Revamp quitting and fix infloops

This fixes some infinite loops that cannot be quitted out of,
e.g., (defun foo () (nth most-positive-fixnum '#1=(1 . #1#)))
when byte-compiled and when run under X.  See:
http://lists.gnu.org/archive/html/emacs-devel/2017-01/msg00577.html
This also attempts to keep the performance improvements I recently
added, as much as possible under the constraint that the infloops
must be caught.  In some cases this fixes infloop bugs recently
introduced when I removed immediate_quit.
* src/alloc.c (Fmake_list):
Use rarely_quit, not maybe_quit, for speed in the usual case.
* src/bytecode.c (exec_byte_code):
* src/editfns.c (Fcompare_buffer_substrings):
* src/fns.c (Fnthcdr):
* src/syntax.c (scan_words, skip_chars, skip_syntaxes)
(Fbackward_prefix_chars):
Use rarely_quit so that users can C-g out of long loops.
* src/callproc.c (call_process_cleanup, call_process):
* src/fileio.c (read_non_regular, Finsert_file_contents):
* src/indent.c (compute_motion):
* src/syntax.c (scan_words, Fforward_comment):
Remove now-unnecessary maybe_quit calls.
* src/callproc.c (call_process):
* src/doc.c (get_doc_string, Fsnarf_documentation):
* src/fileio.c (Fcopy_file, read_non_regular, Finsert_file_contents):
* src/lread.c (safe_to_load_version):
* src/sysdep.c (system_process_attributes) [GNU_LINUX]:
Use emacs_read_quit instead of emacs_read in places where
C-g handling is safe.
* src/eval.c (maybe_quit): Move comment here from lisp.h.
* src/fileio.c (Fcopy_file, e_write):
Use emacs_write_quit instead of emacs_write_sig in places where
C-g handling is safe.
* src/filelock.c (create_lock_file): Use emacs_write, not
plain write, as emacs_write no longer has a problem.
(read_lock_data): Use emacs_read, not read, as emacs_read
no longer has a problem.
* src/fns.c (rarely_quit): Move to lisp.h and rename to
incr_rarely_quit.  All uses changed..
* src/fns.c (Fmemq, Fmemql, Fassq, Frassq, Fplist_put, Fplist_member):
* src/indent.c (compute_motion):
* src/syntax.c (find_defun_start, back_comment, forw_comment)
(Fforward_comment, scan_lists, scan_sexps_forward):
Use incr_rarely_quit so that users can C-g out of long loops.
* src/fns.c (Fnconc): Move incr_rarely_quit call to within
inner loop, so that it catches C-g there too.
* src/keyboard.c (tty_read_avail_input): Remove commented-out
and now-obsolete code dealing with interrupts.
* src/lisp.h (rarely_quit, incr_rarely_quit): New functions,
the latter moved here from fns.c and renamed from rarely_quit.
(emacs_read_quit, emacs_write_quit): New decls.
* src/search.c (find_newline, search_buffer, find_newline1):
Add maybe_quit to catch C-g.
* src/sysdep.c (get_child_status): Always invoke maybe_quit
if interruptible, so that the caller need not bother.
(emacs_nointr_read, emacs_read_quit, emacs_write_quit):
New functions.
(emacs_read): Rewrite in terms of emacs_nointr_read.
Do not handle C-g or signals; that is now for emacs_read_quit.
(emacs_full_write): Replace PROCESS_SIGNALS two-way arg
with INTERRUPTIBLE three-way arg.  All uses changed.

1 files changed, 27 insertions, 13 deletions

diff --git a/src/lisp.h b/src/lisp.h
index a18e4da1cfd..2d67e7edddb 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -3123,24 +3123,36 @@ struct handler
 
 extern Lisp_Object memory_signal_data;
 
-/* Check quit-flag and quit if it is non-nil.  Typing C-g does not
-   directly cause a quit; it only sets Vquit_flag.  So the program
-   needs to call maybe_quit at times when it is safe to quit.  Every
-   loop that might run for a long time or might not exit ought to call
-   maybe_quit at least once, at a safe place.  Unless that is
-   impossible, of course.  But it is very desirable to avoid creating
-   loops where maybe_quit is impossible.
-
-   If quit-flag is set to `kill-emacs' the SIGINT handler has received
-   a request to exit Emacs when it is safe to do.
-
-   When not quitting, process any pending signals.  */
-
 extern void maybe_quit (void);
 
 /* True if ought to quit now.  */
 
 #define QUITP (!NILP (Vquit_flag) && NILP (Vinhibit_quit))
+
+/* Heuristic on how many iterations of a tight loop can be safely done
+   before it's time to do a quit.  This must be a power of 2.  It
+   is nice but not necessary for it to equal USHRT_MAX + 1.  */
+
+enum { QUIT_COUNT_HEURISTIC = 1 << 16 };
+
+/* Process a quit rarely, based on a counter COUNT, for efficiency.
+   "Rarely" means once per QUIT_COUNT_HEURISTIC or per USHRT_MAX + 1
+   times, whichever is smaller (somewhat arbitrary, but often faster).  */
+
+INLINE void
+rarely_quit (unsigned short int count)
+{
+  if (! (count & (QUIT_COUNT_HEURISTIC - 1)))
+    maybe_quit ();
+}
+
+/* Increment *QUIT_COUNT and rarely quit.  */
+
+INLINE void
+incr_rarely_quit (unsigned short int *quit_count)
+{
+  rarely_quit (++*quit_count);
+}
 
 extern Lisp_Object Vascii_downcase_table;
 extern Lisp_Object Vascii_canon_table;
@@ -4216,8 +4228,10 @@ extern int emacs_open (const char *, int, int);
 extern int emacs_pipe (int[2]);
 extern int emacs_close (int);
 extern ptrdiff_t emacs_read (int, void *, ptrdiff_t);
+extern ptrdiff_t emacs_read_quit (int, void *, ptrdiff_t);
 extern ptrdiff_t emacs_write (int, void const *, ptrdiff_t);
 extern ptrdiff_t emacs_write_sig (int, void const *, ptrdiff_t);
+extern ptrdiff_t emacs_write_quit (int, void const *, ptrdiff_t);
 extern void emacs_perror (char const *);
 
 extern void unlock_all_files (void);