src/undo.c

   1 /* undo handling for GNU Emacs.
   2    Copyright (C) 1990, 1993, 1994, 2000, 2001, 2002, 2003, 2004,
   3                  2005, 2006, 2007, 2008, 2009, 2010, 2011  Free Software Foundation, Inc.
   4
   5 This file is part of GNU Emacs.
   6
   7 GNU Emacs is free software: you can redistribute it and/or modify
   8 it under the terms of the GNU General Public License as published by
   9 the Free Software Foundation, either version 3 of the License, or
  10 (at your option) any later version.
  11
  12 GNU Emacs is distributed in the hope that it will be useful,
  13 but WITHOUT ANY WARRANTY; without even the implied warranty of
  14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15 GNU General Public License for more details.
  16
  17 You should have received a copy of the GNU General Public License
  18 along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
  19
  20
  21 #include <config.h>
  22 #include <setjmp.h>
  23 #include "lisp.h"
  24 #include "buffer.h"
  25 #include "commands.h"
  26 #include "window.h"
  27
  28 /* Last buffer for which undo information was recorded.  */
  29 /* BEWARE: This is not traced by the GC, so never dereference it!  */
  30 struct buffer *last_undo_buffer;
  31
  32 /* Position of point last time we inserted a boundary.  */
  33 struct buffer *last_boundary_buffer;
  34 EMACS_INT last_boundary_position;
  35
  36 Lisp_Object Qinhibit_read_only;
  37
  38 /* Marker for function call undo list elements.  */
  39
  40 Lisp_Object Qapply;
  41
  42 /* The first time a command records something for undo.
  43    it also allocates the undo-boundary object
  44    which will be added to the list at the end of the command.
  45    This ensures we can't run out of space while trying to make
  46    an undo-boundary.  */
  47 Lisp_Object pending_boundary;
  48
  49 /* Record point as it was at beginning of this command (if necessary)
  50    and prepare the undo info for recording a change.
  51    PT is the position of point that will naturally occur as a result of the
  52    undo record that will be added just after this command terminates.  */
  53
  54 static void
  55 record_point (EMACS_INT pt)
  56 {
  57   int at_boundary;
  58
  59   /* Don't record position of pt when undo_inhibit_record_point holds.  */
  60   if (undo_inhibit_record_point)
  61     return;
  62
  63   /* Allocate a cons cell to be the undo boundary after this command.  */
  64   if (NILP (pending_boundary))
  65     pending_boundary = Fcons (Qnil, Qnil);
  66
  67   if ((current_buffer != last_undo_buffer)
  68       /* Don't call Fundo_boundary for the first change.  Otherwise we
  69          risk overwriting last_boundary_position in Fundo_boundary with
  70          PT of the current buffer and as a consequence not insert an
  71          undo boundary because last_boundary_position will equal pt in
  72          the test at the end of the present function (Bug#731).  */
  73       && (MODIFF > SAVE_MODIFF))
  74     Fundo_boundary ();
  75   last_undo_buffer = current_buffer;
  76
  77   if (CONSP (current_buffer->undo_list))
  78     {
  79       /* Set AT_BOUNDARY to 1 only when we have nothing other than
  80          marker adjustment before undo boundary.  */
  81
  82       Lisp_Object tail = current_buffer->undo_list, elt;
  83
  84       while (1)
  85         {
  86           if (NILP (tail))
  87             elt = Qnil;
  88           else
  89             elt = XCAR (tail);
  90           if (NILP (elt) || ! (CONSP (elt) && MARKERP (XCAR (elt))))
  91             break;
  92           tail = XCDR (tail);
  93         }
  94       at_boundary = NILP (elt);
  95     }
  96   else
  97     at_boundary = 1;
  98
  99   if (MODIFF <= SAVE_MODIFF)
 100     record_first_change ();
 101
 102   /* If we are just after an undo boundary, and
 103      point wasn't at start of deleted range, record where it was.  */
 104   if (at_boundary
 105       && current_buffer == last_boundary_buffer
 106       && last_boundary_position != pt)
 107     current_buffer->undo_list
 108       = Fcons (make_number (last_boundary_position), current_buffer->undo_list);
 109 }
 110
 111 /* Record an insertion that just happened or is about to happen,
 112    for LENGTH characters at position BEG.
