Simplify WriteUnpreparedTxnReadCallback and fix some comments (#5621)

Summary:
Simplify WriteUnpreparedTxnReadCallback so we just have one function `CalcMaxVisibleSeq`. Also, there's no need for the read callback to hold onto the transaction any more, so just hold the set of unprep_seqs, reducing about of indirection in `IsVisibleFullCheck`.

Also, some comments about using transaction snapshot were out of date, so remove them.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5621

Differential Revision: D16459883

Pulled By: lth

fbshipit-source-id: cd581323fd18982e817d99af57b6eaba59e599bb

utilities/transactions/write_unprepared_txn.cc

@@ -13,15 +13,13 @@
 namespace rocksdb {
 
 bool WriteUnpreparedTxnReadCallback::IsVisibleFullCheck(SequenceNumber seq) {
-  auto unprep_seqs = txn_->GetUnpreparedSequenceNumbers();
-
   // Since unprep_seqs maps prep_seq => prepare_batch_cnt, to check if seq is
   // in unprep_seqs, we have to check if seq is equal to prep_seq or any of
   // the prepare_batch_cnt seq nums after it.
   //
   // TODO(lth): Can be optimized with std::lower_bound if unprep_seqs is
   // large.
-  for (const auto& it : unprep_seqs) {
+  for (const auto& it : unprep_seqs_) {
     if (it.first <= seq && seq < it.first + it.second) {
       return true;
     }
@@ -30,15 +28,6 @@ bool WriteUnpreparedTxnReadCallback::IsVisibleFullCheck(SequenceNumber seq) {
   return db_->IsInSnapshot(seq, wup_snapshot_, min_uncommitted_);
 }
 
-SequenceNumber WriteUnpreparedTxnReadCallback::CalcMaxUnpreparedSequenceNumber(
-    WriteUnpreparedTxn* txn) {
-  const auto& unprep_seqs = txn->GetUnpreparedSequenceNumbers();
-  if (unprep_seqs.size()) {
-    return unprep_seqs.rbegin()->first + unprep_seqs.rbegin()->second - 1;
-  }
-  return 0;
-}
-
 WriteUnpreparedTxn::WriteUnpreparedTxn(WriteUnpreparedTxnDB* txn_db,
                                        const WriteOptions& write_options,
                                        const TransactionOptions& txn_options)
@@ -537,7 +526,7 @@ Status WriteUnpreparedTxn::Get(const ReadOptions& options,
   const bool backed_by_snapshot =
       wupt_db_->AssignMinMaxSeqs(options.snapshot, &min_uncommitted, &snap_seq);
   WriteUnpreparedTxnReadCallback callback(wupt_db_, snap_seq, min_uncommitted,
-                                          this);
+                                          unprep_seqs_);
   auto res = write_batch_.GetFromBatchAndDB(db_, options, column_family, key,
                                             value, &callback);
   if (LIKELY(wupt_db_->ValidateSnapshot(snap_seq, backed_by_snapshot))) {

utilities/transactions/write_unprepared_txn.h

@@ -53,17 +53,17 @@ class WriteUnpreparedTxn;
 //
 class WriteUnpreparedTxnReadCallback : public ReadCallback {
  public:
-  WriteUnpreparedTxnReadCallback(WritePreparedTxnDB* db,
-                                 SequenceNumber snapshot,
-                                 SequenceNumber min_uncommitted,
-                                 WriteUnpreparedTxn* txn)
+  WriteUnpreparedTxnReadCallback(
+      WritePreparedTxnDB* db, SequenceNumber snapshot,
+      SequenceNumber min_uncommitted,
+      const std::map<SequenceNumber, size_t>& unprep_seqs)
       // Pass our last uncommitted seq as the snapshot to the parent class to
       // ensure that the parent will not prematurely filter out own writes. We
       // will do the exact comparison against snapshots in IsVisibleFullCheck
       // override.
-      : ReadCallback(CalcMaxVisibleSeq(txn, snapshot), min_uncommitted),
+      : ReadCallback(CalcMaxVisibleSeq(unprep_seqs, snapshot), min_uncommitted),
         db_(db),
-        txn_(txn),
+        unprep_seqs_(unprep_seqs),
         wup_snapshot_(snapshot) {}
 
   virtual bool IsVisibleFullCheck(SequenceNumber seq) override;
@@ -74,15 +74,18 @@ class WriteUnpreparedTxnReadCallback : public ReadCallback {
   }
 
  private:
-  static SequenceNumber CalcMaxVisibleSeq(WriteUnpreparedTxn* txn,
-                                          SequenceNumber snapshot_seq) {
-    SequenceNumber max_unprepared = CalcMaxUnpreparedSequenceNumber(txn);
+  static SequenceNumber CalcMaxVisibleSeq(
+      const std::map<SequenceNumber, size_t>& unprep_seqs,
+      SequenceNumber snapshot_seq) {
+    SequenceNumber max_unprepared = 0;
+    if (unprep_seqs.size()) {
+      max_unprepared =
+          unprep_seqs.rbegin()->first + unprep_seqs.rbegin()->second - 1;
+    }
     return std::max(max_unprepared, snapshot_seq);
   }
-  static SequenceNumber CalcMaxUnpreparedSequenceNumber(
-      WriteUnpreparedTxn* txn);
   WritePreparedTxnDB* db_;
-  WriteUnpreparedTxn* txn_;
+  const std::map<SequenceNumber, size_t>& unprep_seqs_;
   SequenceNumber wup_snapshot_;
 };
 
