Fix sxhash-equal on bytecodes, markers, etc.

Problem reported by Pip Cet (Bug#38912#14).
* doc/lispref/objects.texi (Equality Predicates):
Document better when ‘equal’ looks inside objects.
* doc/lispref/windows.texi (Window Configurations):
Don’t say that ‘equal’ looks inside window configurations.
* etc/NEWS: Mention the change.
* src/fns.c (internal_equal):
Do not look inside window configurations.
(sxhash_obj): Hash markers, byte-code function objects,
char-tables, and font objects consistently with Fequal.
* src/window.c (compare_window_configurations):
Now static.  Remove last argument.  Caller changed.
* test/lisp/ffap-tests.el (ffap-other-window--bug-25352):
Use compare-window-configurations, not ‘equal’.
* test/src/fns-tests.el (test-sxhash-equal): New test.

9 files changed, 67 insertions, 47 deletions

diff --git a/doc/lispref/objects.texi b/doc/lispref/objects.texi
index 4be2eb6918b..4242223a48e 100644
--- a/doc/lispref/objects.texi
+++ b/doc/lispref/objects.texi
@@ -2336,8 +2336,12 @@ same sequence of character codes and all these codes are in the range
 @end group
 @end example
 
-However, two distinct buffers are never considered @code{equal}, even if
-their textual contents are the same.
+The @code{equal} function recursively compares the contents of objects
+if they are integers, strings, markers, vectors, bool-vectors,
+byte-code function objects, char-tables, records, or font objects.
+Other objects are considered @code{equal} only if they are @code{eq}.
+For example, two distinct buffers are never considered @code{equal},
+even if their textual contents are the same.
 @end defun
 
   For @code{equal}, equality is defined recursively; for example, given
diff --git a/doc/lispref/windows.texi b/doc/lispref/windows.texi
index c9301c9d186..d0791d40196 100644
--- a/doc/lispref/windows.texi
+++ b/doc/lispref/windows.texi
@@ -5915,10 +5915,6 @@ This function compares two window configurations as regards the
 structure of windows, but ignores the values of point and the
 saved scrolling positions---it can return @code{t} even if those
 aspects differ.
-
-The function @code{equal} can also compare two window configurations; it
-regards configurations as unequal if they differ in any respect, even a
-saved point.
 @end defun
 
 @defun window-configuration-frame config
diff --git a/etc/NEWS b/etc/NEWS
index d6cabf8e9e4..0784160ce22 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -42,6 +42,12 @@ applies, and please also update docstrings as needed.
 
 * Incompatible Lisp Changes in Emacs 28.1
 
+** 'equal' no longer examines some contents of window configurations.
+Instead, it considers window configurations to be equal only if they
+are eq.  To compare contents, use compare-window-configurations
+instead.  This change helps fix a bug in sxhash-equal, which returned
+incorrect hashes for window configurations and some other objects.
+
 
 * Lisp Changes in Emacs 28.1
 
diff --git a/src/fns.c b/src/fns.c
index 4a0a8fd96d8..4a463a8feb2 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -2434,6 +2434,9 @@ internal_equal (Lisp_Object o1, Lisp_Object o2, enum equal_kind equal_kind,
            same size.  */
         if (ASIZE (o2) != size)
           return false;
+
+        /* Compare bignums, overlays, markers, and boolvectors
+           specially, by comparing their values.  */
         if (BIGNUMP (o1))
           return mpz_cmp (*xbignum_val (o1), *xbignum_val (o2)) == 0;
         if (OVERLAYP (o1))
@@ -2454,7 +2457,6 @@ internal_equal (Lisp_Object o1, Lisp_Object o2, enum equal_kind equal_kind,
                     && (XMARKER (o1)->buffer == 0
                         || XMARKER (o1)->bytepos == XMARKER (o2)->bytepos));
           }
-        /* Boolvectors are compared much like strings.  */
         if (BOOL_VECTOR_P (o1))
           {
             EMACS_INT size = bool_vector_size (o1);
@@ -2465,11 +2467,6 @@ internal_equal (Lisp_Object o1, Lisp_Object o2, enum equal_kind equal_kind,
               return false;
             return true;
           }
-        if (WINDOW_CONFIGURATIONP (o1))
-          {
-            eassert (equal_kind != EQUAL_NO_QUIT);
-            return compare_window_configurations (o1, o2, false);
-          }
 
         /* Aside from them, only true vectors, char-tables, compiled
            functions, and fonts (font-spec, font-entity, font-object)
@@ -4703,22 +4700,35 @@ sxhash_obj (Lisp_Object obj, int depth)
       hash = sxhash_string (SSDATA (obj), SBYTES (obj));
       break;
 