 113    (It is possible to record an insertion before or after the fact
 114    because we don't need to record the contents.)  */
 115
 116 void
 117 record_insert (EMACS_INT beg, EMACS_INT length)
 118 {
 119   Lisp_Object lbeg, lend;
 120
 121   if (EQ (current_buffer->undo_list, Qt))
 122     return;
 123
 124   record_point (beg);
 125
 126   /* If this is following another insertion and consecutive with it
 127      in the buffer, combine the two.  */
 128   if (CONSP (current_buffer->undo_list))
 129     {
 130       Lisp_Object elt;
 131       elt = XCAR (current_buffer->undo_list);
 132       if (CONSP (elt)
 133           && INTEGERP (XCAR (elt))
 134           && INTEGERP (XCDR (elt))
 135           && XINT (XCDR (elt)) == beg)
 136         {
 137           XSETCDR (elt, make_number (beg + length));
 138           return;
 139         }
 140     }
 141
 142   XSETFASTINT (lbeg, beg);
 143   XSETINT (lend, beg + length);
 144   current_buffer->undo_list = Fcons (Fcons (lbeg, lend),
 145                                      current_buffer->undo_list);
 146 }
 147
 148 /* Record that a deletion is about to take place,
 149    of the characters in STRING, at location BEG.  */
 150
 151 void
 152 record_delete (EMACS_INT beg, Lisp_Object string)
 153 {
 154   Lisp_Object sbeg;
 155
 156   if (EQ (current_buffer->undo_list, Qt))
 157     return;
 158
 159   if (PT == beg + SCHARS (string))
 160     {
 161       XSETINT (sbeg, -beg);
 162       record_point (PT);
 163     }
 164   else
 165     {
 166       XSETFASTINT (sbeg, beg);
 167       record_point (beg);
 168     }
 169
 170   current_buffer->undo_list
 171     = Fcons (Fcons (string, sbeg), current_buffer->undo_list);
 172 }
 173
 174 /* Record the fact that MARKER is about to be adjusted by ADJUSTMENT.
 175    This is done only when a marker points within text being deleted,
 176    because that's the only case where an automatic marker adjustment
 177    won't be inverted automatically by undoing the buffer modification.  */
 178
 179 void
 180 record_marker_adjustment (Lisp_Object marker, EMACS_INT adjustment)
 181 {
 182   if (EQ (current_buffer->undo_list, Qt))
 183     return;
 184
 185   /* Allocate a cons cell to be the undo boundary after this command.  */
 186   if (NILP (pending_boundary))
 187     pending_boundary = Fcons (Qnil, Qnil);
 188
 189   if (current_buffer != last_undo_buffer)
 190     Fundo_boundary ();
 191   last_undo_buffer = current_buffer;
 192
 193   current_buffer->undo_list
 194     = Fcons (Fcons (marker, make_number (adjustment)),
 195              current_buffer->undo_list);
 196 }
 197
 198 /* Record that a replacement is about to take place,
 199    for LENGTH characters at location BEG.
 200    The replacement must not change the number of characters.  */
 201
 202 void
 203 record_change (EMACS_INT beg, EMACS_INT length)
 204 {
 205   record_delete (beg, make_buffer_string (beg, beg + length, 1));
 206   record_insert (beg, length);
 207 }
 208 \f
 209 /* Record that an unmodified buffer is about to be changed.
