[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.

1 files changed, 3 insertions, 3 deletions

diff --git a/gdb/python/python.c b/gdb/python/python.c
index 451c5bd..264f7c8 100644
--- a/gdb/python/python.c
+++ b/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)
     {