[IndexedDB] Transaction limitting

content/browser/indexed_db/indexed_db_transaction_coordinator.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/indexed_db/indexed_db_transaction_coordinator.h"
 
 #include "base/logging.h"
+#include "content/browser/indexed_db/indexed_db_tracing.h"
 #include "content/browser/indexed_db/indexed_db_transaction.h"
 #include "third_party/WebKit/public/platform/modules/indexeddb/WebIDBTypes.h"
 
 namespace content {
+namespace {
+static const size_t kMaxStartedTransactions = 10;

[cmumford, 2017/02/16 23:16:05]

Can you add a comment explaining why this has a value of 10?

[pwnall, 2017/02/16 23:19:40]

Can you explain that this limits the number of transactions per database? It'd
also help if you mention what happens when the limit is reached -- seems like
new transactions will remain queued?

[dmurph, 2017/02/16 23:32:34]

Done.

+}  // namespace
 
 IndexedDBTransactionCoordinator::IndexedDBTransactionCoordinator() {}
 
 IndexedDBTransactionCoordinator::~IndexedDBTransactionCoordinator() {
   DCHECK(queued_transactions_.empty());
   DCHECK(started_transactions_.empty());
 }
 
 void IndexedDBTransactionCoordinator::DidCreateTransaction(
     IndexedDBTransaction* transaction) {
   DCHECK(!queued_transactions_.count(transaction));
   DCHECK(!started_transactions_.count(transaction));
   DCHECK_EQ(IndexedDBTransaction::CREATED, transaction->state());
 
   queued_transactions_.insert(transaction);
   ProcessQueuedTransactions();
 }
 
 // Observer transactions jump to the front of the queue.
 void IndexedDBTransactionCoordinator::DidCreateObserverTransaction(
     IndexedDBTransaction* transaction) {
   DCHECK(!queued_transactions_.count(transaction));
   DCHECK(!started_transactions_.count(transaction));
   DCHECK_EQ(IndexedDBTransaction::CREATED, transaction->state());
 
   started_transactions_.insert_front(transaction);
+  RecordMetrics();
   ProcessQueuedTransactions();
 }
 
 void IndexedDBTransactionCoordinator::DidFinishTransaction(
     IndexedDBTransaction* transaction) {
   if (queued_transactions_.count(transaction)) {
     DCHECK(!started_transactions_.count(transaction));
     queued_transactions_.erase(transaction);
   } else {
     DCHECK(started_transactions_.count(transaction));
     started_transactions_.erase(transaction);
   }
-
+  RecordMetrics();
   ProcessQueuedTransactions();
 }
 
 bool IndexedDBTransactionCoordinator::IsRunningVersionChangeTransaction()
     const {
   return !started_transactions_.empty() &&
          (*started_transactions_.begin())->mode() ==
              blink::WebIDBTransactionModeVersionChange;
 }
 
@@ skipping 16 matching lines @@
 IndexedDBTransactionCoordinator::GetTransactions() const {
   std::vector<const IndexedDBTransaction*> result;
   result.reserve(started_transactions_.size() + queued_transactions_.size());
   result.insert(result.end(), started_transactions_.begin(),
                 started_transactions_.end());
   result.insert(result.end(), queued_transactions_.begin(),
                 queued_transactions_.end());
   return result;
 }
 
+void IndexedDBTransactionCoordinator::RecordMetrics() const {
+  IDB_TRACE_COUNTER2("TransactionCoordinator", "StartedTransactions",
+                     started_transactions_.size(), "QueuedTransactions",
+                     queued_transactions_.size());
+}
+
 void IndexedDBTransactionCoordinator::ProcessQueuedTransactions() {
   if (queued_transactions_.empty())
     return;
 
   DCHECK(!IsRunningVersionChangeTransaction());
 
   // The locked_scope set accumulates the ids of object stores in the scope of
   // running read/write transactions. Other read-write transactions with
   // stores in this set may not be started. Read-only transactions may start,
   // taking a snapshot of the database, which does not include uncommitted
   // data. ("Version change" transactions are exclusive, but handled by the
   // connection sequencing in IndexedDBDatabase.)
   std::set<int64_t> locked_scope;
   for (const auto& transaction : started_transactions_) {
     if (transaction->mode() == blink::WebIDBTransactionModeReadWrite) {
       // Started read/write transactions have exclusive access to the object
       // stores within their scopes.
       locked_scope.insert(transaction->scope().begin(),
                           transaction->scope().end());
     }
   }
 
   auto it = queued_transactions_.begin();
   while (it != queued_transactions_.end()) {
     IndexedDBTransaction* transaction = *it;
     ++it;
-    if (CanStartTransaction(transaction, locked_scope)) {
+    if (CanStartTransaction(transaction, locked_scope,
+                            started_transactions_.size())) {
       DCHECK_EQ(IndexedDBTransaction::CREATED, transaction->state());
       queued_transactions_.erase(transaction);
       started_transactions_.insert(transaction);
       transaction->Start();
       DCHECK_EQ(IndexedDBTransaction::STARTED, transaction->state());
     }
     if (transaction->mode() == blink::WebIDBTransactionModeReadWrite) {
       // Either the transaction started, so it has exclusive access to the
       // stores in its scope, or per the spec the transaction which was
       // created first must get access first, so the stores are also locked.
       locked_scope.insert(transaction->scope().begin(),
                           transaction->scope().end());
     }
   }
+  RecordMetrics();
 }
 
 template<typename T>
 static bool DoSetsIntersect(const std::set<T>& set1,
                             const std::set<T>& set2) {
   typename std::set<T>::const_iterator it1 = set1.begin();
   typename std::set<T>::const_iterator it2 = set2.begin();
   while (it1 != set1.end() && it2 != set2.end()) {
     if (*it1 < *it2)
       ++it1;
     else if (*it2 < *it1)
       ++it2;
     else
       return true;
   }
   return false;
 }
 
 bool IndexedDBTransactionCoordinator::CanStartTransaction(
     IndexedDBTransaction* const transaction,
-    const std::set<int64_t>& locked_scope) const {
+    const std::set<int64_t>& locked_scope,
+    size_t num_started_txns) const {

[cmumford, 2017/02/16 23:16:05]

This is a non-static member function. Why not just access started_transactions_
directly? Seems unnecessarily complex.

[dmurph, 2017/02/16 23:32:34]

Whoops, yes much better.

+  if (num_started_txns >= kMaxStartedTransactions) {
+    return false;
+  }
   DCHECK(queued_transactions_.count(transaction));
   switch (transaction->mode()) {
     case blink::WebIDBTransactionModeVersionChange:
       DCHECK_EQ(1u, queued_transactions_.size());
       DCHECK(started_transactions_.empty());
       DCHECK(locked_scope.empty());
       return true;
 
     case blink::WebIDBTransactionModeReadOnly:
     case blink::WebIDBTransactionModeReadWrite:
       return !DoSetsIntersect(transaction->scope(), locked_scope);
   }
   NOTREACHED();
   return false;
 }
 
 }  // namespace content