 210    Record the file modification date so that when undoing this entry
 211    we can tell whether it is obsolete because the file was saved again.  */
 212
 213 void
 214 record_first_change (void)
 215 {
 216   Lisp_Object high, low;
 217   struct buffer *base_buffer = current_buffer;
 218
 219   if (EQ (current_buffer->undo_list, Qt))
 220     return;
 221
 222   if (current_buffer != last_undo_buffer)
 223     Fundo_boundary ();
 224   last_undo_buffer = current_buffer;
 225
 226   if (base_buffer->base_buffer)
 227     base_buffer = base_buffer->base_buffer;
 228
 229   XSETFASTINT (high, (base_buffer->modtime >> 16) & 0xffff);
 230   XSETFASTINT (low, base_buffer->modtime & 0xffff);
 231   current_buffer->undo_list = Fcons (Fcons (Qt, Fcons (high, low)), current_buffer->undo_list);
 232 }
 233
 234 /* Record a change in property PROP (whose old value was VAL)
 235    for LENGTH characters starting at position BEG in BUFFER.  */
 236
 237 void
 238 record_property_change (EMACS_INT beg, EMACS_INT length,
 239                         Lisp_Object prop, Lisp_Object value,
 240                         Lisp_Object buffer)
 241 {
 242   Lisp_Object lbeg, lend, entry;
 243   struct buffer *obuf = current_buffer, *buf = XBUFFER (buffer);
 244   int boundary = 0;
 245
 246   if (EQ (buf->undo_list, Qt))
 247     return;
 248
 249   /* Allocate a cons cell to be the undo boundary after this command.  */
 250   if (NILP (pending_boundary))
 251     pending_boundary = Fcons (Qnil, Qnil);
 252
 253   if (buf != last_undo_buffer)
 254     boundary = 1;
 255   last_undo_buffer = buf;
 256
 257   /* Switch temporarily to the buffer that was changed.  */
 258   current_buffer = buf;
 259
 260   if (boundary)
 261     Fundo_boundary ();
 262
 263   if (MODIFF <= SAVE_MODIFF)
 264     record_first_change ();
 265
 266   XSETINT (lbeg, beg);
 267   XSETINT (lend, beg + length);
 268   entry = Fcons (Qnil, Fcons (prop, Fcons (value, Fcons (lbeg, lend))));
 269   current_buffer->undo_list = Fcons (entry, current_buffer->undo_list);
 270
 271   current_buffer = obuf;
 272 }
 273
 274 DEFUN ("undo-boundary", Fundo_boundary, Sundo_boundary, 0, 0, 0,
 275        doc: /* Mark a boundary between units of undo.
 276 An undo command will stop at this point,
 277 but another undo command will undo to the previous boundary.  */)
 278   (void)
 279 {
 280   Lisp_Object tem;
 281   if (EQ (current_buffer->undo_list, Qt))
 282     return Qnil;
 283   tem = Fcar (current_buffer->undo_list);
 284   if (!NILP (tem))
 285     {
 286       /* One way or another, cons nil onto the front of the undo list.  */
 287       if (!NILP (pending_boundary))
 288         {
 289           /* If we have preallocated the cons cell to use here,
 290              use that one.  */
 291           XSETCDR (pending_boundary, current_buffer->undo_list);
 292           current_buffer->undo_list = pending_boundary;
 293           pending_boundary = Qnil;
 294         }
 295       else
 296         current_buffer->undo_list = Fcons (Qnil, current_buffer->undo_list);
 297     }
 298   last_boundary_position = PT;
 299   last_boundary_buffer = current_buffer;
 300   return Qnil;
 301 }
 302
 303 /* At garbage collection time, make an undo list shorter at the end,
 304    returning the truncated list.  How this is done depends on the
 305    variables undo-limit, undo-strong-limit and undo-outer-limit.
