PR tui/16138, PR tui/17519, and misc failures to initialize the terminal

PR tui/16138 is about failure to initialize curses resulting in GDB
exiting instead of throwing an error.  E.g.:

 $ TERM=foo gdb
 (gdb) layout asm
 Error opening terminal: foo.
 $

The problem is that we're calling initscr to initialize the screen.
As mentioned in
http://pubs.opengroup.org/onlinepubs/7908799/xcurses/initscr.html:

 If errors occur, initscr() writes an appropriate error message to
 standard error and exits.
                    ^^^^^

Instead, we should use newterm:

 "A program that needs an indication of error conditions, so it can
 continue to run in a line-oriented mode if the terminal cannot support
 a screen-oriented program, would also use this function."

After the patch:

 $ TERM=foo gdb -q -nx
 (gdb) layout asm
 Cannot enable the TUI: error opening terminal [TERM=foo]
 (gdb)

And then PR tui/17519 is about GDB not validating whether the terminal
has the necessary capabilities when enabling the TUI.  If one tries to
enable the TUI with TERM=dumb (and e.g., from a shell within emacs),
GDB ends up with a clear screen, the cursor is placed at the
bottom/right corner of the screen, there's no prompt, typing shows no
echo, and there's no indication of what's going on.  c-x,a gets you
out of the TUI, but it's completely non-obvious.

After the patch, we get:

 $ TERM=dumb gdb -q -nx
 (gdb) layout asm
 Cannot enable the TUI: terminal doesn't support cursor addressing [TERM=dumb]
 (gdb)

While at it, I've moved all the tui_allowed_p validation to
tui_enable, and expanded the error messages.  Previously we'd get:

 $ gdb -q -nx -i=mi
 (gdb)
 layout asm
 &"layout asm\n"
 &"TUI mode not allowed\n"
 ^error,msg="TUI mode not allowed"

and:

 $ gdb -q -nx -ex "layout asm" > foo
 TUI mode not allowed

While now we get:

 $ gdb -q -nx -i=mi
 (gdb)
 layout asm
 &"layout asm\n"
 &"Cannot enable the TUI when the interpreter is 'mi'\n"
 ^error,msg="Cannot enable the TUI when the interpreter is 'mi'"
 (gdb)

and:

 $ gdb -q -nx -ex "layout asm" > foo
 Cannot enable the TUI when output is not a terminal

Tested on x86_64 Fedora 20.

gdb/
2014-10-29  Pedro Alves  <palves@redhat.com>

	PR tui/16138
	PR tui/17519
	* tui/tui-interp.c (tui_is_toplevel): Delete global.
	(tui_allowed_p): Delete function.
	* tui/tui.c: Include "interps.h".
	(tui_enable): Don't use tui_allowed_p.  Error out here with
	detailed error messages if the TUI is the top level interpreter,
	or if output is not a terminal.  Use newterm instead of initscr,
	and error out if initializing the terminal fails.  Also error out if
	the terminal doesn't support cursor addressing.
	* tui/tui.h (tui_allowed_p): Delete declaration.

gdb/tui/tui-interp.c

@@ -51,9 +51,6 @@ tui_exit (void)
   tui_disable ();
 }
 
-/* True if TUI is the top-level interpreter.  */
-static int tui_is_toplevel = 0;
-
 /* Observers for several run control events.  If the interpreter is
    quiet (i.e., another interpreter is being run with
    interpreter-exec), print nothing.  */
@@ -126,8 +123,6 @@ tui_on_command_error (void)
 static void *
 tui_init (struct interp *self, int top_level)
 {
-  tui_is_toplevel = top_level;
-
   /* Install exit handler to leave the screen in a good shape.  */
   atexit (tui_exit);
 
@@ -150,18 +145,6 @@ tui_init (struct interp *self, int top_level)
   return NULL;
 }
 
-/* True if enabling the TUI is allowed.  Example, if the top level
-   interpreter is MI, enabling curses will certainly lose.  */
-
-int
-tui_allowed_p (void)
-{
-  /* Only if TUI is the top level interpreter.  Also don't try to
-     setup curses (and print funny control characters) if we're not
-     outputting to a terminal.  */
-  return tui_is_toplevel && ui_file_isatty (gdb_stdout);
-}
-
 static int
 tui_resume (void *data)
 {

gdb/tui/tui.c

@@ -48,6 +48,7 @@
 #include <setjmp.h>
 
 #include "gdb_curses.h"
+#include "interps.h"
 
 /* This redefines CTRL if it is not already defined, so it must come
    after terminal state releated include files like <term.h> and
@@ -361,6 +362,20 @@ tui_initialize_readline (void)
   rl_bind_key_in_map ('s', tui_rl_next_keymap, tui_ctlx_keymap);
 }
 
+/* Return the TERM variable from the environment, or "<unset>"
+   if not set.  */
+
+static const char *
+gdb_getenv_term (void)
+{
+  const char *term;
+
+  term = getenv ("TERM");
+  if (term != NULL)
+    return term;
+  return "<unset>";
+}
+
 /* Enter in the tui mode (curses).
    When in normal mode, it installs the tui hooks in gdb, redirects
    the gdb output, configures the readline to work in tui mode.
@@ -368,8 +383,7 @@ tui_initialize_readline (void)
 void
 tui_enable (void)
 {
-  if (!tui_allowed_p ())
-    error (_("TUI mode not allowed"));
+  struct interp *interp;
 
   if (tui_active)
     return;
@@ -380,8 +394,39 @@ tui_enable (void)
   if (tui_finish_init)
     {
       WINDOW *w;
+      SCREEN *s;
+      const char *cap;
+      const char *interp;
 
-      w = initscr ();
+      /* If the top level interpreter is not the console/tui (e.g.,
+	 MI), enabling curses will certainly lose.  */
+      interp = interp_name (top_level_interpreter ());
+      if (strcmp (interp, INTERP_TUI) != 0)
+	error (_("Cannot enable the TUI when the interpreter is '%s'"), interp);
+
+      /* Don't try to setup curses (and print funny control
+	 characters) if we're not outputting to a terminal.  */
+      if (!ui_file_isatty (gdb_stdout))
+	error (_("Cannot enable the TUI when output is not a terminal"));
+
+      s = newterm (NULL, NULL, NULL);
+      if (s == NULL)
+	{
+	  error (_("Cannot enable the TUI: error opening terminal [TERM=%s]"),
+		 gdb_getenv_term ());
+	}
+      w = stdscr;
+
+      /* Check required terminal capabilities.  */
+      cap = tigetstr ("cup");
+      if (cap == NULL || cap == (char *) -1 || *cap == '\0')
+	{
+	  endwin ();
+	  delscreen (s);
+	  error (_("Cannot enable the TUI: "
+		   "terminal doesn't support cursor addressing [TERM=%s]"),
+		 gdb_getenv_term ());
+	}
 
       cbreak ();
       noecho ();

gdb/tui/tui.h

@@ -64,10 +64,6 @@ extern int tui_get_command_dimension (unsigned int *width,
    key shortcut.  */
 extern void tui_initialize_readline (void);
 
-/* True if enabling the TUI is allowed.  Example, if the top level
-   interpreter is MI, enabling curses will certainly lose.  */
-extern int tui_allowed_p (void);
-
 /* Enter in the tui mode (curses).  */
 extern void tui_enable (void);