[BP] MB-57400: "estimated" items_remaining drops to 0 when cursor settles

[[backport to 7.1.x for maitanence patch]]

"estimate" previously introduced in MB-57400. Here we introduce the
invariant that "estimate" drops to 0 when the cursor reaches the end
of checkpoint.

That also fixes test issues like MB-57808.

(cherry picked from commit e903cfe)

Change-Id: Ieb65be994e279571e7c79d76a4b5df7f5042b9bc
Reviewed-on: https://review.couchbase.org/c/kv_engine/+/195023
Well-Formed: Restriction Checker
Reviewed-by: Paolo Cocchi <[email protected]>
Tested-by: Dave Rigby <[email protected]>

Author: paolococchi

engines/ep/src/checkpoint_iterator.h

@@ -146,12 +146,12 @@ class CheckpointIterator {
         return iter;
     }
 
-private:
     /// Is the iterator currently pointing to the "end" element.
     bool isAtEnd() const {
         return (iter == container.get().end());
     }
 
+private:
     /// Is the iterator currently pointing to the first element,
     bool isAtStart() const {
         return (iter == container.get().begin());

engines/ep/src/checkpoint_manager.cc

@@ -1269,8 +1269,16 @@ size_t CheckpointManager::getNumItemsForCursor(
         bool accurate) const {
     if (cursor && cursor->valid()) {
         CheckpointList::const_iterator chkptIterator(cursor->getCheckpoint());
+        // Notes:
+        // - Estimate introduced in MB-57400 for avoiding expensive O(N) calls
+        //   to cursor->getRemainingItemsCount()
+        // - MB-57808 improves that Estimate by introducing the invariant that
+        //   the quantity drops to 0 when the cursor reaches the end of the
+        //   checkpoint
         size_t items = accurate ? cursor->getRemainingItemsCount()
-                                : (*chkptIterator)->getNumItems();
+                                : (std::next(cursor->getPos()).isAtEnd()
+                                           ? 0
+                                           : (*chkptIterator)->getNumItems());
 
         if (chkptIterator != checkpointList.end()) {
             ++chkptIterator;

engines/ep/tests/module_tests/checkpoint_test.cc

@@ -2763,15 +2763,17 @@ TEST_P(CheckpointTest, getNumItemsForCursor_Accurate_vs_Estimate) {
     EXPECT_FALSE(res.moreAvailable);
     EXPECT_EQ(3, items.size());
 
-    // Now we start to see a differnce, which can increase as the checkpoint
-    // grows.
+    // Note: The new Estimate is still precise in the case where the cursor has
+    // reached the end of the checkpoint.
     EXPECT_EQ(0, manager->getNumItemsForCursor(cursor, true));
-    EXPECT_EQ(2, manager->getNumItemsForCursor(cursor, false));
+    EXPECT_EQ(0, manager->getNumItemsForCursor(cursor, false));
 
     for (auto i : {1, 2}) {
         EXPECT_TRUE(this->queueNewItem("b" + std::to_string(i)));
     }
 
+    // Now we start to see a difference, which can increase as the checkpoint
+    // grows.
     EXPECT_EQ(2, manager->getNumItemsForCursor(cursor, true));
     EXPECT_EQ(4, manager->getNumItemsForCursor(cursor, false));