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mem: AXNode tree, query and isIgnore walks -> iterative

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Follow-up to the SemanticTree change: the CDP accessibility writer still
recursed once per DOM level in writeNodeChildren (getFullAXTree /
getPartialAXTree), walkQuery (queryAXTree) and isIgnore (chains of
generic div/span containers). In a debug build, a 100k-deep chain
overflows the 8MB main-thread stack in the tree writer and in isIgnore.

The tree and query walks now share an allocation-free pre-order Walker
that counts aria-hidden ancestors instead of threading the flag through
the recursion. isIgnore splits into a shallow per-node verdict
(ignoreSelf) and a TreeWalker pass over generic containers that skips
ignored subtrees.

Output is byte-identical to the recursive version.
This commit is contained in:
Adrià Arrufat committed 2026-09-24 15:39:16 +02:00
1 parent 1151eaebf3
commit 140bedf39f
1 file changed
+112 -64
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src/server/cdp/AXNode.zig
+112 -64
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@@ -82,13 +82,11 @@ pub const Writer = struct {
fn toJSON(self: *const Writer, w: anytype) !void {
try w.beginArray();
if (self.filter != null) {
try self.walkQuery(self.root.dom, false, w);
try self.walkQuery(self.root.dom, w);
} else {
const root = AXNode.fromNode(self.root.dom);
const root_hidden = if (self.root.dom.is(DOMNode.Element)) |el| isHidden(el, self.frame, .{}) else false;
if (try self.writeNode(self.root.id, root, false, root_hidden, w)) {
try self.writeNodeChildren(root, false, w);
}
try self.writeTree(root, root_hidden, w);
}
return w.endArray();
}
@@ -118,28 +116,27 @@ pub const Writer = struct {
try w.write(s);
}
fn writeNodeChildren(self: *const Writer, parent: AXNode, in_aria_hidden: bool, w: anytype) !void {
// Add ListMarker for listitem elements
if (parent.dom.is(DOMNode.Element)) |parent_el| {
if (parent_el.getTag() == .li) {
try self.writeListMarker(parent.dom, w);
fn writeTree(self: *const Writer, root: AXNode, root_hidden: bool, w: anytype) !void {
var walker: Walker = .init(root.dom);
var axn = root;
var descend = try self.writeNode(self.root.id, root, false, root_hidden, w);
while (true) {
if (descend) {
if (axn.dom.is(DOMNode.Element)) |el| {
if (el.getTag() == .li) {
try self.writeListMarker(axn.dom, w);
}
}
}
}
const child_in_aria_hidden = in_aria_hidden or blk: {
const parent_el = parent.dom.is(DOMNode.Element) orelse break :blk false;
break :blk hasAriaHiddenTrue(parent_el);
};
var it = parent.dom.childrenIterator();
const ignore_text = ignoreText(parent.dom);
while (it.next()) |dom_node| {
const dom_node = walker.next(descend) orelse return;
descend = false;
switch (dom_node._type) {
.cdata => {
if (dom_node.is(DOMNode.CData.Text) == null) {
continue;
}
if (ignore_text) {
if (ignoreText(dom_node._parent.?)) {
continue;
}
},
@@ -147,8 +144,8 @@ pub const Writer = struct {
// Prune hidden subtrees entirely (display:none,
// visibility:hidden, aria-hidden, hidden, inert). Matches
// Chromium: these elements aren't exposed to the AX tree.
const child_el = dom_node.as(DOMNode.Element);
if (child_in_aria_hidden or isHidden(child_el, self.frame, .{ .ancestors = false })) {
const el = dom_node.as(DOMNode.Element);
if (walker.inAriaHidden() or isHidden(el, self.frame, .{ .ancestors = false })) {
continue;
}
},
@@ -156,10 +153,8 @@ pub const Writer = struct {
}
const node = try self.registry.register(dom_node);
const axn = AXNode.fromNode(node.dom);
if (try self.writeNode(node.id, axn, child_in_aria_hidden, false, w)) {
try self.writeNodeChildren(axn, child_in_aria_hidden, w);
}
axn = AXNode.fromNode(node.dom);
descend = try self.writeNode(node.id, axn, walker.inAriaHidden(), false, w);
}
}
@@ -591,7 +586,7 @@ pub const Writer = struct {
}
// Skip hidden element children so childIds matches the
// subtree-pruning done in writeNodeChildren.
// subtree-pruning done in writeTree.
if (child.is(DOMNode.Element)) |child_el| {
if (child_in_aria_hidden or isHidden(child_el, self.frame, .{ .ancestors = false })) {
continue;
@@ -648,25 +643,21 @@ pub const Writer = struct {
// Query-mode walk. Visits every node under `root` (including AX-ignored
// ones, per the queryAXTree spec) and defers emission to emitMatch.
fn walkQuery(self: *const Writer, node: *DOMNode, in_aria_hidden: bool, w: anytype) !void {
const axn = AXNode.fromNode(node);
try self.emitMatch(axn, in_aria_hidden, w);
fn walkQuery(self: *const Writer, root: *DOMNode, w: anytype) !void {
var walker: Walker = .init(root);
var node = root;
while (true) {
const axn = AXNode.fromNode(node);
try self.emitMatch(axn, walker.inAriaHidden(), w);
// <head>, <script>, <style> never expose AX content — skip recursion.
if (axn.ignoreChildren()) return;
const child_in_aria_hidden = in_aria_hidden or blk: {
const parent_el = node.is(DOMNode.Element) orelse break :blk false;
break :blk hasAriaHiddenTrue(parent_el);
};
