SERVER-54552 [SBE] Reverse index scan may produce wrong results

3 files changed, 40 insertions, 14 deletions

diff --git a/src/mongo/db/query/sbe_stage_builder_index_scan.cpp b/src/mongo/db/query/sbe_stage_builder_index_scan.cpp
index 863a2824ed8..1eee1a968f7 100644
--- a/src/mongo/db/query/sbe_stage_builder_index_scan.cpp
+++ b/src/mongo/db/query/sbe_stage_builder_index_scan.cpp
@@ -233,15 +233,20 @@ makeIntervalsFromIndexBounds(const IndexBounds& bounds,
                     "Generated interval [lowKey, highKey]",
                     "lowKey"_attr = lowKey,
                     "highKey"_attr = highKey);
-        // For high keys use the opposite rule as a normal seek because a forward scan should end
-        // after the key if inclusive, and before if exclusive.
-        const auto inclusive = forward != highKeyInclusive;
-        result.push_back({std::make_unique<KeyString::Value>(
-                              IndexEntryComparison::makeKeyStringFromBSONKeyForSeek(
-                                  lowKey, version, ordering, forward, lowKeyInclusive)),
-                          std::make_unique<KeyString::Value>(
-                              IndexEntryComparison::makeKeyStringFromBSONKeyForSeek(
-                                  highKey, version, ordering, forward, inclusive))});
+        // Note that 'makeKeyFromBSONKeyForSeek()' is intended to compute the "start" key for an
+        // index scan. The logic for computing a "discriminator" for an "end" key is reversed, which
+        // is why we use 'makeKeyStringFromBSONKey()' to manually specify the discriminator for the
+        // end key.
+        result.push_back(
+            {std::make_unique<KeyString::Value>(
+                 IndexEntryComparison::makeKeyStringFromBSONKeyForSeek(
+                     lowKey, version, ordering, forward, lowKeyInclusive)),
+             std::make_unique<KeyString::Value>(IndexEntryComparison::makeKeyStringFromBSONKey(
+                 highKey,
+                 version,
+                 ordering,
+                 forward != highKeyInclusive ? KeyString::Discriminator::kExclusiveBefore
+                                             : KeyString::Discriminator::kExclusiveAfter))});
     }
     return result;
 }
diff --git a/src/mongo/db/storage/index_entry_comparison.cpp b/src/mongo/db/storage/index_entry_comparison.cpp
index a1b42051f15..965a4e8d796 100644
--- a/src/mongo/db/storage/index_entry_comparison.cpp
+++ b/src/mongo/db/storage/index_entry_comparison.cpp
@@ -167,12 +167,20 @@ KeyString::Value IndexEntryComparison::makeKeyStringFromBSONKeyForSeek(const BSO
                                                                        Ordering ord,
                                                                        bool isForward,
                                                                        bool inclusive) {
+    return makeKeyStringFromBSONKey(bsonKey,
+                                    version,
+                                    ord,
+                                    isForward == inclusive
+                                        ? KeyString::Discriminator::kExclusiveBefore
+                                        : KeyString::Discriminator::kExclusiveAfter);
+}
+
+KeyString::Value IndexEntryComparison::makeKeyStringFromBSONKey(const BSONObj& bsonKey,
+                                                                KeyString::Version version,
+                                                                Ordering ord,
+                                                                KeyString::Discriminator discrim) {
     BSONObj finalKey = BSONObj::stripFieldNames(bsonKey);
-    KeyString::Builder builder(version,
-                               finalKey,
-                               ord,
-                               isForward == inclusive ? KeyString::Discriminator::kExclusiveBefore
-                                                      : KeyString::Discriminator::kExclusiveAfter);
+    KeyString::Builder builder(version, finalKey, ord, discrim);
     return builder.getValueCopy();
 }
 
diff --git a/src/mongo/db/storage/index_entry_comparison.h b/src/mongo/db/storage/index_entry_comparison.h
index 6a1574c7360..767385283cf 100644
--- a/src/mongo/db/storage/index_entry_comparison.h
+++ b/src/mongo/db/storage/index_entry_comparison.h
@@ -257,6 +257,19 @@ public:
                                                             bool isForward,
                                                             bool inclusive);
 
+    /**
+     * Encodes the BSON Key into a KeyString object to pass in to SortedDataInterface::seek()
+     * or SortedDataInterface::setEndPosition().
+     *
+     * This funcition is similar to IndexEntryComparison::makeKeyStringFromBSONKeyForSeek()
+     * but allows you to pick your own KeyString::Discriminator based on wether or not the
+     * resulting KeyString is for the start key or end key of a seek.
+     */
+    static KeyString::Value makeKeyStringFromBSONKey(const BSONObj& bsonKey,
+                                                     KeyString::Version version,
+                                                     Ordering ord,
+                                                     KeyString::Discriminator discrim);
+
 private:
     // Ordering is used in comparison() to compare BSONElements
     const Ordering _order;