Revert of Move sqlite_channel_id_store from chrome/browser/net to net/extras.

chrome/browser/net/sqlite_channel_id_store.cc

Index: chrome/browser/net/sqlite_channel_id_store.cc
diff --git a/chrome/browser/net/sqlite_channel_id_store.cc b/chrome/browser/net/sqlite_channel_id_store.cc
new file mode 100644
index 0000000000000000000000000000000000000000..15bad3dc31ae909ed3d5d4d1ed73ba8f86058cc3
--- /dev/null
+++ b/chrome/browser/net/sqlite_channel_id_store.cc
@@ -0,0 +1,641 @@
+// Copyright 2014 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 "chrome/browser/net/sqlite_channel_id_store.h"
+
+#include <list>
+#include <set>
+
+#include "base/basictypes.h"
+#include "base/bind.h"
+#include "base/file_util.h"
+#include "base/files/file_path.h"
+#include "base/logging.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/metrics/histogram.h"
+#include "base/strings/string_util.h"
+#include "base/threading/thread.h"
+#include "base/threading/thread_restrictions.h"
+#include "net/cert/x509_certificate.h"
+#include "net/cookies/cookie_util.h"
+#include "net/ssl/ssl_client_cert_type.h"
+#include "sql/error_delegate_util.h"
+#include "sql/meta_table.h"
+#include "sql/statement.h"
+#include "sql/transaction.h"
+#include "third_party/sqlite/sqlite3.h"
+#include "url/gurl.h"
+#include "webkit/browser/quota/special_storage_policy.h"
+
+// This class is designed to be shared between any calling threads and the
+// background task runner. It batches operations and commits them on a timer.
+class SQLiteChannelIDStore::Backend
+    : public base::RefCountedThreadSafe<SQLiteChannelIDStore::Backend> {
+ public:
+  Backend(
+      const base::FilePath& path,
+      const scoped_refptr<base::SequencedTaskRunner>& background_task_runner,
+      quota::SpecialStoragePolicy* special_storage_policy)
+      : path_(path),
+        num_pending_(0),
+        force_keep_session_state_(false),
+        background_task_runner_(background_task_runner),
+        special_storage_policy_(special_storage_policy),
+        corruption_detected_(false) {}
+
+  // Creates or loads the SQLite database.
+  void Load(const LoadedCallback& loaded_callback);
+
+  // Batch a channel ID addition.
+  void AddChannelID(
+      const net::DefaultChannelIDStore::ChannelID& channel_id);
+
+  // Batch a channel ID deletion.
+  void DeleteChannelID(
+      const net::DefaultChannelIDStore::ChannelID& channel_id);
+
+  // Commit any pending operations and close the database.  This must be called
+  // before the object is destructed.
+  void Close();
+
+  void SetForceKeepSessionState();
+
+ private:
+  void LoadOnDBThread(
+      ScopedVector<net::DefaultChannelIDStore::ChannelID>* channel_ids);
+
+  friend class base::RefCountedThreadSafe<SQLiteChannelIDStore::Backend>;
+
+  // You should call Close() before destructing this object.
+  ~Backend() {
+    DCHECK(!db_.get()) << "Close should have already been called.";
+    DCHECK(num_pending_ == 0 && pending_.empty());
+  }
+
+  // Database upgrade statements.
+  bool EnsureDatabaseVersion();
+
+  class PendingOperation {
+   public:
+    typedef enum {
+      CHANNEL_ID_ADD,
+      CHANNEL_ID_DELETE
+    } OperationType;
+
+    PendingOperation(
+        OperationType op,
+        const net::DefaultChannelIDStore::ChannelID& channel_id)
+        : op_(op), channel_id_(channel_id) {}
+
+    OperationType op() const { return op_; }
+    const net::DefaultChannelIDStore::ChannelID& channel_id() const {
+        return channel_id_;
+    }
+
+   private:
+    OperationType op_;
+    net::DefaultChannelIDStore::ChannelID channel_id_;
+  };
+
+ private:
+  // Batch a channel id operation (add or delete).
+  void BatchOperation(
+      PendingOperation::OperationType op,
+      const net::DefaultChannelIDStore::ChannelID& channel_id);
+  // Commit our pending operations to the database.
+  void Commit();
+  // Close() executed on the background thread.
