WebContent: Don't assume start nodes for finding elements remain valid

When timeouts are implemented, the start node used to find elements may
not remain valid for the entire duration of the timeout. For example,
the active document element may change, or the start node may be removed
from the DOM.

To handle this, we will need to re-evaluate the start node on each
iteration of the find() operation. This patch wraps the steps to do so
in a lambda to be executed on each iteration.

Userland/Services/WebContent/WebDriverConnection.cpp

@@ -678,15 +678,19 @@ Messages::WebDriverClient::FindElementResponse WebDriverConnection::find_element
     // 6. Handle any user prompts and return its value if it is an error.
     TRY(handle_any_user_prompts());
 
-    // 7. Let start node be the current browsing context’s document element.
-    auto* start_node = m_page_host.page().top_level_browsing_context().active_document();
+    auto start_node_getter = [this]() -> StartNodeGetter::ReturnType {
+        // 7. Let start node be the current browsing context’s document element.
+        auto* start_node = m_page_host.page().top_level_browsing_context().active_document();
 
-    // 8. If start node is null, return error with error code no such element.
-    if (!start_node)
-        return Web::WebDriver::Error::from_code(Web::WebDriver::ErrorCode::NoSuchElement, "document element does not exist"sv);
+        // 8. If start node is null, return error with error code no such element.
+        if (!start_node)
+            return Web::WebDriver::Error::from_code(Web::WebDriver::ErrorCode::NoSuchElement, "document element does not exist"sv);
+
+        return start_node;
+    };
 
     // 9. Let result be the result of trying to Find with start node, location strategy, and selector.
-    auto result = TRY(find(*start_node, *location_strategy, selector));
+    auto result = TRY(find(move(start_node_getter), *location_strategy, selector));
 
     // 10. If result is empty, return error with error code no such element. Otherwise, return the first element of result.
     if (result.is_empty())
@@ -716,15 +720,19 @@ Messages::WebDriverClient::FindElementsResponse WebDriverConnection::find_elemen
     // 6. Handle any user prompts and return its value if it is an error.
     TRY(handle_any_user_prompts());
 
-    // 7. Let start node be the current browsing context’s document element.
-    auto* start_node = m_page_host.page().top_level_browsing_context().active_document();
+    auto start_node_getter = [this]() -> StartNodeGetter::ReturnType {
+        // 7. Let start node be the current browsing context’s document element.
+        auto* start_node = m_page_host.page().top_level_browsing_context().active_document();
 
-    // 8. If start node is null, return error with error code no such element.
-    if (!start_node)
-        return Web::WebDriver::Error::from_code(Web::WebDriver::ErrorCode::NoSuchElement, "document element does not exist"sv);
+        // 8. If start node is null, return error with error code no such element.
+        if (!start_node)
+            return Web::WebDriver::Error::from_code(Web::WebDriver::ErrorCode::NoSuchElement, "document element does not exist"sv);
+
+        return start_node;
+    };
 
     // 9. Return the result of trying to Find with start node, location strategy, and selector.
-    return TRY(find(*start_node, *location_strategy, selector));
+    return TRY(find(move(start_node_getter), *location_strategy, selector));
 }
 
 // 12.3.4 Find Element From Element, https://w3c.github.io/webdriver/#dfn-find-element-from-element
@@ -748,11 +756,13 @@ Messages::WebDriverClient::FindElementFromElementResponse WebDriverConnection::f
     // 6. Handle any user prompts and return its value if it is an error.
     TRY(handle_any_user_prompts());
 
-    // 7. Let start node be the result of trying to get a known connected element with url variable element id.
-    auto* start_node = TRY(get_known_connected_element(element_id));
+    auto start_node_getter = [&]() -> StartNodeGetter::ReturnType {
+        // 7. Let start node be the result of trying to get a known connected element with url variable element id.
+        return TRY(get_known_connected_element(element_id));
+    };
 
     // 8. Let result be the value of trying to Find with start node, location strategy, and selector.
-    auto result = TRY(find(*start_node, *location_strategy, selector));
+    auto result = TRY(find(move(start_node_getter), *location_strategy, selector));
 
     // 9. If result is empty, return error with error code no such element. Otherwise, return the first element of result.
     if (result.is_empty())
@@ -782,11 +792,13 @@ Messages::WebDriverClient::FindElementsFromElementResponse WebDriverConnection::
     // 6. Handle any user prompts and return its value if it is an error.
     TRY(handle_any_user_prompts());
 
-    // 7. Let start node be the result of trying to get a known connected element with url variable element id.
-    auto* start_node = TRY(get_known_connected_element(element_id));
+    auto start_node_getter = [&]() -> StartNodeGetter::ReturnType {
+        // 7. Let start node be the result of trying to get a known connected element with url variable element id.
+        return TRY(get_known_connected_element(element_id));
+    };
 
