itree: Reproduce markers's behavior more faithfully (bug#58928)

The most obvious problem was the lack of support for
`insert-before-markers`, but the behavior was also different in a few
other cases.

* src/itree.h (itree_insert_gap):
* src/itree.c (itree_insert_gap): Add `before_markers` arg.
* src/lisp.h (adjust_overlays_for_insert):
* src/buffer.c (adjust_overlays_for_insert): Add `before_markers` arg.

* src/insdel.c (adjust_markers_for_replace, adjust_markers_for_insert)
(adjust_markers_for_delete): Adjust overlays directly from here.
(insert_1_both, insert_from_string_1, insert_from_gap)
(insert_from_buffer_1, adjust_after_replace, replace_range)
(replace_range_2, del_range_2): Don't adjust overlays explicitly here
any more.

* test/src/buffer-tests.el (test-overlay-insert-before-markers-empty)
(test-overlay-insert-before-markers-non-empty): New tests.

1 files changed, 16 insertions, 9 deletions

diff --git a/src/itree.c b/src/itree.c
index 3137cb6358f..611f6d46845 100644
--- a/src/itree.c
+++ b/src/itree.c
@@ -1186,7 +1186,7 @@ itree_iterator_finish (struct itree_iterator *iter)
 
 void
 itree_insert_gap (struct itree_tree *tree,
-                  ptrdiff_t pos, ptrdiff_t length)
+                  ptrdiff_t pos, ptrdiff_t length, bool before_markers)
 {
   if (!tree || length <= 0 || tree->root == NULL)
     return;
@@ -1195,14 +1195,19 @@ itree_insert_gap (struct itree_tree *tree,
   /* FIXME: Don't allocate iterator/stack anew every time. */
 
   /* Nodes with front_advance starting at pos may mess up the tree
-     order, so we need to remove them first. */
+     order, so we need to remove them first.  This doesn't apply for
+     `before_markers` since in that case, all positions move identically
+     regardless of `front_advance` or `rear_advance`.  */
   struct interval_stack *saved = interval_stack_create (0);
   struct itree_node *node = NULL;
-  ITREE_FOREACH (node, tree, pos, pos + 1, PRE_ORDER)
+  if (!before_markers)
     {
-      if (node->begin == pos && node->front_advance
-          && (node->begin != node->end || node->rear_advance))
-        interval_stack_push (saved, node);
+      ITREE_FOREACH (node, tree, pos, pos + 1, PRE_ORDER)
+        {
+          if (node->begin == pos && node->front_advance
+              && (node->begin != node->end || node->rear_advance))
+            interval_stack_push (saved, node);
+        }
     }
   for (size_t i = 0; i < saved->length; ++i)
     itree_remove (tree, nav_nodeptr (saved->nodes[i]));
@@ -1235,10 +1240,12 @@ itree_insert_gap (struct itree_tree *tree,
               && pos <= node->left->limit + node->left->offset)
             interval_stack_push (stack, node->left);
 
-          /* node->begin == pos implies no front-advance. */
-          if (node->begin > pos)
+          if (before_markers
+              ? node->begin >= pos
+              : node->begin > pos) /* node->begin == pos => !front-advance  */
             node->begin += length;
-          if (node->end > pos || (node->end == pos && node->rear_advance))
+          if (node->end > pos
+              || (node->end == pos && (before_markers || node->rear_advance)))
             {
               node->end += length;
               eassert (node != NULL);