chore: remove tail field from qlist (#4220)

Also update license's change date.

Signed-off-by: Roman Gershman <roman@dragonflydb.io>

LICENSE.md

@@ -6,7 +6,7 @@
 
 <u>Licensed Work</u>: Dragonfly including the software components, or any portion of them, and any modification.
 
-<u>Change Date</u>: September 1, 2028
+<u>Change Date</u>: March 1, 2029
 
 <u>Change License</u>: [Apache License, Version
 2.0](https://www.apache.org/licenses/LICENSE-2.0), as published by the

src/core/qlist.cc

@@ -60,7 +60,7 @@ namespace dfly {
 
 namespace {
 
-static_assert(sizeof(QList) == 32);
+static_assert(sizeof(QList) == 24);
 
 enum IterDir : uint8_t { FWD = 1, REV = 0 };
 
@@ -305,13 +305,12 @@ QList::QList(int fill, int compress) : fill_(fill), compress_(compress), bookmar
 
 QList::QList(QList&& other)
     : head_(other.head_),
-      tail_(other.tail_),
       count_(other.count_),
       len_(other.len_),
       fill_(other.fill_),
       compress_(other.compress_),
       bookmark_count_(other.bookmark_count_) {
-  other.head_ = other.tail_ = nullptr;
+  other.head_ = nullptr;
   other.len_ = other.count_ = 0;
 }
 
@@ -323,14 +322,13 @@ QList& QList::operator=(QList&& other) {
   if (this != &other) {
     Clear();
     head_ = other.head_;
-    tail_ = other.tail_;
     len_ = other.len_;
     count_ = other.count_;
     fill_ = other.fill_;
     compress_ = other.compress_;
     bookmark_count_ = other.bookmark_count_;
 
-    other.head_ = other.tail_ = nullptr;
+    other.head_ = nullptr;
     other.len_ = other.count_ = 0;
   }
   return *this;
@@ -348,28 +346,24 @@ void QList::Clear() {
     len_--;
     current = next;
   }
-  head_ = tail_ = nullptr;
+  head_ = nullptr;
   count_ = 0;
 }
 
 void QList::Push(string_view value, Where where) {
   /* The head and tail should never be compressed (we don't attempt to decompress them) */
-  if (head_)
+  if (head_) {
     DCHECK(head_->encoding != QUICKLIST_NODE_ENCODING_LZF);
-  if (tail_)
+    DCHECK(head_->prev->encoding != QUICKLIST_NODE_ENCODING_LZF);
-    DCHECK(tail_->encoding != QUICKLIST_NODE_ENCODING_LZF);
+  }
-
   PushSentinel(value, where);
 }
 
 string QList::Pop(Where where) {
   DCHECK_GT(count_, 0u);
-  quicklistNode* node;
+  quicklistNode* node = head_;
-  if (where == HEAD) {
+  if (where == TAIL) {
-    node = head_;
+    node = head_->prev;
-  } else {
-    DCHECK_EQ(TAIL, where);
-    node = tail_;
   }
 
   /* The head and tail should never be compressed */
@@ -402,7 +396,7 @@ void QList::AppendListpack(unsigned char* zl) {
   node->count = lpLength(node->entry);
   node->sz = lpBytes(zl);
 
-  InsertNode(tail_, node, AFTER);
+  InsertNode(_Tail(), node, AFTER);
   count_ += node->count;
 }
 
@@ -414,7 +408,7 @@ void QList::AppendPlain(unsigned char* data, size_t sz) {
   node->sz = sz;
   node->container = QUICKLIST_NODE_CONTAINER_PLAIN;
 
-  InsertNode(tail_, node, AFTER);
+  InsertNode(_Tail(), node, AFTER);
   ++count_;
 }
 
@@ -469,7 +463,11 @@ void QList::Iterate(IterateFunc cb, long start, long end) const {
 }
 
 bool QList::PushSentinel(string_view value, Where where) {
-  quicklistNode* orig = where == HEAD ? head_ : tail_;
+  quicklistNode* orig = head_;
+  if (where == TAIL && orig) {
+    orig = orig->prev;
+  }
+
   InsertOpt opt = where == HEAD ? BEFORE : AFTER;
 
   size_t sz = value.size();
@@ -507,23 +505,27 @@ void QList::InsertNode(quicklistNode* old_node, quicklistNode* new_node, InsertO
       if (old_node->next)
         old_node->next->prev = new_node;
       old_node->next = new_node;
+      if (head_->prev == old_node)  // if old_node is tail, update the tail to the new node.
+        head_->prev = new_node;
     }
-    if (tail_ == old_node)
-      tail_ = new_node;
   } else {
     new_node->next = old_node;
     if (old_node) {
       new_node->prev = old_node->prev;
-      if (old_node->prev)
+      // if old_node is not head, link its prev to the new node.
+      // head->prev is tail, so we don't need to update it.
+      if (old_node != head_)
         old_node->prev->next = new_node;
       old_node->prev = new_node;
     }
     if (head_ == old_node)
       head_ = new_node;
   }
+
   /* If this insert creates the only element so far, initialize head/tail. */
   if (len_ == 0) {
-    head_ = tail_ = new_node;
+    head_ = new_node;
+    head_->prev = new_node;
   }
 
