Fix PR20494 - User input stops being echoed in CLI

This patch fixes a problem that problem triggers if you start an
inferior, e.g., with the "start" command, in a UI created with the
new-ui command, and then run a foreground execution command in the
main UI.  Once the program stops for the latter command, typing in the
main UI no longer echoes back to the user.

The problem revolves around this:

- gdb_has_a_terminal computes its result lazily, on first call.

  that is what saves gdb's initial main UI terminal state (the UI
  associated with stdin):

          our_terminal_info.ttystate = serial_get_tty_state (stdin_serial);

  This is the state that target_terminal_ours() restores.

- In this scenario, the gdb_has_a_terminal function happens to be
  first ever called from within the target_terminal_init call in
  startup_inferior:

      (top-gdb) bt
      #0  gdb_has_a_terminal () at src/gdb/inflow.c:157
      #1  0x000000000079db22 in child_terminal_init_with_pgrp () at src/gdb/inflow.c:217
       [...]
      #4  0x000000000065bacb in target_terminal_init () at src/gdb/target.c:456
      #5  0x00000000004676d2 in startup_inferior () at src/gdb/fork-child.c:531
       [...]
      #7  0x000000000046b168 in linux_nat_create_inferior () at src/gdb/linux-nat.c:1112
       [...]
      #9  0x00000000005f20c9 in start_command (args=0x0, from_tty=1) at src/gdb/infcmd.c:657

If the command to start the inferior is issued on the main UI, then
readline will have deprepped the terminal when we reach the above, and
the problem doesn't appear.

If however the command is issued on a non-main UI, then when we reach
that gdb_has_a_terminal call, the main UI's terminal state is still
set to whatever readline has sets it to in rl_prep_terminal, which
happens to have echo disabled.  Later, when the following synchronous
execution command finishes, we'll call target_terminal_ours to restore
gdb's the main UI's terminal settings, and that restores the terminal
state with echo disabled...

Conceptually, the fix is to move the gdb_has_a_terminal call earlier,
to someplace during GDB initialization, before readline/ncurses have
had a chance to change terminal settings.  Turns out that
"set_initial_gdb_ttystate" is exactly such a place.

I say conceptually, because the fix actually inlines the
gdb_has_a_terminal part that saves the terminal state in
set_initial_gdb_ttystate and then simplifies gdb_has_a_terminal, since
there's no point in making gdb_has_a_terminal do lazy computation.

gdb/ChangeLog:
2016-08-23  Pedro Alves  <palves@redhat.com>

	PR gdb/20494
	* inflow.c (our_terminal_info, initial_gdb_ttystate): Update
	comments.
	(enum gdb_has_a_terminal_flag_enum, gdb_has_a_terminal_flag):
	Delete.
	(set_initial_gdb_ttystate): Record our_terminal_info here too,
	instead of ...
	(gdb_has_a_terminal): ... here.  Reimplement in terms of
	initial_gdb_ttystate.  Make static.
	* terminal.h (gdb_has_a_terminal): Delete declaration.
	(set_initial_gdb_ttystate): Add comment.
	* top.c (show_interactive_mode): Use input_interactive_p instead
	of gdb_has_a_terminal.

gdb/testsuite/ChangeLog:
2016-08-23  Pedro Alves  <palves@redhat.com>

	PR gdb/20494
	* gdb.base/new-ui-echo.c: New file.
	* gdb.base/new-ui-echo.exp: New file.

1 files changed, 24 insertions, 44 deletions

diff --git a/gdb/inflow.c b/gdb/inflow.c
index 27ce0b0..5b55cec 100644
--- a/gdb/inflow.c
+++ b/gdb/inflow.c
@@ -74,13 +74,17 @@ struct terminal_info
 };
 
 /* Our own tty state, which we restore every time we need to deal with
-   the terminal.  This is only set once, when GDB first starts.  The
-   settings of flags which readline saves and restores and
+   the terminal.  This is set once, when GDB first starts, and then
+   whenever we enter/leave TUI mode (gdb_save_tty_state).  The
+   settings of flags which readline saves and restores are
    unimportant.  */
 static struct terminal_info our_terminal_info;
 
-/* Snapshot of our own tty state taken during initialization of GDB.
-   This is used as the initial tty state given to each new inferior.  */
+/* Snapshot of the initial tty state taken during initialization of
+   GDB, before readline/ncurses have had a chance to change it.  This
+   is used as the initial tty state given to each new spawned
+   inferior.  Unlike our_terminal_info, this is only ever set
+   once.  */
 static serial_ttystate initial_gdb_ttystate;
 
 static struct terminal_info *get_inflow_inferior_data (struct inferior *);
@@ -136,61 +140,37 @@ gdb_getpgrp (void)
 }
 #endif
 
-enum gdb_has_a_terminal_flag_enum
-  {
-    yes, no, have_not_checked
-  }
-gdb_has_a_terminal_flag = have_not_checked;
-
-/* Set the initial tty state that is to be inherited by new inferiors.  */
+/* See terminal.h.  */
 
 void
 set_initial_gdb_ttystate (void)
 {
+  /* Note we can't do any of this in _initialize_inflow because at
+     that point stdin_serial has not been created yet.  */
+
   initial_gdb_ttystate = serial_get_tty_state (stdin_serial);
-}
 
-/* Does GDB have a terminal (on stdin)?  */
-int
-gdb_has_a_terminal (void)
-{
-  switch (gdb_has_a_terminal_flag)
+  if (initial_gdb_ttystate != NULL)
     {
-    case yes:
-      return 1;
-    case no:
-      return 0;
-    case have_not_checked:
-      /* Get all the current tty settings (including whether we have a
-         tty at all!).  Can't do this in _initialize_inflow because
-         serial_fdopen() won't work until the serial_ops_list is
-         initialized.  */
-
+      our_terminal_info.ttystate
+	= serial_copy_tty_state (stdin_serial, initial_gdb_ttystate);
 #ifdef F_GETFL
       our_terminal_info.tflags = fcntl (0, F_GETFL, 0);
 #endif
-
-      gdb_has_a_terminal_flag = no;
-      if (stdin_serial != NULL)
-	{
-	  our_terminal_info.ttystate = serial_get_tty_state (stdin_serial);
-
-	  if (our_terminal_info.ttystate != NULL)
-	    {
-	      gdb_has_a_terminal_flag = yes;
 #ifdef PROCESS_GROUP_TYPE
-	      our_terminal_info.process_group = gdb_getpgrp ();
+      our_terminal_info.process_group = gdb_getpgrp ();
 #endif
-	    }
-	}
-
-      return gdb_has_a_terminal_flag == yes;
-    default:
-      /* "Can't happen".  */
-      return 0;
     }
 }
 
+/* Does GDB have a terminal (on stdin)?  */
+
+static int
+gdb_has_a_terminal (void)
+{
+  return initial_gdb_ttystate != NULL;
+}
+
 /* Macro for printing errors from ioctl operations */
 
 #define	OOPSY(what)	\