rework _element_attr_listeners

Merges caching and lazy evaluation logic in `_element_attr_listeners`; this introduce allocations for inline function values, which is a price we pay for future compat w/ modern browsers.

Spec also require us to keep a list of **internal raw uncompiled handler**, which is a fancy way to say store-as-bytes-evaluate-later.

src/browser/Page.zig

@@ -276,7 +276,6 @@ fn reset(self: *Page, comptime initializing: bool) !void {
             self._arena_pool_leak_track.clearRetainingCapacity();
         }
 
-
         // We force a garbage collection between page navigations to keep v8
         // memory usage as low as possible.
         self._session.browser.env.memoryPressureNotification(.moderate);
@@ -1173,7 +1172,10 @@ pub fn setAttrListener(
     self: *Page,
     element: *Element,
     listener_type: Element.KnownListener,
-    listener_callback: JS.Function.Global,
+    /// This can be;
+    /// []const u8 or []u8 (memory will be duped anyway) or,
+    /// JS function (persisted).
+    raw_or_function: anytype,
 ) !void {
     if (comptime IS_DEBUG) {
         log.debug(.event, "Page.setAttrListener", .{
@@ -1184,16 +1186,53 @@ pub fn setAttrListener(
 
     const key = element.calcAttrListenerKey(listener_type);
     const gop = try self._element_attr_listeners.getOrPut(self.arena, key);
-    gop.value_ptr.* = listener_callback;
+    switch (@TypeOf(raw_or_function)) {
+        []const u8, []u8 => gop.value_ptr.* = .{ .raw = try self.arena.dupe(u8, raw_or_function) },
+        JS.Function.Global => gop.value_ptr.* = .{ .function = raw_or_function },
+        else => |T| @panic("setAttrListener: unknown type " ++ @typeName(T)),
+    }
 }
 
-/// Returns the inline event listener by an element and listener type.
+/// Returns the inline event listener function by an element and listener type.
 pub fn getAttrListener(
-    self: *const Page,
+    self: *Page,
     element: *Element,
     listener_type: Element.KnownListener,
 ) ?JS.Function.Global {
-    return self._element_attr_listeners.get(element.calcAttrListenerKey(listener_type));
+    const listeners = &self._element_attr_listeners;
+    // Check if there's such attr listener.
+    const key = element.calcAttrListenerKey(listener_type);
+    const listener = listeners.getPtr(key) orelse return null;
+
+    return switch (listener.*) {
+        // Fast path.
+        .function => |function| function,
+        // Lazy evaluation.
+        .raw => |untrusted| {
+            // First time access to this getter; try to compile as function and cache result.
+            const function = self.js.stringToPersistedFunction(untrusted) catch |err| {
+                // Not a valid expression; log this to find out if its something
+                // that we should be supporting.
+                log.warn(.unknown_prop, "Page.getAttrListener", .{
+                    .expression = untrusted,
+                    .err = err,
+                });
+                // We can remove this safely.
+                const result = listeners.remove(key);
+                lp.assert(result == true, "Page.getAttrListener: unexpected result", .{});
+                // Remove invalid bytes.
+                self.arena.free(untrusted);
+
+                return null;
+            };
+
+            // Now that we obtained a function; this has no use.
+            self.arena.free(untrusted);
+            // Cache the resulting function.
+            listener.* = .{ .function = function };
+            return function;
+        },
+    };
 }
 
 pub fn registerPerformanceObserver(self: *Page, observer: *PerformanceObserver) !void {
@@ -2242,14 +2281,12 @@ fn populateElementAttributes(self: *Page, element: *Element, list: anytype) !voi
         const has_on_prefix = @as(u16, @bitCast([2]u8{ name.ptr[0], name.ptr[1 % name.len] })) == asUint("on");
         // We may have found an event handler.
         if (has_on_prefix) {
-            // Must be usable as function.
-            const func = self.js.stringToPersistedFunction(attr.value.slice()) catch continue;
-
             // Longest known listener kind is 32 bytes long.
             const remaining: u6 = @truncate(name.len -| 2);
             const unsafe = name.ptr + 2;
             const Vec16x8 = @Vector(16, u8);
             const Vec32x8 = @Vector(32, u8);
+            const func = attr.value.slice();
 
             switch (remaining) {
                 3 => if (@as(u24, @bitCast(unsafe[0..3].*)) == asUint("cut")) {

src/browser/webapi/Element.zig

@@ -53,8 +53,15 @@ pub const AssignedSlotLookup = std.AutoHashMapUnmanaged(*Element, *Html.Slot);
 ///
 /// See `calcAttrListenerKey` to obtain one.
 const AttrListenerKey = u64;
+/// We lazily evaluate functions in order to not to create garbage.
+const AttrListener = union(enum(u1)) {
+    /// Raw bytes; this may or may not be a valid JS expression.
+    raw: []const u8,
+    /// A valid JS function; can be executed directly.
+    function: js.Function.Global,
+};
 /// Use `getAttrListenerKey` to create a key.
-pub const AttrListenerLookup = std.AutoHashMapUnmanaged(AttrListenerKey, js.Function.Global);
+pub const AttrListenerLookup = std.AutoHashMapUnmanaged(AttrListenerKey, AttrListener);
 
 /// Enum of known event listeners; increasing the size of it (u7)
 /// can cause `AttrListenerKey` to behave incorrectly.

src/browser/webapi/element/Html.zig

@@ -339,30 +339,7 @@ fn getAttributeFunction(
     listener_type: Element.KnownListener,
     page: *Page,
 ) ?js.Function.Global {
-    const element = self.asElement();
-    // Check if we've already cached this.
-    if (page.getAttrListener(element, listener_type)) |cached_func| {
-        return cached_func;
-    }
-
-    // Not found in cache; parse from attribute list.
-    const js_expression = element.getAttributeSafe(.wrap(@tagName(listener_type))) orelse {
-        return null;
-    };
-
-    const callback = page.js.stringToPersistedFunction(js_expression) catch {
-        // Not a valid expression; log this to find out if its something
-        // that we should be supporting.
-        log.warn(.unknown_prop, "Html.getAttributeFunction", .{ .expression = js_expression });
-        return null;
-    };
-
-    // Cache the function for future calls.
-    page.setAttrListener(element, listener_type, callback) catch {
-        // This is fine :tm: we likely hit out of memory.
-    };
-
-    return callback;
+    return page.getAttrListener(self.asElement(), listener_type);
 }
 
 pub fn setOnAbort(self: *HtmlElement, callback: js.Function.Global, page: *Page) !void {