 306    In some cases this works by calling undo-outer-limit-function.  */
 307
 308 void
 309 truncate_undo_list (struct buffer *b)
 310 {
 311   Lisp_Object list;
 312   Lisp_Object prev, next, last_boundary;
 313   int size_so_far = 0;
 314
 315   /* Make sure that calling undo-outer-limit-function
 316      won't cause another GC.  */
 317   int count = inhibit_garbage_collection ();
 318
 319   /* Make the buffer current to get its local values of variables such
 320      as undo_limit.  Also so that Vundo_outer_limit_function can
 321      tell which buffer to operate on.  */
 322   record_unwind_protect (set_buffer_if_live, Fcurrent_buffer ());
 323   set_buffer_internal (b);
 324
 325   list = b->undo_list;
 326
 327   prev = Qnil;
 328   next = list;
 329   last_boundary = Qnil;
 330
 331   /* If the first element is an undo boundary, skip past it.  */
 332   if (CONSP (next) && NILP (XCAR (next)))
 333     {
 334       /* Add in the space occupied by this element and its chain link.  */
 335       size_so_far += sizeof (struct Lisp_Cons);
 336
 337       /* Advance to next element.  */
 338       prev = next;
 339       next = XCDR (next);
 340     }
 341
 342   /* Always preserve at least the most recent undo record
 343      unless it is really horribly big.
 344
 345      Skip, skip, skip the undo, skip, skip, skip the undo,
 346      Skip, skip, skip the undo, skip to the undo bound'ry.  */
 347
 348   while (CONSP (next) && ! NILP (XCAR (next)))
 349     {
 350       Lisp_Object elt;
 351       elt = XCAR (next);
 352
 353       /* Add in the space occupied by this element and its chain link.  */
 354       size_so_far += sizeof (struct Lisp_Cons);
 355       if (CONSP (elt))
 356         {
 357           size_so_far += sizeof (struct Lisp_Cons);
 358           if (STRINGP (XCAR (elt)))
 359             size_so_far += (sizeof (struct Lisp_String) - 1
 360                             + SCHARS (XCAR (elt)));
 361         }
 362
 363       /* Advance to next element.  */
 364       prev = next;
 365       next = XCDR (next);
 366     }
 367
 368   /* If by the first boundary we have already passed undo_outer_limit,
 369      we're heading for memory full, so offer to clear out the list.  */
 370   if (INTEGERP (Vundo_outer_limit)
 371       && size_so_far > XINT (Vundo_outer_limit)
 372       && !NILP (Vundo_outer_limit_function))
 373     {
 374       Lisp_Object tem;
 375       struct buffer *temp = last_undo_buffer;
 376
 377       /* Normally the function this calls is undo-outer-limit-truncate.  */
 378       tem = call1 (Vundo_outer_limit_function, make_number (size_so_far));
 379       if (! NILP (tem))
 380         {
 381           /* The function is responsible for making
 382              any desired changes in buffer-undo-list.  */
 383           unbind_to (count, Qnil);
 384           return;
 385         }
 386       /* That function probably used the minibuffer, and if so, that
 387          changed last_undo_buffer.  Change it back so that we don't
 388          force next change to make an undo boundary here.  */
 389       last_undo_buffer = temp;
 390     }
 391
 392   if (CONSP (next))
 393     last_boundary = prev;
 394
 395   /* Keep additional undo data, if it fits in the limits.  */
 396   while (CONSP (next))
 397     {
 398       Lisp_Object elt;
 399       elt = XCAR (next);
 400
 401       /* When we get to a boundary, decide whether to truncate
 402          either before or after it.  The lower threshold, undo_limit,
 403          tells us to truncate after it.  If its size pushes past
 404          the higher threshold undo_strong_limit, we truncate before it.  */
 405       if (NILP (elt))
 406         {
 407           if (size_so_far > undo_strong_limit)
 408             break;
 409           last_boundary = prev;
 410           if (size_so_far > undo_limit)
 411             break;
 412         }
 413
 414       /* Add in the space occupied by this element and its chain link.  */
 415       size_so_far += sizeof (struct Lisp_Cons);
 416       if (CONSP (elt))
 417         {
 418           size_so_far += sizeof (struct Lisp_Cons);
 419           if (STRINGP (XCAR (elt)))
 420             size_so_far += (sizeof (struct Lisp_String) - 1
 421                             + SCHARS (XCAR (elt)));
 422         }
 423
 424       /* Advance to next element.  */
 425       prev = next;
 426       next = XCDR (next);
 427     }
 428
 429   /* If we scanned the whole list, it is short enough; don't change it.  */
 430   if (NILP (next))
 431     ;
 432   /* Truncate at the boundary where we decided to truncate.  */
 433   else if (!NILP (last_boundary))
 434     XSETCDR (last_boundary, Qnil);
 435   /* There's nothing we decided to keep, so clear it out.  */
 436   else
 437     b->undo_list = Qnil;
 438
 439   unbind_to (count, Qnil);
 440 }
 441 \f
 442 DEFUN ("primitive-undo", Fprimitive_undo, Sprimitive_undo, 2, 2, 0,
 443        doc: /* Undo N records from the front of the list LIST.