   /* Update len first, so in Compress we know exactly len */
@@ -698,7 +700,7 @@ void QList::Compress(quicklistNode* node) {
     return;
 
   /* The head and tail should never be compressed (we should not attempt to recompress them) */
-  assert(head_->recompress == 0 && tail_->recompress == 0);
+  DCHECK(head_->recompress == 0 && head_->prev->recompress == 0);
 
   /* If length is less than our compress depth (from both sides),
    * we can't compress anything. */
@@ -709,7 +711,7 @@ void QList::Compress(quicklistNode* node) {
    * Note: because we do length checks at the *top* of this function,
    *       we can skip explicit null checks below. Everything exists. */
   quicklistNode* forward = head_;
-  quicklistNode* reverse = tail_;
+  quicklistNode* reverse = head_->prev;
   int depth = 0;
   int in_depth = 0;
   while (depth++ < compress_) {
@@ -837,12 +839,10 @@ void QList::DelNode(quicklistNode* node) {
   if (node->prev)
     node->prev->next = node->next;
 
-  if (node == tail_) {
-    tail_ = node->prev;
-  }
-
   if (node == head_) {
     head_ = node->next;
+  } else if (node == head_->prev) {  // tail
+    head_->prev = node->prev;
   }
 
   /* Update len first, so in Compress we know exactly len */
@@ -892,7 +892,7 @@ auto QList::GetIterator(Where where) const -> Iterator {
     it.offset_ = 0;
     it.direction_ = FWD;
   } else {
-    it.current_ = tail_;
+    it.current_ = _Tail();
     it.offset_ = -1;
     it.direction_ = REV;
   }
@@ -918,7 +918,7 @@ auto QList::GetIterator(long idx) const -> Iterator {
     seek_index = count_ - 1 - index;
   }
 
-  n = seek_forward ? head_ : tail_;
+  n = seek_forward ? head_ : head_->prev;
   while (ABSL_PREDICT_TRUE(n)) {
     if ((accum + n->count) > seek_index) {
       break;
@@ -1119,9 +1119,8 @@ bool QList::Iterator::Next() {
   } else {
     /* Reverse traversal, Jumping to end of previous node */
     DCHECK_EQ(REV, direction_);
-
-    current_ = current_->prev;
     offset_ = -1;
+    current_ = (current_ == owner_->head_) ? nullptr : current_->prev;
   }
 
   return current_ ? Next() : false;

src/core/qlist.h

@@ -144,7 +144,7 @@ class QList {
   }
 
   const quicklistNode* Tail() const {
-    return tail_;
+    return _Tail();
   }
 
   void set_fill(int fill) {
@@ -158,6 +158,10 @@ class QList {
     return compress_ != 0;
   }
 
+  quicklistNode* _Tail() const {
+    return head_ ? head_->prev : nullptr;
+  }
+
   // Returns false if used existing sentinel, true if a new sentinel was created.
   bool PushSentinel(std::string_view value, Where where);
 
@@ -176,7 +180,7 @@ class QList {
   bool DelPackedIndex(quicklistNode* node, uint8_t** p);
 
   quicklistNode* head_ = nullptr;
-  quicklistNode* tail_ = nullptr;
+
   uint32_t count_ = 0;                   /* total count of all entries in all listpacks */
   uint32_t len_ = 0;                     /* number of quicklistNodes */
   signed int fill_ : QL_FILL_BITS;       /* fill factor for individual nodes */

src/core/qlist_test.cc

@@ -90,7 +90,7 @@ static int ql_verify(const QList& ql, uint32_t nc, uint32_t count, uint32_t head
   node_size = 0;
   while (node) {
     node_size += node->count;
-    node = node->prev;
+    node = (node == ql.Head()) ? nullptr : node->prev;
   }
   if (node_size != ql.Size()) {
     LOG(ERROR) << "has different forward count than reverse count!  "
@@ -165,7 +165,6 @@ TEST_F(QListTest, Basic) {
   EXPECT_EQ(0, ql_.Size());
   ql_.Push("abc", QList::HEAD);
   EXPECT_EQ(1, ql_.Size());
-  EXPECT_TRUE(ql_.Head()->prev == nullptr);
   EXPECT_TRUE(ql_.Tail() == ql_.Head());
 
   auto it = ql_.GetIterator(QList::HEAD);
@@ -517,6 +516,10 @@ TEST_P(OptionsTest, DelNeg100From500) {
   for (int i = 0; i < 500; i++)
     ql_.Push(StrCat("hello", i + 1), QList::TAIL);
   ql_.Erase(-100, 100);
+
+  QList::Iterator it = ql_.GetIterator(QList::TAIL);
+  ASSERT_TRUE(it.Next());
+  ASSERT_EQ("hello400", it.Get());
   ASSERT_EQ(0, ql_verify(ql_, 13, 400, 32, 16));
 }