Fix lifetime problems in AXPlatformNodeCocoa.

ui/accessibility/platform/ax_platform_node_mac.mm

Index: ui/accessibility/platform/ax_platform_node_mac.mm
diff --git a/ui/accessibility/platform/ax_platform_node_mac.mm b/ui/accessibility/platform/ax_platform_node_mac.mm
index 2ae6c78132ee1ff34f62c2de0896c8775ba2f2ee..bf9813298bf8bbf562628d538eb3adf1dbaa754b 100644
--- a/ui/accessibility/platform/ax_platform_node_mac.mm
+++ b/ui/accessibility/platform/ax_platform_node_mac.mm
@@ -280,6 +280,9 @@ void NotifyMacEvent(AXPlatformNodeCocoa* target, ui::AXEvent event_type) {
 }
 
 - (NSString*)getStringAttribute:(ui::AXStringAttribute)attribute {
+  if (!node_)

[dmazzoni, 2017/06/20 17:47:16]

Isn't this an internal helper method? It seems like it
shouldn't be necessary to check at the top of every method,
only at the top of public APIs (below where it says
"NSAccessibility informal protocol implementation".

[tapted, 2017/06/20 23:30:52]

Removed - I think this was just left over from earlier iterations.

+    return nil;
+
   std::string attributeValue;
   if (node_->GetStringAttribute(attribute, &attributeValue))
     return base::SysUTF8ToNSString(attributeValue);
@@ -294,6 +297,9 @@ void NotifyMacEvent(AXPlatformNodeCocoa* target, ui::AXEvent event_type) {
 // NSAccessibility informal protocol implementation.
 
 - (BOOL)accessibilityIsIgnored {
+  if (!node_)
+    return YES;
+
   return [[self AXRole] isEqualToString:NSAccessibilityUnknownRole] ||
          node_->GetData().HasState(ui::AX_STATE_INVISIBLE);
 }
@@ -313,10 +319,13 @@ void NotifyMacEvent(AXPlatformNodeCocoa* target, ui::AXEvent event_type) {
 }
 
 - (id)accessibilityFocusedUIElement {
-  return node_->GetDelegate()->GetFocus();
+  return node_ ? node_->GetDelegate()->GetFocus() : nil;
 }
 
 - (NSArray*)accessibilityActionNames {
+  if (!node_)
+    return @[];
+
   base::scoped_nsobject<NSMutableArray> axActions(
       [[NSMutableArray alloc] init]);
 
@@ -340,7 +349,11 @@ void NotifyMacEvent(AXPlatformNodeCocoa* target, ui::AXEvent event_type) {
 }
 
 - (void)accessibilityPerformAction:(NSString*)action {
-  DCHECK([[self accessibilityActionNames] containsObject:action]);
+  // Actions are performed asynchronously, so it's always possible for an object
+  // to change its mind after previously reporting an action as available.
+  if (![[self accessibilityActionNames] containsObject:action])
+    return;
+
   ui::AXActionData data;
   for (const ActionMap::value_type& entry : GetActionMap()) {
     if ([action isEqualToString:entry.second]) {
@@ -358,6 +371,9 @@ void NotifyMacEvent(AXPlatformNodeCocoa* target, ui::AXEvent event_type) {
 }
 
 - (NSArray*)accessibilityAttributeNames {
+  if (!node_)
+    return @[];
+
   // These attributes are required on all accessibility objects.
   NSArray* const kAllRoleAttributes = @[
     NSAccessibilityChildrenAttribute,
@@ -426,6 +442,9 @@ void NotifyMacEvent(AXPlatformNodeCocoa* target, ui::AXEvent event_type) {
 }
 
 - (BOOL)accessibilityIsAttributeSettable:(NSString*)attributeName {
+  if (!node_)
+    return NO;
+
   if (node_->GetData().HasState(ui::AX_STATE_DISABLED))
     return NO;
 
@@ -463,6 +482,9 @@ void NotifyMacEvent(AXPlatformNodeCocoa* target, ui::AXEvent event_type) {
 }
 
 - (void)accessibilitySetValue:(id)value forAttribute:(NSString*)attribute {
+  if (!node_)
+    return;
+
   ui::AXActionData data;
 
   // Check for attributes first. Only the |data.action| should be set here - any
@@ -501,6 +523,9 @@ void NotifyMacEvent(AXPlatformNodeCocoa* target, ui::AXEvent event_type) {
 }
 
 - (id)accessibilityAttributeValue:(NSString*)attribute {
+  if (!node_)
+    return nil;  // Return nil when detached. Even for AXRole.
+
   SEL selector = NSSelectorFromString(attribute);
   if ([self respondsToSelector:selector])
     return [self performSelector:selector];
@@ -513,6 +538,7 @@ void NotifyMacEvent(AXPlatformNodeCocoa* target, ui::AXEvent event_type) {
 - (NSString*)AXRole {
   if (!node_)
     return nil;
+
   return [[self class] nativeRoleFromAXRole:node_->GetData().role];
 }
 
@@ -577,7 +603,8 @@ void NotifyMacEvent(AXPlatformNodeCocoa* target, ui::AXEvent event_type) {
 
 - (NSArray*)AXChildren {
   if (!node_)
-    return nil;
+    return @[];
+
   int count = node_->GetChildCount();
   NSMutableArray* children = [NSMutableArray arrayWithCapacity:count];
   for (int i = 0; i < count; ++i)