bug#17406: 25.2; repeatable crash when hitting C-g twice

[Paul Eggert <eggert@cs.ucla.edu>]

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Eli's suggestion looks good. I tweaked his patch slightly to avoid some calls 
and installed the attached into the emacs-26 branch to implement it. Please give 
it a try.

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From e56681085ee0575358b2193d69dd1a3c2ecf3dee Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Sun, 17 Sep 2017 22:01:56 -0700
Subject: [PATCH] Avoid crash with C-g C-g in GC

Problem reported by Richard Stallman (Bug#17406).
Based on fix suggested by Eli Zaretskii (Bug#28279#16).
* src/term.c (tty_send_additional_strings):
Use only safe accessors, to avoid crash when C-g C-g in GC.
---
 src/term.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/src/term.c b/src/term.c
index a2ae8c2..065bce4 100644
--- a/src/term.c
+++ b/src/term.c
@@ -155,12 +155,16 @@ tty_ring_bell (struct frame *f)
 static void
 tty_send_additional_strings (struct terminal *terminal, Lisp_Object sym)
 {
-  Lisp_Object lisp_terminal;
-  Lisp_Object extra_codes;
+  /* Use only accessors like CDR_SAFE and assq_no_quit to avoid any
+     form of quitting or signaling an error, since this function can
+     run as part of the "emergency escape" procedure invoked in the
+     middle of GC, where quitting means crashing (Bug#17406).  */
+  if (! terminal->name)
+    return;
   struct tty_display_info *tty = terminal->display_info.tty;
 
-  XSETTERMINAL (lisp_terminal, terminal);
-  for (extra_codes = Fterminal_parameter (lisp_terminal, sym);
+  for (Lisp_Object extra_codes
+	 = CDR_SAFE (assq_no_quit (sym, terminal->param_alist));
        CONSP (extra_codes);
        extra_codes = XCDR (extra_codes))
     {
-- 
2.7.4