-      /* This can be everything from a vector to an overlay.  */
     case Lisp_Vectorlike:
-      if (BIGNUMP (obj))
-        hash = sxhash_bignum (obj);
-      else if (VECTORP (obj) || RECORDP (obj))
-        /* According to the CL HyperSpec, two arrays are equal only if
-           they are `eq', except for strings and bit-vectors.  In
-           Emacs, this works differently.  We have to compare element
-           by element.  Same for records.  */
-        hash = sxhash_vector (obj, depth);
-      else if (BOOL_VECTOR_P (obj))
-        hash = sxhash_bool_vector (obj);
-      else
-        /* Others are `equal' if they are `eq', so let's take their
-           address as hash.  */
-        hash = XHASH (obj);
+      {
+        enum pvec_type pvec_type = PSEUDOVECTOR_TYPE (XVECTOR (obj));
+        if (! (PVEC_NORMAL_VECTOR < pvec_type && pvec_type < PVEC_COMPILED))
+          {
+            /* According to the CL HyperSpec, two arrays are equal only if
+               they are 'eq', except for strings and bit-vectors.  In
+               Emacs, this works differently.  We have to compare element
+               by element.  Same for pseudovectors that internal_equal
+               examines the Lisp contents of.  */
+            hash = sxhash_vector (obj, depth);
+            break;
+          }
+        else if (pvec_type == PVEC_BIGNUM)
+          hash = sxhash_bignum (obj);
+        else if (pvec_type == PVEC_MARKER)
+          {
+            ptrdiff_t bytepos
+              = XMARKER (obj)->buffer ? XMARKER (obj)->bytepos : 0;
+            hash = sxhash_combine ((intptr_t) XMARKER (obj)->buffer, bytepos);
+            hash = SXHASH_REDUCE (hash);
+          }
+        else if (pvec_type == PVEC_BOOL_VECTOR)
+          hash = sxhash_bool_vector (obj);
+        else
+          /* Others are 'equal' if they are 'eq', so take their
+             address as hash.  */
+          hash = XHASH (obj);
+      }
       break;
 
     case Lisp_Cons:
diff --git a/src/lisp.h b/src/lisp.h
index 1a1ae0399be..3681b7b2a7c 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -1069,7 +1069,7 @@ DEFINE_GDB_SYMBOL_END (PSEUDOVECTOR_FLAG)
    with PVEC_TYPE_MASK to indicate the actual type.  */
 enum pvec_type
 {
-  PVEC_NORMAL_VECTOR,
+  PVEC_NORMAL_VECTOR,   /* Should be first, for sxhash_obj.  */
   PVEC_FREE,
   PVEC_BIGNUM,
   PVEC_MARKER,
@@ -1094,7 +1094,7 @@ enum pvec_type
   PVEC_CONDVAR,
   PVEC_MODULE_FUNCTION,
 
-  /* These should be last, check internal_equal to see why.  */
+  /* These should be last, for internal_equal and sxhash_obj.  */
   PVEC_COMPILED,
   PVEC_CHAR_TABLE,
   PVEC_SUB_CHAR_TABLE,
diff --git a/src/window.c b/src/window.c
index ff17cd88f38..8cdad27b664 100644
--- a/src/window.c
+++ b/src/window.c
@@ -7976,19 +7976,17 @@ foreach_window_1 (struct window *w, bool (*fn) (struct window *, void *),
 /* Return true if window configurations CONFIGURATION1 and CONFIGURATION2
    describe the same state of affairs.  This is used by Fequal.
 
-   IGNORE_POSITIONS means ignore non-matching scroll positions
-   and the like.
+   Ignore non-matching scroll positions and the like.
 
    This ignores a couple of things like the dedication status of
    window, combination_limit and the like.  This might have to be
    fixed.  */
 
-bool
+static bool
 compare_window_configurations (Lisp_Object configuration1,
-                               Lisp_Object configuration2,
-                               bool ignore_positions)
+                               Lisp_Object configuration2)
 {
-  register struct save_window_data *d1, *d2;
+  struct save_window_data *d1, *d2;
   struct Lisp_Vector *sws1, *sws2;
   ptrdiff_t i;
 