@@ -124,8 +127,6 @@ class WriteUnpreparedTxn : public WritePreparedTxn {
 
   virtual Status RebuildFromWriteBatch(WriteBatch*) override;
 
-  const std::map<SequenceNumber, size_t>& GetUnpreparedSequenceNumbers();
-
  protected:
   void Initialize(const TransactionOptions& txn_options) override;
 
@@ -156,6 +157,8 @@ class WriteUnpreparedTxn : public WritePreparedTxn {
   friend class WriteUnpreparedTransactionTest_UnpreparedBatch_Test;
   friend class WriteUnpreparedTxnDB;
 
+  const std::map<SequenceNumber, size_t>& GetUnpreparedSequenceNumbers();
+
   Status MaybeFlushWriteBatchToDB();
   Status FlushWriteBatchToDB(bool prepared);
   Status HandleWrite(std::function<Status()> do_write);

utilities/transactions/write_unprepared_txn_db.cc

@@ -348,7 +348,8 @@ struct WriteUnpreparedTxnDB::IteratorState {
   IteratorState(WritePreparedTxnDB* txn_db, SequenceNumber sequence,
                 std::shared_ptr<ManagedSnapshot> s,
                 SequenceNumber min_uncommitted, WriteUnpreparedTxn* txn)
-      : callback(txn_db, sequence, min_uncommitted, txn), snapshot(s) {}
+      : callback(txn_db, sequence, min_uncommitted, txn->unprep_seqs_),
+        snapshot(s) {}
   SequenceNumber MaxVisibleSeq() { return callback.max_visible_seq(); }
 
   WriteUnpreparedTxnReadCallback callback;
@@ -384,27 +385,22 @@ Iterator* WriteUnpreparedTxnDB::NewIterator(const ReadOptions& options,
   // foo: v5 5
   //
   // Then 1, 2, 3 will be visible, but 4 will be non-visible, so we return v3,
-  // which is the last visible key.
+  // which is the last visible value.
   //
   // For unprepared transactions, if we have snap_seq = 3, but the current
-  // transaction has unprep_seq 5, then returning the first non-visible key
+  // transaction has unprep_seq 5, then returning the first non-visible value
   // would be incorrect, as we should return v5, and not v3. The problem is that
-  // there are committed keys at snapshot_seq < commit_seq < unprep_seq.
+  // there are committed values at snapshot_seq < commit_seq < unprep_seq.
   //
   // Snapshot validation can prevent this problem by ensuring that no committed
-  // keys exist at snapshot_seq < commit_seq, and thus any value with a sequence
-  // number greater than snapshot_seq must be unprepared keys. For example, if
-  // the transaction had a snapshot at 3, then snapshot validation would be
-  // performed during the Put(v5) call. It would find v4, and the Put would fail
-  // with snapshot validation failure.
-  //
-  // Because of this, if any writes have occurred, then the transaction snapshot
-  // must be used for the iterator. If no writes have occurred though, we can
-  // simply create a snapshot. Later writes would not be visible though, but we
-  // don't support iterating while writing anyway.
+  // values exist at snapshot_seq < commit_seq, and thus any value with a
+  // sequence number greater than snapshot_seq must be unprepared values. For
+  // example, if the transaction had a snapshot at 3, then snapshot validation
+  // would be performed during the Put(v5) call. It would find v4, and the Put
+  // would fail with snapshot validation failure.
   //
   // TODO(lth): Improve Prev() logic to continue iterating until
-  // max_visible_seq, and then return the last visible key, so that this
+  // max_visible_seq, and then return the last visible value, so that this
   // restriction can be lifted.
   const Snapshot* snapshot = nullptr;
   if (options.snapshot == nullptr) {
@@ -418,9 +414,9 @@ Iterator* WriteUnpreparedTxnDB::NewIterator(const ReadOptions& options,
   assert(snapshot_seq != kMaxSequenceNumber);
   // Iteration is safe as long as largest_validated_seq <= snapshot_seq. We are
   // guaranteed that for keys that were modified by this transaction (and thus
-  // might have unprepared versions), no committed versions exist at
+  // might have unprepared values), no committed values exist at
   // largest_validated_seq < commit_seq (or the contrapositive: any committed
-  // version must exist at commit_seq <= largest_validated_seq). This implies
+  // value must exist at commit_seq <= largest_validated_seq). This implies
   // that commit_seq <= largest_validated_seq <= snapshot_seq or commit_seq <=
   // snapshot_seq. As explained above, the problem with Prev() only happens when
   // snapshot_seq < commit_seq.