     // 8. Return the result of trying to Find with start node, location strategy, and selector.
-    return TRY(find(*start_node, *location_strategy, selector));
+    return TRY(find(move(start_node_getter), *location_strategy, selector));
 }
 
 // 12.3.6 Find Element From Shadow Root, https://w3c.github.io/webdriver/#find-element-from-shadow-root
@@ -810,11 +822,13 @@ Messages::WebDriverClient::FindElementFromShadowRootResponse WebDriverConnection
     // 6. Handle any user prompts and return its value if it is an error.
     TRY(handle_any_user_prompts());
 
-    // 7. Let start node be the result of trying to get a known shadow root with url variable shadow id.
-    auto* start_node = TRY(get_known_shadow_root(shadow_id));
+    auto start_node_getter = [&]() -> StartNodeGetter::ReturnType {
+        // 7. Let start node be the result of trying to get a known shadow root with url variable shadow id.
+        return TRY(get_known_shadow_root(shadow_id));
+    };
 
     // 8. Let result be the value of trying to Find with start node, location strategy, and selector.
-    auto result = TRY(find(*start_node, *location_strategy, selector));
+    auto result = TRY(find(move(start_node_getter), *location_strategy, selector));
 
     // 9. If result is empty, return error with error code no such element. Otherwise, return the first element of result.
     if (result.is_empty())
@@ -844,11 +858,13 @@ Messages::WebDriverClient::FindElementsFromShadowRootResponse WebDriverConnectio
     // 6. Handle any user prompts and return its value if it is an error.
     TRY(handle_any_user_prompts());
 
-    // 7. Let start node be the result of trying to get a known shadow root with url variable shadow id.
-    auto* start_node = TRY(get_known_shadow_root(shadow_id));
+    auto start_node_getter = [&]() -> StartNodeGetter::ReturnType {
+        // 7. Let start node be the result of trying to get a known shadow root with url variable shadow id.
+        return TRY(get_known_shadow_root(shadow_id));
+    };
 
     // 8. Return the result of trying to Find with start node, location strategy, and selector.
-    return TRY(find(*start_node, *location_strategy, selector));
+    return TRY(find(move(start_node_getter), *location_strategy, selector));
 }
 
 // 12.3.8 Get Active Element, https://w3c.github.io/webdriver/#get-active-element
@@ -1615,7 +1631,7 @@ Gfx::IntRect WebDriverConnection::iconify_the_window()
 }
 
 // https://w3c.github.io/webdriver/#dfn-find
-ErrorOr<JsonArray, Web::WebDriver::Error> WebDriverConnection::find(Web::DOM::ParentNode& start_node, Web::WebDriver::LocationStrategy using_, StringView value)
+ErrorOr<JsonArray, Web::WebDriver::Error> WebDriverConnection::find(StartNodeGetter&& start_node_getter, Web::WebDriver::LocationStrategy using_, StringView value)
 {
     // FIXME: 1. Let end time be the current time plus the session implicit wait timeout.
 
@@ -1626,7 +1642,7 @@ ErrorOr<JsonArray, Web::WebDriver::Error> WebDriverConnection::find(Web::DOM::Pa
     auto selector = value;
 
     // 4. Let elements returned be the result of trying to call the relevant element location strategy with arguments start node, and selector.
-    auto elements = Web::WebDriver::invoke_location_strategy(location_strategy, start_node, selector);
+    auto elements = Web::WebDriver::invoke_location_strategy(location_strategy, *TRY(start_node_getter()), selector);
 
     // 5. If a DOMException, SyntaxError, XPathException, or other error occurs during the execution of the element location strategy, return error invalid selector.
     if (elements.is_error())

Userland/Services/WebContent/WebDriverConnection.h

@@ -8,6 +8,7 @@
 
 #pragma once
 
+#include <AK/Function.h>
 #include <AK/HashMap.h>
 #include <AK/String.h>
 #include <LibIPC/ConnectionToServer.h>
@@ -93,7 +94,9 @@ private:
     void restore_the_window();
     Gfx::IntRect maximize_the_window();
     Gfx::IntRect iconify_the_window();
-    ErrorOr<JsonArray, Web::WebDriver::Error> find(Web::DOM::ParentNode& start_node, Web::WebDriver::LocationStrategy using_, StringView value);
+
+    using StartNodeGetter = Function<ErrorOr<Web::DOM::ParentNode*, Web::WebDriver::Error>()>;
+    ErrorOr<JsonArray, Web::WebDriver::Error> find(StartNodeGetter&& start_node_getter, Web::WebDriver::LocationStrategy using_, StringView value);
 
     struct ScriptArguments {
         String script;