +3028,8 @@ frame_apply_command_count (const char *which_command,
 	       set to the selected frame.  */
 	    scoped_restore_current_thread restore_fi_current_frame;
 
-	    cmd_result = execute_command_to_string
-	      (cmd, from_tty, gdb_stdout->term_out ());
+	    execute_command_to_string
+	      (cmd_result, cmd, from_tty, gdb_stdout->term_out ());
 	  }
 	  fi = get_selected_frame (_("frame apply "
 				     "unable to get selected frame."));

gdb/thread.c

@@ -1450,8 +1450,9 @@ thr_try_catch_cmd (thread_info *thr, const char *cmd, int from_tty,
 
   try
     {
-      std::string cmd_result = execute_command_to_string
-	(cmd, from_tty, gdb_stdout->term_out ());
+      std::string cmd_result;
+      execute_command_to_string
+	(cmd_result, cmd, from_tty, gdb_stdout->term_out ());
       if (!flags.silent || cmd_result.length () > 0)
 	{
 	  if (!flags.quiet)

gdb/top.c

@@ -724,13 +724,25 @@ execute_fn_to_ui_file (struct ui_file *file, std::function<void(void)> fn)
 
 /* See gdbcmd.h.  */
 
-std::string
-execute_fn_to_string (std::function<void(void)> fn, bool term_out)
+void
+execute_fn_to_string (std::string &res, std::function<void(void)> fn,
+		      bool term_out)
 {
   string_file str_file (term_out);
 
-  execute_fn_to_ui_file (&str_file, fn);
-  return std::move (str_file.string ());
+  try
+    {
+      execute_fn_to_ui_file (&str_file, fn);
+    }
+  catch (...)
+    {
+      /* Finally.  */
+      res = std::move (str_file.string ());
+      throw;
+    }
+
+  /* And finally.  */
+  res = std::move (str_file.string ());
 }
 
 /* See gdbcmd.h.  */
@@ -744,12 +756,23 @@ execute_command_to_ui_file (struct ui_file *file,
 
 /* See gdbcmd.h.  */
 
-std::string
+void
+execute_command_to_string (std::string &res, const char *p, int from_tty,
+			   bool term_out)
+{
+  execute_fn_to_string (res, [=]() { execute_command (p, from_tty); },
+			term_out);
+}
+
+/* See gdbcmd.h.  */
+
+void
 execute_command_to_string (const char *p, int from_tty,
 			   bool term_out)
 {
-  return
-    execute_fn_to_string ([=]() { execute_command (p, from_tty); }, term_out);
+  std::string dummy;
+  execute_fn_to_string (dummy, [=]() { execute_command (p, from_tty); },
+			term_out);
 }
 
 /* When nonzero, cause dont_repeat to do nothing.  This should only be