Summary:
If `NeedToSeekImmutable()` returns false, `SeekInternal()` won't reset the
contents of `immutable_min_heap_`. However, since it calls `UpdateCurrent()`
unconditionally, if `current_` is one of immutable iterators (previously popped
from `immutable_min_heap_`), `UpdateCurrent()` will overwrite it. As a result,
if old `current_` in fact pointed to the smallest entry, forward iterator will
skip some records.
Fix implemented in this diff pushes `current_` back to `immutable_min_heap_`
before calling `UpdateCurrent()`.
Test Plan:
New unit test (courtesy of @lovro):
$ ROCKSDB_TESTS=TailingIteratorSeekToSame ./db_test
Reviewers: igor, dhruba, haobo, ljin
Reviewed By: ljin
Subscribers: lovro, leveldb
Differential Revision: https://reviews.facebook.net/D19653
3b97ee96c4