 444 Return what remains of the list.  */)
 445   (Lisp_Object n, Lisp_Object list)
 446 {
 447   struct gcpro gcpro1, gcpro2;
 448   Lisp_Object next;
 449   int count = SPECPDL_INDEX ();
 450   register int arg;
 451   Lisp_Object oldlist;
 452   int did_apply = 0;
 453
 454 #if 0  /* This is a good feature, but would make undo-start
 455           unable to do what is expected.  */
 456   Lisp_Object tem;
 457
 458   /* If the head of the list is a boundary, it is the boundary
 459      preceding this command.  Get rid of it and don't count it.  */
 460   tem = Fcar (list);
 461   if (NILP (tem))
 462     list = Fcdr (list);
 463 #endif
 464
 465   CHECK_NUMBER (n);
 466   arg = XINT (n);
 467   next = Qnil;
 468   GCPRO2 (next, list);
 469   /* I don't think we need to gcpro oldlist, as we use it only
 470      to check for EQ.  ++kfs  */
 471
 472   /* In a writable buffer, enable undoing read-only text that is so
 473      because of text properties.  */
 474   if (NILP (current_buffer->read_only))
 475     specbind (Qinhibit_read_only, Qt);
 476
 477   /* Don't let `intangible' properties interfere with undo.  */
 478   specbind (Qinhibit_point_motion_hooks, Qt);
 479
 480   oldlist = current_buffer->undo_list;
 481
 482   while (arg > 0)
 483     {
 484       while (CONSP (list))
 485         {
 486           next = XCAR (list);
 487           list = XCDR (list);
 488           /* Exit inner loop at undo boundary.  */
 489           if (NILP (next))
 490             break;
 491           /* Handle an integer by setting point to that value.  */
 492           if (INTEGERP (next))
 493             SET_PT (clip_to_bounds (BEGV, XINT (next), ZV));
 494           else if (CONSP (next))
 495             {
 496               Lisp_Object car, cdr;
 497
 498               car = XCAR (next);
 499               cdr = XCDR (next);
 500               if (EQ (car, Qt))
 501                 {
 502                   /* Element (t high . low) records previous modtime.  */
 503                   Lisp_Object high, low;
 504                   int mod_time;
 505                   struct buffer *base_buffer = current_buffer;
 506
 507                   high = Fcar (cdr);
 508                   low = Fcdr (cdr);
 509                   mod_time = (XFASTINT (high) << 16) + XFASTINT (low);
 510
 511                   if (current_buffer->base_buffer)
 512                     base_buffer = current_buffer->base_buffer;
 513
 514                   /* If this records an obsolete save
 515                      (not matching the actual disk file)
 516                      then don't mark unmodified.  */
 517                   if (mod_time != base_buffer->modtime)
 518                     continue;
 519 #ifdef CLASH_DETECTION
 520                   Funlock_buffer ();
 521 #endif /* CLASH_DETECTION */
 522                   Fset_buffer_modified_p (Qnil);
 523                 }
 524               else if (EQ (car, Qnil))
 525                 {
 526                   /* Element (nil PROP VAL BEG . END) is property change.  */
 527                   Lisp_Object beg, end, prop, val;
 528
 529                   prop = Fcar (cdr);
 530                   cdr = Fcdr (cdr);
 531                   val = Fcar (cdr);
 532                   cdr = Fcdr (cdr);
 533                   beg = Fcar (cdr);
 534                   end = Fcdr (cdr);
 535
 536                   if (XINT (beg) < BEGV || XINT (end) > ZV)
 537                     error ("Changes to be undone are outside visible portion of buffer");
 538                   Fput_text_property (beg, end, prop, val, Qnil);
 539                 }
 540               else if (INTEGERP (car) && INTEGERP (cdr))
 541                 {
 542                   /* Element (BEG . END) means range was inserted.  */
 543
 544                   if (XINT (car) < BEGV
 545                       || XINT (cdr) > ZV)
