search: fix viewport not moving if we tried to move it past the scrollback start

If the match was somewhere near the scrollback beginning, and if the
entire scrollback hadn't yet been filled, we ended up trying to move
the viewport past the beginning of the scrollback, which we then
adjusted in the wrong direction, causing the viewport to not move at
all.

Besides being a bad user experience, since the new match wasn't
visible, foot would also crash if you manually scrolled up to the
match.

CHANGELOG.md

@@ -57,6 +57,9 @@
 * Mouse cursor style in top and left margins.
 * Selection is now **updated** when the cursor moves outside the grid
   (https://codeberg.org/dnkl/foot/issues/70).
+* Viewport sometimes not moving when doing a scrollback search.
+* Crash when cancelling a scrollback search and the window had been
+  resized while searching.
 
 
 ### Security

search.c

@@ -18,6 +18,44 @@
 #include "shm.h"
 #include "util.h"
 
+/*
+ * Ensures a "new" viewport doesn't contain any unallocated rows.
+ *
+ * This is done by first checking if the *first* row is NULL. If so,
+ * we move the viewport *forward*, until the first row is non-NULL. At
+ * this point, the entire viewport should be allocated rows only.
+ *
+ * If the first row already was non-NULL, we instead check the *last*
+ * row, and if it is NULL, we move the viewport *backward* until the
+ * last row is non-NULL.
+ */
+static int
+ensure_view_is_allocated(struct terminal *term, int new_view)
+{
+    int view_end = (new_view + term->rows - 1) & (term->grid->num_rows - 1);
+
+    if (term->grid->rows[new_view] == NULL) {
+        while (term->grid->rows[new_view] == NULL)
+            new_view = (new_view + 1) & (term->grid->num_rows - 1);
+    }
+
+    else if (term->grid->rows[view_end] == NULL) {
+        while (term->grid->rows[view_end] == NULL) {
+            new_view--;
+            if (new_view < 0)
+                new_view += term->grid->num_rows;
+            view_end = (new_view + term->rows - 1) & (term->grid->num_rows - 1);
+        }
+    }
+
+#if defined(_DEBUG)
+    for (size_t r = 0; r < term->rows; r++)
+        assert(term->grid->rows[(new_view + r) & (term->grid->num_rows - 1)] != NULL);
+#endif
+
+    return new_view;
+}
+
 static bool
 search_ensure_size(struct terminal *term, size_t wanted_size)
 {
@@ -124,23 +162,10 @@ search_update_selection(struct terminal *term,
         while (new_view < 0)
             new_view += term->grid->num_rows;
 
-        /* Prevent scrolling in uninitialized rows */
-        bool all_initialized = false;
-        do {
-            all_initialized = true;
-
-            for (int i = 0; i < term->rows; i++) {
-                int row_no = (new_view + i) % term->grid->num_rows;
-                if (term->grid->rows[row_no] == NULL) {
-                    all_initialized = false;
-                    new_view--;
-                    break;
-                }
-            }
-        } while (!all_initialized);
+        new_view = ensure_view_is_allocated(term, new_view);
 
         /* Don't scroll past scrollback history */
-        int end = (term->grid->offset + term->rows - 1) % term->grid->num_rows;
+        int end = (term->grid->offset + term->rows - 1) & (term->grid->num_rows - 1);
         if (end >= term->grid->offset) {
             /* Not wrapped */
             if (new_view >= term->grid->offset && new_view <= end)
@@ -150,6 +175,12 @@ search_update_selection(struct terminal *term,
                 new_view = term->grid->offset;
         }
 
+#if defined(_DEBUG)
+        /* Verify all to-be-visible rows have been allocated */
+        for (int r = 0; r < term->rows; r++)
+            assert(term->grid->rows[(new_view + r) & (term->grid->num_rows - 1)] != NULL);
+#endif
+
         /* Update view */
         term->grid->view = new_view;
         if (new_view != old_view)
@@ -221,8 +252,8 @@ search_find_next(struct terminal *term)
             backward ? "backward" : "forward", start_row, start_col,
             term->grid->offset, term->grid->view);
 
-#define ROW_DEC(_r) ((_r) = ((_r) - 1 + term->grid->num_rows) % term->grid->num_rows)
-#define ROW_INC(_r) ((_r) = ((_r) + 1) % term->grid->num_rows)
+#define ROW_DEC(_r) ((_r) = ((_r) - 1 + term->grid->num_rows) & (term->grid->num_rows - 1))
+#define ROW_INC(_r) ((_r) = ((_r) + 1) & (term->grid->num_rows - 1))
 
     /* Scan backward from current end-of-output */
     /* TODO: don't search "scrollback" in alt screen? */
@@ -316,7 +347,7 @@ search_match_to_end_of_word(struct terminal *term, bool spaces_only)
     /* Calculate end coord - note: assumed to be valid */
     for (size_t i = 0; i < len; i++) {
         if (++end_col >= term->cols) {
-            end_row = (end_row + 1) % term->grid->num_rows;
+            end_row = (end_row + 1) & (term->grid->num_rows - 1);
             end_col = 0;
         }
     }
@@ -328,7 +359,7 @@ search_match_to_end_of_word(struct terminal *term, bool spaces_only)
 
     for (size_t r = 0;
          r < term->grid->num_rows;
-         end_row = (end_row + 1) % term->grid->num_rows, r++)
+         end_row = (end_row + 1) & (term->grid->num_rows - 1), r++)
     {
         const struct row *row = term->grid->rows[end_row];
         if (row == NULL)
@@ -440,8 +471,10 @@ execute_binding(struct seat *seat, struct terminal *term,
     case BIND_ACTION_SEARCH_CANCEL:
         if (term->search.view_followed_offset)
             term->grid->view = term->grid->offset;
-        else
-            term->grid->view = term->search.original_view;
+        else {
+            term->grid->view = ensure_view_is_allocated(
+                term, term->search.original_view);
+        }
         term_damage_view(term);
         search_cancel(term);
         return true;