var it = node.childrenIterator();
while (it.next()) |child| {
switch (child._type) {
.element, .cdata => {},
else => continue,
}
try self.walkQuery(child, child_in_aria_hidden, w);
// <head>, <script>, <style> never expose AX content — skip their children.
var descend = !axn.ignoreChildren();
node = while (walker.next(descend)) |next| {
switch (next._type) {
.element, .cdata => break next,
else => descend = false,
}
} else return;
}
}
@@ -1131,6 +1122,51 @@ fn writeAccessibleNameFallback(node: *DOMNode, writer: *std.Io.Writer) !void {
}
}
/// Pre-order walk of a subtree's descendants that tracks whether one of the
/// current node's ancestors (the root included) is aria-hidden. Iterative, so
/// a deep DOM can't overflow the native stack.
const Walker = struct {
root: *DOMNode,
current: *DOMNode,
aria_hidden_ancestors: u32 = 0,
fn init(root: *DOMNode) Walker {
return .{ .root = root, .current = root };
}
/// The node after the current one, entering its children only if `descend`.
fn next(self: *Walker, descend: bool) ?*DOMNode {
if (descend) {
if (self.current.firstChild()) |child| {
if (isAriaHidden(self.current)) self.aria_hidden_ancestors += 1;
self.current = child;
return child;
}
}
var node = self.current;
while (node != self.root) {
if (node.nextSibling()) |sibling| {
self.current = sibling;
return sibling;
}
node = node._parent.?;
if (isAriaHidden(node)) self.aria_hidden_ancestors -= 1;
}
self.current = self.root;
return null;
}
fn inAriaHidden(self: *const Walker) bool {
return self.aria_hidden_ancestors > 0;
}
fn isAriaHidden(node: *DOMNode) bool {
const el = node.is(DOMNode.Element) orelse return false;
return hasAriaHiddenTrue(el);
}
};
fn hasAriaHiddenTrue(elt: *DOMNode.Element) bool {
if (elt.getAttributeInterned("aria-hidden")) |value| {
return std.mem.eql(u8, value, "true");
@@ -1325,14 +1361,36 @@ fn ignoreChildren(self: AXNode) bool {
// `hidden` comes from the caller: the tree walk prunes, so its children never
// are; the root and the query walk probe the whole chain.
fn isIgnore(self: AXNode, frame: *Frame, in_aria_hidden: bool, hidden: bool) bool {
return switch (self.ignoreSelf(in_aria_hidden, hidden)) {
.ignored => true,
.exposed => false,
// A generic container is ignored unless it has a non-ignored
// descendant, reached through other generic containers.
.generic => {
var tw = TreeWalker.FullExcludeSelf.init(self.dom, .{});
while (tw.next()) |node| {
const node_hidden = if (node.is(DOMNode.Element)) |el| isHidden(el, frame, .{ .ancestors = false }) else false;
switch (AXNode.fromNode(node).ignoreSelf(in_aria_hidden, node_hidden)) {
.ignored => tw.skipChildren(),
.exposed => return false,
.generic => {},
}
}
return true;
},
};
}
/// isIgnore for `self` alone, leaving generic containers to the caller.
fn ignoreSelf(self: AXNode, in_aria_hidden: bool, hidden: bool) enum { ignored, exposed, generic } {
const node = self.dom;
const role_attr = self.role_attr;
// Don't ignore non-Element node: CData, Document...
const elt = node.is(DOMNode.Element) orelse return in_aria_hidden;
const elt = node.is(DOMNode.Element) orelse return if (in_aria_hidden) .ignored else .exposed;
// Ignore non-HTML elements: svg...
if (elt._type != .html) {
return true;
return .ignored;
}
const tag = elt.getTag();
@@ -1341,20 +1399,20 @@ fn isIgnore(self: AXNode, frame: *Frame, in_aria_hidden: bool, hidden: bool) boo
.script, .style, .meta, .link, .title, .base, .head, .noscript,
.template, .param, .source, .track, .datalist, .col, .colgroup, .html,
.body
=> return true,
=> return .ignored,
// zig fmt: on
.img => {
// Check for empty decorative images
const alt_ = elt.getAttributeInterned("alt");
if (alt_ == null or alt_.?.len == 0) {
return true;
return .ignored;
}
},
.input => {
// Check for hidden inputs
const input = elt.as(DOMNode.Element.Html.Input);
if (input._input_type == .hidden) {
return true;
return .ignored;
}
},
else => {},
@@ -1362,16 +1420,16 @@ fn isIgnore(self: AXNode, frame: *Frame, in_aria_hidden: bool, hidden: bool) boo
if (role_attr) |role| {
if (std.ascii.eqlIgnoreCase(role, "none") or std.ascii.eqlIgnoreCase(role, "presentation")) {
return true;
return .ignored;
}
}
if (in_aria_hidden) {
return true;
return .ignored;
}
if (hidden) {
return true;
return .ignored;
}
// Generic containers with no semantic value
@@ -1381,21 +1439,11 @@ fn isIgnore(self: AXNode, frame: *Frame, in_aria_hidden: bool, hidden: bool) boo
const has_aria_labelledby = elt.hasAttributeSafe(.wrap("aria-labelledby"));
if (!has_role and !has_aria_label and !has_aria_labelledby) {
// Check if it has any non-ignored children.
var it = node.childrenIterator();
while (it.next()) |child| {
const axn = AXNode.fromNode(child);
const child_hidden = if (child.is(DOMNode.Element)) |child_el| isHidden(child_el, frame, .{ .ancestors = false }) else false;
if (!axn.isIgnore(frame, in_aria_hidden, child_hidden)) {
return false;
}
}
return true;
return .generic;
}
}
return false;
return .exposed;
}
pub fn getRole(self: AXNode) ![]const u8 {