 546                     error ("Changes to be undone are outside visible portion of buffer");
 547                   /* Set point first thing, so that undoing this undo
 548                      does not send point back to where it is now.  */
 549                   Fgoto_char (car);
 550                   Fdelete_region (car, cdr);
 551                 }
 552               else if (EQ (car, Qapply))
 553                 {
 554                   /* Element (apply FUN . ARGS) means call FUN to undo.  */
 555                   struct buffer *save_buffer = current_buffer;
 556
 557                   car = Fcar (cdr);
 558                   cdr = Fcdr (cdr);
 559                   if (INTEGERP (car))
 560                     {
 561                       /* Long format: (apply DELTA START END FUN . ARGS).  */
 562                       Lisp_Object delta = car;
 563                       Lisp_Object start = Fcar (cdr);
 564                       Lisp_Object end   = Fcar (Fcdr (cdr));
 565                       Lisp_Object start_mark = Fcopy_marker (start, Qnil);
 566                       Lisp_Object end_mark   = Fcopy_marker (end, Qt);
 567
 568                       cdr = Fcdr (Fcdr (cdr));
 569                       apply1 (Fcar (cdr), Fcdr (cdr));
 570
 571                       /* Check that the function did what the entry said it
 572                          would do.  */
 573                       if (!EQ (start, Fmarker_position (start_mark))
 574                           || (XINT (delta) + XINT (end)
 575                               != marker_position (end_mark)))
 576                         error ("Changes to be undone by function different than announced");
 577                       Fset_marker (start_mark, Qnil, Qnil);
 578                       Fset_marker (end_mark, Qnil, Qnil);
 579                     }
 580                   else
 581                     apply1 (car, cdr);
 582
 583                   if (save_buffer != current_buffer)
 584                     error ("Undo function switched buffer");
 585                   did_apply = 1;
 586                 }
 587               else if (STRINGP (car) && INTEGERP (cdr))
 588                 {
 589                   /* Element (STRING . POS) means STRING was deleted.  */
 590                   Lisp_Object membuf;
 591                   EMACS_INT pos = XINT (cdr);
 592
 593                   membuf = car;
 594                   if (pos < 0)
 595                     {
 596                       if (-pos < BEGV || -pos > ZV)
 597                         error ("Changes to be undone are outside visible portion of buffer");
 598                       SET_PT (-pos);
 599                       Finsert (1, &membuf);
 600                     }
 601                   else
 602                     {
 603                       if (pos < BEGV || pos > ZV)
 604                         error ("Changes to be undone are outside visible portion of buffer");
 605                       SET_PT (pos);
 606
 607                       /* Now that we record marker adjustments
 608                          (caused by deletion) for undo,
 609                          we should always insert after markers,
 610                          so that undoing the marker adjustments
 611                          put the markers back in the right place.  */
 612                       Finsert (1, &membuf);
 613                       SET_PT (pos);
 614                     }
 615                 }
 616               else if (MARKERP (car) && INTEGERP (cdr))
 617                 {
 618                   /* (MARKER . INTEGER) means a marker MARKER
 619                      was adjusted by INTEGER.  */
 620                   if (XMARKER (car)->buffer)
 621                     Fset_marker (car,
 622                                  make_number (marker_position (car) - XINT (cdr)),