+  void InternalBackgroundClose();
+
+  void DeleteCertificatesOnShutdown();
+
+  void DatabaseErrorCallback(int error, sql::Statement* stmt);
+  void KillDatabase();
+
+  base::FilePath path_;
+  scoped_ptr<sql::Connection> db_;
+  sql::MetaTable meta_table_;
+
+  typedef std::list<PendingOperation*> PendingOperationsList;
+  PendingOperationsList pending_;
+  PendingOperationsList::size_type num_pending_;
+  // True if the persistent store should skip clear on exit rules.
+  bool force_keep_session_state_;
+  // Guard |pending_|, |num_pending_| and |force_keep_session_state_|.
+  base::Lock lock_;
+
+  // Cache of origins we have channel IDs stored for.
+  std::set<std::string> channel_id_origins_;
+
+  scoped_refptr<base::SequencedTaskRunner> background_task_runner_;
+
+  scoped_refptr<quota::SpecialStoragePolicy> special_storage_policy_;
+
+  // Indicates if the kill-database callback has been scheduled.
+  bool corruption_detected_;
+
+  DISALLOW_COPY_AND_ASSIGN(Backend);
+};
+
+// Version number of the database.
+static const int kCurrentVersionNumber = 4;
+static const int kCompatibleVersionNumber = 1;
+
+namespace {
+
+// Initializes the certs table, returning true on success.
+bool InitTable(sql::Connection* db) {
+  // The table is named "origin_bound_certs" for backwards compatability before
+  // we renamed this class to SQLiteChannelIDStore.  Likewise, the primary
+  // key is "origin", but now can be other things like a plain domain.
+  if (!db->DoesTableExist("origin_bound_certs")) {
+    if (!db->Execute("CREATE TABLE origin_bound_certs ("
+                     "origin TEXT NOT NULL UNIQUE PRIMARY KEY,"
+                     "private_key BLOB NOT NULL,"
+                     "cert BLOB NOT NULL,"
+                     "cert_type INTEGER,"
+                     "expiration_time INTEGER,"
+                     "creation_time INTEGER)"))
+      return false;
+  }
+
+  return true;
+}
+
+}  // namespace
+
+void SQLiteChannelIDStore::Backend::Load(
+    const LoadedCallback& loaded_callback) {
+  // This function should be called only once per instance.
+  DCHECK(!db_.get());
+  scoped_ptr<ScopedVector<net::DefaultChannelIDStore::ChannelID> >
+      channel_ids(new ScopedVector<net::DefaultChannelIDStore::ChannelID>());
+  ScopedVector<net::DefaultChannelIDStore::ChannelID>* channel_ids_ptr =
+      channel_ids.get();
+
+  background_task_runner_->PostTaskAndReply(
+      FROM_HERE,
+      base::Bind(&Backend::LoadOnDBThread, this, channel_ids_ptr),
+      base::Bind(loaded_callback, base::Passed(&channel_ids)));
+}
+
+void SQLiteChannelIDStore::Backend::LoadOnDBThread(
+    ScopedVector<net::DefaultChannelIDStore::ChannelID>* channel_ids) {
+  DCHECK(background_task_runner_->RunsTasksOnCurrentThread());
+
+  // This method should be called only once per instance.
+  DCHECK(!db_.get());
+
+  base::TimeTicks start = base::TimeTicks::Now();
+
+  // Ensure the parent directory for storing certs is created before reading
+  // from it.
+  const base::FilePath dir = path_.DirName();
+  if (!base::PathExists(dir) && !base::CreateDirectory(dir))
+    return;
+
+  int64 db_size = 0;
+  if (base::GetFileSize(path_, &db_size))
+    UMA_HISTOGRAM_COUNTS("DomainBoundCerts.DBSizeInKB", db_size / 1024 );
+
+  db_.reset(new sql::Connection);
+  db_->set_histogram_tag("DomainBoundCerts");
+
+  // Unretained to avoid a ref loop with db_.
+  db_->set_error_callback(
+      base::Bind(&SQLiteChannelIDStore::Backend::DatabaseErrorCallback,
+                 base::Unretained(this)));
+
+  if (!db_->Open(path_)) {
+    NOTREACHED() << "Unable to open cert DB.";
+    if (corruption_detected_)
+      KillDatabase();
+    db_.reset();
+    return;
+  }
+
+  if (!EnsureDatabaseVersion() || !InitTable(db_.get())) {
+    NOTREACHED() << "Unable to open cert DB.";
+    if (corruption_detected_)
+      KillDatabase();
+    meta_table_.Reset();
+    db_.reset();
+    return;
+  }
+
+  db_->Preload();
+
+  // Slurp all the certs into the out-vector.
