PR python/17372 - Python hangs when displaying help()

This is more of a readline/terminal issue than a Python one.

PR17372 is a regression in 7.8 caused by the fix for PR17072:

 commit 0017922d0292d8c374584f6100874580659c9973
 Author: Pedro Alves <palves@redhat.com>
 Date:   Mon Jul 14 19:55:32 2014 +0100

    Background execution + pagination aborts readline/gdb

    gdb_readline_wrapper_line removes the handler after a line is
    processed.  Usually, we'll end up re-displaying the prompt, and that
    reinstalls the handler.  But if the output is coming out of handling
    a stop event, we don't re-display the prompt, and nothing restores the
    handler.  So the next input wakes up the event loop and calls into
    readline, which aborts.
...
    gdb/
    2014-07-14  Pedro Alves  <palves@redhat.com>

        PR gdb/17072
        * top.c (gdb_readline_wrapper_line): Tweak comment.
        (gdb_readline_wrapper_cleanup): If readline is enabled, reinstall
        the input handler callback.

The problem is that installing the input handler callback also preps
the terminal, putting it in raw mode and with echo disabled, which is
bad if we're going to call a command that assumes cooked/canonical
mode, and echo enabled, like in the case of the PR, Python's
interactive shell.  Another example I came up with that doesn't depend
on Python is starting a subshell with "(gdb) shell /bin/sh" from a
multi-line command.  Tests covering both these examples are added.

The fix is to revert the original fix for PR gdb/17072, and instead
restore the callback handler after processing an asynchronous target
event.

Furthermore, calling rl_callback_handler_install when we already have
some input in readline's line buffer discards that input, which is
obviously a bad thing to do while the user is typing.  No specific
test is added for that, because I first tried calling it even if the
callback handler was still installed and that resulted in hundreds of
failures in the testsuite.

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

	PR python/17372
	* event-top.c (change_line_handler): Call
	gdb_rl_callback_handler_remove instead of
	rl_callback_handler_remove.
	(callback_handler_installed): New global.
	(gdb_rl_callback_handler_remove, gdb_rl_callback_handler_install)
	(gdb_rl_callback_handler_reinstall): New functions.
	(display_gdb_prompt): Call gdb_rl_callback_handler_remove and
	gdb_rl_callback_handler_install instead of
	rl_callback_handler_remove and rl_callback_handler_install.
	(gdb_disable_readline): Call gdb_rl_callback_handler_remove
	instead of rl_callback_handler_remove.
	* event-top.h (gdb_rl_callback_handler_remove)
	(gdb_rl_callback_handler_install)
	(gdb_rl_callback_handler_reinstall): New declarations.
	* infrun.c (reinstall_readline_callback_handler_cleanup): New
	cleanup function.
	(fetch_inferior_event): Install it.
	* top.c (gdb_readline_wrapper_line) Call
	gdb_rl_callback_handler_remove instead of
	rl_callback_handler_remove.
	(gdb_readline_wrapper_cleanup): Don't call
	rl_callback_handler_install.

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

	PR python/17372
	* gdb.python/python.exp: Test a multi-line command that spawns
	interactive Python.
	* gdb.base/multi-line-starts-subshell.exp: New file.

1 files changed, 13 insertions, 6 deletions

diff --git a/gdb/top.c b/gdb/top.c
index 5a947d7..83d858a 100644
--- a/gdb/top.c
+++ b/gdb/top.c
@@ -763,9 +763,14 @@ gdb_readline_wrapper_line (char *line)
 
   /* Prevent parts of the prompt from being redisplayed if annotations
      are enabled, and readline's state getting out of sync.  We'll
-     restore it in gdb_readline_wrapper_cleanup.  */
+     reinstall the callback handler, which puts the terminal in raw
+     mode (or in readline lingo, in prepped state), when we're next
+     ready to process user input, either in display_gdb_prompt, or if
+     we're handling an asynchronous target event and running in the
+     background, just before returning to the event loop to process
+     further input (or more target events).  */
   if (async_command_editing_p)
-    rl_callback_handler_remove ();
+    gdb_rl_callback_handler_remove ();
 }
 
 struct gdb_readline_wrapper_cleanup
@@ -787,10 +792,12 @@ gdb_readline_wrapper_cleanup (void *arg)
   gdb_assert (input_handler == gdb_readline_wrapper_line);
   input_handler = cleanup->handler_orig;
 
-  /* Reinstall INPUT_HANDLER in readline, without displaying a
-     prompt.  */
-  if (async_command_editing_p)
-    rl_callback_handler_install (NULL, input_handler);
+  /* Don't restore our input handler in readline yet.  That would make
+     readline prep the terminal (putting it in raw mode), while the
+     line we just read may trigger execution of a command that expects
+     the terminal in the default cooked/canonical mode, such as e.g.,
+     running Python's interactive online help utility.  See
+     gdb_readline_wrapper_line for when we'll reinstall it.  */
 
   gdb_readline_wrapper_result = NULL;
   gdb_readline_wrapper_done = 0;