Split slot index calculation into separate functions

The original GetSlotIndexForTabModelIndex served two purposes: getting
the slot for an existing tab, and getting the insertion index for a
new slot. This splits it into separate methods with common logic
remaining in a shared method.

The logic had a bug when requesting the first tab of a group while
tabs just before it are animating closed. This bug is fixed.

To help debug the associated crash, CHECKs are added.

Bug: 1138748
Change-Id: I6e0d797bb70e9aa1da813dbf1fecf228290e55b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2515363
Commit-Queue: Collin Baker <collinbaker@chromium.org>
Reviewed-by: Taylor Bergquist <tbergquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824672}

chrome/browser/ui/views/tabs/tab_strip.cc

@@ -949,7 +949,7 @@ class TabStrip::TabDragContextImpl : public TabDragContext {
     if (dragging_group == other_group) {
       // |dragging_tabs| can only be empty if dragging in from another window,
       // in which case |dragging_group| can't be the same as |other_group|.
-      DCHECK(dragging_tabs.size() > 0);
+      DCHECK_GT(dragging_tabs.size(), 0u);
       if (candidate_index <= dragging_tabs.front() ||
           dragging_tabs.back() >= GetTabCount() - 1)
         return dragging_tabs.front();
@@ -1010,8 +1010,6 @@ TabStrip::TabStrip(std::unique_ptr<TabStripController> controller)
       layout_helper_(std::make_unique<TabStripLayoutHelper>(
           controller_.get(),
           base::BindRepeating(&TabStrip::tabs_view_model,
-                              base::Unretained(this)),
-          base::BindRepeating(&TabStrip::GetGroupHeaders,
                               base::Unretained(this)))),
       drag_context_(std::make_unique<TabDragContextImpl>(this)) {
   Init();

chrome/browser/ui/views/tabs/tab_strip_layout_helper.h

@@ -5,7 +5,6 @@
 #ifndef CHROME_BROWSER_UI_VIEWS_TABS_TAB_STRIP_LAYOUT_HELPER_H_
 #define CHROME_BROWSER_UI_VIEWS_TABS_TAB_STRIP_LAYOUT_HELPER_H_
 
-#include <map>
 #include <vector>
 
 #include "base/callback.h"
@@ -33,12 +32,9 @@ class TabGroupId;
 class TabStripLayoutHelper {
  public:
   using GetTabsCallback = base::RepeatingCallback<views::ViewModelT<Tab>*()>;
-  using GetGroupHeadersCallback = base::RepeatingCallback<
-      std::map<tab_groups::TabGroupId, TabGroupHeader*>()>;
 
   TabStripLayoutHelper(const TabStripController* controller,
-                       GetTabsCallback get_tabs_callback,
-                       GetGroupHeadersCallback get_group_headers_callback);
+                       GetTabsCallback get_tabs_callback);
   TabStripLayoutHelper(const TabStripLayoutHelper&) = delete;
   TabStripLayoutHelper& operator=(const TabStripLayoutHelper&) = delete;
   ~TabStripLayoutHelper();
@@ -132,12 +128,26 @@ class TabStripLayoutHelper {
   std::vector<gfx::Rect> CalculateIdealBounds(
       base::Optional<int> available_width);
 
-  // Given a tab's |model_index| and |group|, returns the index of its
-  // corresponding TabSlot in |slots_|.
-  int GetSlotIndexForTabModelIndex(
-      int model_index,
+  // Given |model_index| for a tab already present in |slots_|, return
+  // the corresponding index in |slots_|.
+  int GetSlotIndexForExistingTab(int model_index) const;
+
+  // For a new tab at |new_model_index|, get the insertion index in
+  // |slots_|. |group| is the new tab's group.
+  int GetSlotInsertionIndexForNewTab(
+      int new_model_index,
       base::Optional<tab_groups::TabGroupId> group) const;
 