+  sql::Statement smt(db_->GetUniqueStatement(
+      "SELECT origin, private_key, cert, cert_type, expiration_time, "
+      "creation_time FROM origin_bound_certs"));
+  if (!smt.is_valid()) {
+    if (corruption_detected_)
+      KillDatabase();
+    meta_table_.Reset();
+    db_.reset();
+    return;
+  }
+
+  while (smt.Step()) {
+    net::SSLClientCertType type =
+        static_cast<net::SSLClientCertType>(smt.ColumnInt(3));
+    if (type != net::CLIENT_CERT_ECDSA_SIGN)
+      continue;
+    std::string private_key_from_db, cert_from_db;
+    smt.ColumnBlobAsString(1, &private_key_from_db);
+    smt.ColumnBlobAsString(2, &cert_from_db);
+    scoped_ptr<net::DefaultChannelIDStore::ChannelID> channel_id(
+        new net::DefaultChannelIDStore::ChannelID(
+            smt.ColumnString(0),  // origin
+            base::Time::FromInternalValue(smt.ColumnInt64(5)),
+            base::Time::FromInternalValue(smt.ColumnInt64(4)),
+            private_key_from_db,
+            cert_from_db));
+    channel_id_origins_.insert(channel_id->server_identifier());
+    channel_ids->push_back(channel_id.release());
+  }
+
+  UMA_HISTOGRAM_COUNTS_10000("DomainBoundCerts.DBLoadedCount",
+                             channel_ids->size());
+  base::TimeDelta load_time = base::TimeTicks::Now() - start;
+  UMA_HISTOGRAM_CUSTOM_TIMES("DomainBoundCerts.DBLoadTime",
+                             load_time,
+                             base::TimeDelta::FromMilliseconds(1),
+                             base::TimeDelta::FromMinutes(1),
+                             50);
+  DVLOG(1) << "loaded " << channel_ids->size() << " in "
+           << load_time.InMilliseconds() << " ms";
+}
+
+bool SQLiteChannelIDStore::Backend::EnsureDatabaseVersion() {
+  // Version check.
+  if (!meta_table_.Init(
+      db_.get(), kCurrentVersionNumber, kCompatibleVersionNumber)) {
+    return false;
+  }
+
+  if (meta_table_.GetCompatibleVersionNumber() > kCurrentVersionNumber) {
+    LOG(WARNING) << "Server bound cert database is too new.";
+    return false;
+  }
+
+  int cur_version = meta_table_.GetVersionNumber();
+  if (cur_version == 1) {
+    sql::Transaction transaction(db_.get());
+    if (!transaction.Begin())
+      return false;
+    if (!db_->Execute("ALTER TABLE origin_bound_certs ADD COLUMN cert_type "
+                      "INTEGER")) {
+      LOG(WARNING) << "Unable to update server bound cert database to "
+                   << "version 2.";
+      return false;
+    }
+    // All certs in version 1 database are rsa_sign, which are unsupported.
+    // Just discard them all.
+    if (!db_->Execute("DELETE from origin_bound_certs")) {
+      LOG(WARNING) << "Unable to update server bound cert database to "
+                   << "version 2.";
+      return false;
+    }
+    ++cur_version;
+    meta_table_.SetVersionNumber(cur_version);
+    meta_table_.SetCompatibleVersionNumber(
+        std::min(cur_version, kCompatibleVersionNumber));
+    transaction.Commit();
+  }
+
+  if (cur_version <= 3) {
+    sql::Transaction transaction(db_.get());
+    if (!transaction.Begin())
+      return false;
+
+    if (cur_version == 2) {
+      if (!db_->Execute("ALTER TABLE origin_bound_certs ADD COLUMN "
+                        "expiration_time INTEGER")) {
+        LOG(WARNING) << "Unable to update server bound cert database to "
+                     << "version 4.";
+        return false;
+      }
+    }
+
+    if (!db_->Execute("ALTER TABLE origin_bound_certs ADD COLUMN "
+                      "creation_time INTEGER")) {
+      LOG(WARNING) << "Unable to update server bound cert database to "
+                   << "version 4.";
+      return false;
+    }
+
+    sql::Statement smt(db_->GetUniqueStatement(
+        "SELECT origin, cert FROM origin_bound_certs"));
+    sql::Statement update_expires_smt(db_->GetUniqueStatement(
+        "UPDATE origin_bound_certs SET expiration_time = ? WHERE origin = ?"));
+    sql::Statement update_creation_smt(db_->GetUniqueStatement(
+        "UPDATE origin_bound_certs SET creation_time = ? WHERE origin = ?"));
+    if (!smt.is_valid() ||
+        !update_expires_smt.is_valid() ||
+        !update_creation_smt.is_valid()) {
+      LOG(WARNING) << "Unable to update server bound cert database to "
+                   << "version 4.";
+      return false;
+    }
+
+    while (smt.Step()) {
+      std::string origin = smt.ColumnString(0);
+      std::string cert_from_db;
+      smt.ColumnBlobAsString(1, &cert_from_db);
+      // Parse the cert and extract the real value and then update the DB.
