From f235f4b0a3236037905ef5fec2a28bbb74121770 Mon Sep 17 00:00:00 2001
From: Yanqin Jin <yanqin@fb.com>
Date: Tue, 24 Aug 2021 18:17:32 -0700
Subject: [PATCH] Fix a bug of secondary instance sequence going backward
 (#8653)

Summary:
Recent refactor of `ReactiveVersionSet::ReadAndApply()` uses
`ManifestTailer` whose `Iterate()` method can cause the db's
`last_sequence_` to go backward. Consequently, read requests can see
out-dated data. For example, latest changes to the primary will not be
seen on the secondary even after a `TryCatchUpWithPrimary()` if no new
write batches are read from the WALs and no new MANIFEST entries are
read from the MANIFEST.

Fix the bug so that `VersionEditHandler::CheckIterationResult` will
never decrease `last_sequence_`, `last_allocated_sequence_` and
`last_published_sequence_`.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8653

Test Plan: make check

Reviewed By: jay-zhuang

Differential Revision: D30272084

Pulled By: riversand963

fbshipit-source-id: c6a49c534b2509b93ef62d8936ed0acd5b860eaa
---
 HISTORY.md                 |  1 +
 db/db_secondary_test.cc    | 33 +++++++++++++++++++++++++++++++++
 db/version_edit_handler.cc | 19 ++++++++++++++-----
 3 files changed, 48 insertions(+), 5 deletions(-)

diff --git a/HISTORY.md b/HISTORY.md
index ba89774c52..25c1d79992 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -2,6 +2,7 @@
 ## Unreleased
 ### Bug Fixes
 * Allow secondary instance to refresh iterator. Assign read seq after referencing SuperVersion.
+* Fixed a bug of secondary instance's last_sequence going backward, and reads on the secondary fail to see recent updates from the primary.
 
 ### New Features
 * RemoteCompaction's interface now includes `db_name`, `db_id`, `session_id`, which could help the user uniquely identify compaction job between db instances and sessions.
diff --git a/db/db_secondary_test.cc b/db/db_secondary_test.cc
index 178844ff77..902f1ec2cd 100644
--- a/db/db_secondary_test.cc
+++ b/db/db_secondary_test.cc
@@ -560,6 +560,39 @@ TEST_F(DBSecondaryTest, OpenAsSecondaryWALTailing) {
   verify_db_func("new_foo_value_1", "new_bar_value");
 }
 
+TEST_F(DBSecondaryTest, SecondaryTailingBug_ISSUE_8467) {
+  Options options;
+  options.env = env_;
+  Reopen(options);
+  for (int i = 0; i < 3; ++i) {
+    ASSERT_OK(Put("foo", "foo_value" + std::to_string(i)));
+    ASSERT_OK(Put("bar", "bar_value" + std::to_string(i)));
+  }
+
+  Options options1;
+  options1.env = env_;
+  options1.max_open_files = -1;
+  OpenSecondary(options1);
+
+  const auto verify_db = [&](const std::string& foo_val,
+                             const std::string& bar_val) {
+    std::string value;
+    ReadOptions ropts;
+    Status s = db_secondary_->Get(ropts, "foo", &value);
+    ASSERT_OK(s);
+    ASSERT_EQ(foo_val, value);
+
+    s = db_secondary_->Get(ropts, "bar", &value);
+    ASSERT_OK(s);
+    ASSERT_EQ(bar_val, value);
+  };
+
+  for (int i = 0; i < 2; ++i) {
+    ASSERT_OK(db_secondary_->TryCatchUpWithPrimary());
+    verify_db("foo_value2", "bar_value2");
+  }
+}
+
 TEST_F(DBSecondaryTest, RefreshIterator) {
   Options options;
   options.env = env_;
diff --git a/db/version_edit_handler.cc b/db/version_edit_handler.cc
index 7a2996a59e..5631cda48e 100644
--- a/db/version_edit_handler.cc
+++ b/db/version_edit_handler.cc
@@ -427,11 +427,20 @@ void VersionEditHandler::CheckIterationResult(const log::Reader& reader,
     assert(version_set_->manifest_file_size_ > 0);
     version_set_->next_file_number_.store(
         version_edit_params_.next_file_number_ + 1);
-    version_set_->last_allocated_sequence_ =
-        version_edit_params_.last_sequence_;
-    version_set_->last_published_sequence_ =
-        version_edit_params_.last_sequence_;
-    version_set_->last_sequence_ = version_edit_params_.last_sequence_;
+    SequenceNumber last_seq = version_edit_params_.last_sequence_;
+    assert(last_seq != kMaxSequenceNumber);
+    if (last_seq != kMaxSequenceNumber &&
+        last_seq > version_set_->last_allocated_sequence_.load()) {
+      version_set_->last_allocated_sequence_.store(last_seq);
+    }
+    if (last_seq != kMaxSequenceNumber &&
+        last_seq > version_set_->last_published_sequence_.load()) {
+      version_set_->last_published_sequence_.store(last_seq);
+    }
+    if (last_seq != kMaxSequenceNumber &&
+        last_seq > version_set_->last_sequence_.load()) {
+      version_set_->last_sequence_.store(last_seq);
+    }
     version_set_->prev_log_number_ = version_edit_params_.prev_log_number_;
   }
 }