 623                                  Fmarker_buffer (car));
 624                 }
 625             }
 626         }
 627       arg--;
 628     }
 629
 630
 631   /* Make sure an apply entry produces at least one undo entry,
 632      so the test in `undo' for continuing an undo series
 633      will work right.  */
 634   if (did_apply
 635       && EQ (oldlist, current_buffer->undo_list))
 636     current_buffer->undo_list
 637       = Fcons (list3 (Qapply, Qcdr, Qnil), current_buffer->undo_list);
 638
 639   UNGCPRO;
 640   return unbind_to (count, list);
 641 }
 642 \f
 643 void
 644 syms_of_undo (void)
 645 {
 646   Qinhibit_read_only = intern_c_string ("inhibit-read-only");
 647   staticpro (&Qinhibit_read_only);
 648
 649   Qapply = intern_c_string ("apply");
 650   staticpro (&Qapply);
 651
 652   pending_boundary = Qnil;
 653   staticpro (&pending_boundary);
 654
 655   last_undo_buffer = NULL;
 656   last_boundary_buffer = NULL;
 657
 658   defsubr (&Sprimitive_undo);
 659   defsubr (&Sundo_boundary);
 660
 661   DEFVAR_INT ("undo-limit", undo_limit,
 662               doc: /* Keep no more undo information once it exceeds this size.
 663 This limit is applied when garbage collection happens.
 664 When a previous command increases the total undo list size past this
 665 value, the earlier commands that came before it are forgotten.
 666
 667 The size is counted as the number of bytes occupied,
 668 which includes both saved text and other data.  */);
 669   undo_limit = 80000;
 670
 671   DEFVAR_INT ("undo-strong-limit", undo_strong_limit,
 672               doc: /* Don't keep more than this much size of undo information.
 673 This limit is applied when garbage collection happens.
 674 When a previous command increases the total undo list size past this
 675 value, that command and the earlier commands that came before it are forgotten.
 676 However, the most recent buffer-modifying command's undo info
 677 is never discarded for this reason.
 678
 679 The size is counted as the number of bytes occupied,
 680 which includes both saved text and other data.  */);
 681   undo_strong_limit = 120000;
 682
 683   DEFVAR_LISP ("undo-outer-limit", Vundo_outer_limit,
 684               doc: /* Outer limit on size of undo information for one command.
 685 At garbage collection time, if the current command has produced
 686 more than this much undo information, it discards the info and displays
 687 a warning.  This is a last-ditch limit to prevent memory overflow.
 688
 689 The size is counted as the number of bytes occupied, which includes
 690 both saved text and other data.  A value of nil means no limit.  In
 691 this case, accumulating one huge undo entry could make Emacs crash as
 692 a result of memory overflow.
 693
 694 In fact, this calls the function which is the value of
 695 `undo-outer-limit-function' with one argument, the size.
 696 The text above describes the behavior of the function
 697 that variable usually specifies.  */);
 698   Vundo_outer_limit = make_number (12000000);
 699
 700   DEFVAR_LISP ("undo-outer-limit-function", Vundo_outer_limit_function,
 701                doc: /* Function to call when an undo list exceeds `undo-outer-limit'.
 702 This function is called with one argument, the current undo list size
 703 for the most recent command (since the last undo boundary).
 704 If the function returns t, that means truncation has been fully handled.
 705 If it returns nil, the other forms of truncation are done.
 706
 707 Garbage collection is inhibited around the call to this function,
 708 so it must make sure not to do a lot of consing.  */);
 709   Vundo_outer_limit_function = Qnil;
 710
 711   DEFVAR_BOOL ("undo-inhibit-record-point", undo_inhibit_record_point,
 712                doc: /* Non-nil means do not record `point' in `buffer-undo-list'.  */);
 713   undo_inhibit_record_point = 0;
 714 }
 715