+      scoped_refptr<net::X509Certificate> cert(
+          net::X509Certificate::CreateFromBytes(
+              cert_from_db.data(), cert_from_db.size()));
+      if (cert.get()) {
+        if (cur_version == 2) {
+          update_expires_smt.Reset(true);
+          update_expires_smt.BindInt64(0,
+                                       cert->valid_expiry().ToInternalValue());
+          update_expires_smt.BindString(1, origin);
+          if (!update_expires_smt.Run()) {
+            LOG(WARNING) << "Unable to update server bound cert database to "
+                         << "version 4.";
+            return false;
+          }
+        }
+
+        update_creation_smt.Reset(true);
+        update_creation_smt.BindInt64(0, cert->valid_start().ToInternalValue());
+        update_creation_smt.BindString(1, origin);
+        if (!update_creation_smt.Run()) {
+          LOG(WARNING) << "Unable to update server bound cert database to "
+                       << "version 4.";
+          return false;
+        }
+      } else {
+        // If there's a cert we can't parse, just leave it.  It'll get replaced
+        // with a new one if we ever try to use it.
+        LOG(WARNING) << "Error parsing cert for database upgrade for origin "
+                     << smt.ColumnString(0);
+      }
+    }
+
+    cur_version = 4;
+    meta_table_.SetVersionNumber(cur_version);
+    meta_table_.SetCompatibleVersionNumber(
+        std::min(cur_version, kCompatibleVersionNumber));
+    transaction.Commit();
+  }
+
+  // Put future migration cases here.
+
+  // When the version is too old, we just try to continue anyway, there should
+  // not be a released product that makes a database too old for us to handle.
+  LOG_IF(WARNING, cur_version < kCurrentVersionNumber) <<
+      "Server bound cert database version " << cur_version <<
+      " is too old to handle.";
+
+  return true;
+}
+
+void SQLiteChannelIDStore::Backend::DatabaseErrorCallback(
+    int error,
+    sql::Statement* stmt) {
+  DCHECK(background_task_runner_->RunsTasksOnCurrentThread());
+
+  if (!sql::IsErrorCatastrophic(error))
+    return;
+
+  // TODO(shess): Running KillDatabase() multiple times should be
+  // safe.
+  if (corruption_detected_)
+    return;
+
+  corruption_detected_ = true;
+
+  // TODO(shess): Consider just calling RazeAndClose() immediately.
+  // db_ may not be safe to reset at this point, but RazeAndClose()
+  // would cause the stack to unwind safely with errors.
+  background_task_runner_->PostTask(FROM_HERE,
+                                    base::Bind(&Backend::KillDatabase, this));
+}
+
+void SQLiteChannelIDStore::Backend::KillDatabase() {
+  DCHECK(background_task_runner_->RunsTasksOnCurrentThread());
+
+  if (db_) {
+    // This Backend will now be in-memory only. In a future run the database
+    // will be recreated. Hopefully things go better then!