+  // Used internally in the above two functions. For a tabstrip with N
+  // tabs, this takes 0 <= |model_index| <= N and returns the first
+  // possible slot corresponding to this model index.
+  //
+  // This means that if |model_index| is the first tab in a group, the
+  // returned slot index will point to the group header. For other tabs,
+  // the slot index corresponding to that tab will be returned. Finally,
+  // if |model_index| = N, slots_.size() will be returned.
+  int GetFirstSlotIndexForTabModelIndex(int model_index) const;
+
   // Given a group ID, returns the index of its header's corresponding TabSlot
   // in |slots_|.
   int GetSlotIndexForGroupHeader(tab_groups::TabGroupId group) const;
@@ -164,9 +174,8 @@ class TabStripLayoutHelper {
   // The owning tabstrip's controller.
   const TabStripController* const controller_;
 
-  // Callbacks to get the necessary View objects from the owning tabstrip.
+  // Callback to get the necessary View objects from the owning tabstrip.
   GetTabsCallback get_tabs_callback_;
-  GetGroupHeadersCallback get_group_headers_callback_;
 
   // Current collation of tabs and group headers, along with necessary data to
   // run layout and animations for those Views.

chrome/browser/ui/views/tabs/tab_strip_unittest.cc

@@ -1348,4 +1348,55 @@ TEST_P(TabStripTest, DISABLED_NewTabButtonFlushWithTopOfTabStrip) {
   // EXPECT_EQ(0, tab_strip_->new_tab_button()->bounds().y());
 }
 
+// Regression test for a crash when closing a tab under certain
+// conditions. If the first tab in a group was animating closed,
+// attempting to close the next tab could result in a crash. This was
+// due to TabStripLayoutHelper mistakenly mapping the next tab's model
+// index to the closing tab's slot. See https://crbug.com/1138748 for a
+// related crash.
+TEST_P(TabStripTest, CloseTabInGroupWhilePreviousTabAnimatingClosed) {
+  controller_->AddTab(0, true);
+  controller_->AddTab(1, false);
+  controller_->AddTab(2, false);
+
+  auto group_id = tab_groups::TabGroupId::GenerateNew();
+  controller_->MoveTabIntoGroup(1, group_id);
+  controller_->MoveTabIntoGroup(2, group_id);
+
+  CompleteAnimationAndLayout();
+  ASSERT_EQ(3, tab_strip_->tab_count());
+  ASSERT_EQ(3, tab_strip_->GetModelCount());
+  EXPECT_EQ(base::nullopt, tab_strip_->tab_at(0)->group());
+  EXPECT_EQ(group_id, tab_strip_->tab_at(1)->group());
+  EXPECT_EQ(group_id, tab_strip_->tab_at(2)->group());
+
+  // We have the following tabs:
+  // 1. An ungrouped tab with model index 0
+  // 2. A tab in |group_id| with model index 1
+  // 3. A tab in |group_id| with model index 2
+  controller_->RemoveTab(1);
+
+  // After closing the first tab, we now have:
+  // 1. An ungrouped tab with model index 0
+  // 2. A closing tab in |group_id| with no model index
+  // 3. A tab in |group_id| with model index 1.
+  //
+  // Closing the tab at model index 1 should result in (3) above being
+  // closed.
+  controller_->RemoveTab(1);
+
+  // We should now have:
+  // 1. An ungrouped tab with model index 0
+  // 2. A closing tab in |group_id| with no model index
+  // 3. A closing tab in |group_id| with no model index.
+
+  CompleteAnimationAndLayout();
+
+  // After finishing animations, there should be exactly 1 tab in no
+  // group.
+  EXPECT_EQ(1, tab_strip_->tab_count());
+  EXPECT_EQ(base::nullopt, tab_strip_->tab_at(0)->group());
+  EXPECT_EQ(1, tab_strip_->GetModelCount());
+}
+
 INSTANTIATE_TEST_SUITE_P(All, TabStripTest, ::testing::Values(false, true));