@@ -8006,9 +8004,6 @@ compare_window_configurations (Lisp_Object configuration1,
       || d1->frame_menu_bar_lines != d2->frame_menu_bar_lines
       || !EQ (d1->selected_frame, d2->selected_frame)
       || !EQ (d1->f_current_buffer, d2->f_current_buffer)
-      || (!ignore_positions
-          && (!EQ (d1->minibuf_scroll_window, d2->minibuf_scroll_window)
-              || !EQ (d1->minibuf_selected_window, d2->minibuf_selected_window)))
       || !EQ (d1->focus_frame, d2->focus_frame)
       /* Verify that the two configurations have the same number of windows.  */
       || sws1->header.size != sws2->header.size)
@@ -8041,12 +8036,6 @@ compare_window_configurations (Lisp_Object configuration1,
              equality.  */
           || !EQ (sw1->parent, sw2->parent)
           || !EQ (sw1->prev, sw2->prev)
-          || (!ignore_positions
-              && (!EQ (sw1->hscroll, sw2->hscroll)
-                  || !EQ (sw1->min_hscroll, sw2->min_hscroll)
-                  || !EQ (sw1->start_at_line_beg, sw2->start_at_line_beg)
-                  || NILP (Fequal (sw1->start, sw2->start))
-                  || NILP (Fequal (sw1->pointm, sw2->pointm))))
           || !EQ (sw1->left_margin_cols, sw2->left_margin_cols)
           || !EQ (sw1->right_margin_cols, sw2->right_margin_cols)
           || !EQ (sw1->left_fringe_width, sw2->left_fringe_width)
@@ -8071,7 +8060,7 @@ This function ignores details such as the values of point
 and scrolling positions.  */)
   (Lisp_Object x, Lisp_Object y)
 {
-  if (compare_window_configurations (x, y, true))
+  if (compare_window_configurations (x, y))
     return Qt;
   return Qnil;
 }
diff --git a/src/window.h b/src/window.h
index aa8d2c8d1d2..167d1be7abb 100644
--- a/src/window.h
+++ b/src/window.h
@@ -1184,7 +1184,6 @@ extern Lisp_Object window_list (void);
 extern Lisp_Object window_parameter (struct window *, Lisp_Object parameter);
 extern struct window *decode_live_window (Lisp_Object);
 extern struct window *decode_any_window (Lisp_Object);
-extern bool compare_window_configurations (Lisp_Object, Lisp_Object, bool);
 extern void mark_window_cursors_off (struct window *);
 extern bool window_wants_mode_line (struct window *);
 extern bool window_wants_header_line (struct window *);
diff --git a/test/lisp/ffap-tests.el b/test/lisp/ffap-tests.el
index eaf39680e48..30c8f794577 100644
--- a/test/lisp/ffap-tests.el
+++ b/test/lisp/ffap-tests.el
@@ -74,7 +74,7 @@ left alone when opening a URL in an external browser."
              (urls nil)
              (ffap-url-fetcher (lambda (url) (push url urls) nil)))
     (should-not (ffap-other-window "https://www.gnu.org"))
-    (should (equal (current-window-configuration) old))
+    (should (compare-window-configurations (current-window-configuration) old))
     (should (equal urls '("https://www.gnu.org")))))
 
 (provide 'ffap-tests)
diff --git a/test/src/fns-tests.el b/test/src/fns-tests.el
index 60be2c6c2d7..c6ceae4a00e 100644
--- a/test/src/fns-tests.el
+++ b/test/src/fns-tests.el
@@ -858,6 +858,22 @@
        (puthash k k h)))
     (should (= 100 (hash-table-count h)))))
 
+(ert-deftest test-sxhash-equal ()
+  (should (= (sxhash-equal (* most-positive-fixnum most-negative-fixnum))
+             (sxhash-equal (* most-positive-fixnum most-negative-fixnum))))
+  (should (= (sxhash-equal (make-string 1000 ?a))
+             (sxhash-equal (make-string 1000 ?a))))
+  (should (= (sxhash-equal (point-marker))
+             (sxhash-equal (point-marker))))
+  (should (= (sxhash-equal (make-vector 1000 (make-string 10 ?a)))
+             (sxhash-equal (make-vector 1000 (make-string 10 ?a)))))
+  (should (= (sxhash-equal (make-bool-vector 1000 t))
+             (sxhash-equal (make-bool-vector 1000 t))))
+  (should (= (sxhash-equal (make-char-table nil (make-string 10 ?a)))
+             (sxhash-equal (make-char-table nil (make-string 10 ?a)))))
+  (should (= (sxhash-equal (record 'a (make-string 10 ?a)))
+             (sxhash-equal (record 'a (make-string 10 ?a))))))
+
 (ert-deftest test-secure-hash ()
   (should (equal (secure-hash 'md5    "foobar")
                  "3858f62230ac3c915f300c664312c63f"))