tui: don't overwrite a secondary prompt that was given no input

This patch fixes the following bug in TUI:

  (gdb) break foo
  No symbol table is loaded.  Use the "file" command.
  Make breakpoint pending on future shared library load? (y or [n]) <ENTER>

By submitting an empty command line to a secondary prompt, the line
corresponding to the secondary prompt is undesirably cleared and
overwritten.  Outside of a secondary prompt, clearing the prompt line
after submitting an empty command line is intended behavior which
complements GDB's repeat-command shorthand.  But inside a secondary
prompt, this behavior is undesired since the shorthand is not applicable
in that case.  We should retain the secondary-prompt line even when it's
given no input.

This patch makes sure that a prompt that was given an empty command line
is cleared and overwritten only if it's not a secondary prompt.  To
acheive this, a new predicate is defined which informs us whether the
current input handler is a secondary prompt.

gdb/ChangeLog:

	* top.h (gdb_in_secondary_prompt_p): Declare.
	* top.c (gdb_secondary_prompt_depth): Define.
	(gdb_in_secondary_prompt_p): Define.
	(gdb_readline_wrapper_cleanup): Decrement
	gdb_secondary_prompt_depth.
	(gdb_readline_wrapper): Increment gdb_secondary_prompt_depth.
	* tui/tui-io.c (tui_getc): Don't clear the prompt line if we
	are in a secondary prompt.

1 files changed, 18 insertions, 0 deletions

diff --git a/gdb/top.c b/gdb/top.c
index 061b52f..cb3c761 100644
--- a/gdb/top.c
+++ b/gdb/top.c
@@ -754,6 +754,21 @@ static char *gdb_readline_wrapper_result;
    return.  */
 static void (*saved_after_char_processing_hook) (void);
 
+
+/* The number of nested readline secondary prompts that are currently
+   active.  */
+
+static int gdb_secondary_prompt_depth = 0;
+
+/* See top.h.  */
+
+int
+gdb_in_secondary_prompt_p (void)
+{
+  return gdb_secondary_prompt_depth > 0;
+}
+
+
 /* This function is called when readline has seen a complete line of
    text.  */
 
@@ -808,6 +823,8 @@ gdb_readline_wrapper_cleanup (void *arg)
 
   gdb_readline_wrapper_result = NULL;
   gdb_readline_wrapper_done = 0;
+  gdb_secondary_prompt_depth--;
+  gdb_assert (gdb_secondary_prompt_depth >= 0);
 
   after_char_processing_hook = saved_after_char_processing_hook;
   saved_after_char_processing_hook = NULL;
@@ -833,6 +850,7 @@ gdb_readline_wrapper (const char *prompt)
 
   cleanup->target_is_async_orig = target_is_async_p ();
 
+  gdb_secondary_prompt_depth++;
   back_to = make_cleanup (gdb_readline_wrapper_cleanup, cleanup);
 
   if (cleanup->target_is_async_orig)