+    bool success = db_->RazeAndClose();
+    UMA_HISTOGRAM_BOOLEAN("DomainBoundCerts.KillDatabaseResult", success);
+    meta_table_.Reset();
+    db_.reset();
+  }
+}
+
+void SQLiteChannelIDStore::Backend::AddChannelID(
+    const net::DefaultChannelIDStore::ChannelID& channel_id) {
+  BatchOperation(PendingOperation::CHANNEL_ID_ADD, channel_id);
+}
+
+void SQLiteChannelIDStore::Backend::DeleteChannelID(
+    const net::DefaultChannelIDStore::ChannelID& channel_id) {
+  BatchOperation(PendingOperation::CHANNEL_ID_DELETE, channel_id);
+}
+
+void SQLiteChannelIDStore::Backend::BatchOperation(
+    PendingOperation::OperationType op,
+    const net::DefaultChannelIDStore::ChannelID& channel_id) {
+  // Commit every 30 seconds.
+  static const int kCommitIntervalMs = 30 * 1000;
+  // Commit right away if we have more than 512 outstanding operations.
+  static const size_t kCommitAfterBatchSize = 512;
+
+  // We do a full copy of the cert here, and hopefully just here.
+  scoped_ptr<PendingOperation> po(new PendingOperation(op, channel_id));
+
+  PendingOperationsList::size_type num_pending;
+  {
+    base::AutoLock locked(lock_);
+    pending_.push_back(po.release());
+    num_pending = ++num_pending_;
+  }
+
+  if (num_pending == 1) {
+    // We've gotten our first entry for this batch, fire off the timer.
+    background_task_runner_->PostDelayedTask(
+        FROM_HERE,
+        base::Bind(&Backend::Commit, this),
+        base::TimeDelta::FromMilliseconds(kCommitIntervalMs));
+  } else if (num_pending == kCommitAfterBatchSize) {
+    // We've reached a big enough batch, fire off a commit now.
+    background_task_runner_->PostTask(FROM_HERE,
+                                      base::Bind(&Backend::Commit, this));
+  }
+}
+
+void SQLiteChannelIDStore::Backend::Commit() {
+  DCHECK(background_task_runner_->RunsTasksOnCurrentThread());
+
+  PendingOperationsList ops;
+  {
+    base::AutoLock locked(lock_);
+    pending_.swap(ops);
+    num_pending_ = 0;
+  }
+
+  // Maybe an old timer fired or we are already Close()'ed.
+  if (!db_.get() || ops.empty())
+    return;
+
+  sql::Statement add_smt(db_->GetCachedStatement(SQL_FROM_HERE,
+      "INSERT INTO origin_bound_certs (origin, private_key, cert, cert_type, "
+      "expiration_time, creation_time) VALUES (?,?,?,?,?,?)"));
+  if (!add_smt.is_valid())
+    return;
+
+  sql::Statement del_smt(db_->GetCachedStatement(SQL_FROM_HERE,
+                             "DELETE FROM origin_bound_certs WHERE origin=?"));
+  if (!del_smt.is_valid())
+    return;
+
+  sql::Transaction transaction(db_.get());
+  if (!transaction.Begin())
+    return;
+
+  for (PendingOperationsList::iterator it = ops.begin();
+       it != ops.end(); ++it) {
+    // Free the certs as we commit them to the database.
+    scoped_ptr<PendingOperation> po(*it);
+    switch (po->op()) {
+      case PendingOperation::CHANNEL_ID_ADD: {
+        channel_id_origins_.insert(po->channel_id().server_identifier());
+        add_smt.Reset(true);
+        add_smt.BindString(0, po->channel_id().server_identifier());
+        const std::string& private_key = po->channel_id().private_key();
+        add_smt.BindBlob(1, private_key.data(), private_key.size());
+        const std::string& cert = po->channel_id().cert();
+        add_smt.BindBlob(2, cert.data(), cert.size());
+        add_smt.BindInt(3, net::CLIENT_CERT_ECDSA_SIGN);
+        add_smt.BindInt64(4,
+                          po->channel_id().expiration_time().ToInternalValue());
+        add_smt.BindInt64(5,
+                          po->channel_id().creation_time().ToInternalValue());
+        if (!add_smt.Run())
+          NOTREACHED() << "Could not add a server bound cert to the DB.";
+        break;
+      }
+      case PendingOperation::CHANNEL_ID_DELETE:
+        channel_id_origins_.erase(po->channel_id().server_identifier());
+        del_smt.Reset(true);
+        del_smt.BindString(0, po->channel_id().server_identifier());
+        if (!del_smt.Run())
+          NOTREACHED() << "Could not delete a server bound cert from the DB.";
+        break;
+
+      default:
+        NOTREACHED();
+        break;
+    }
+  }
+  transaction.Commit();
+}
+
+// Fire off a close message to the background thread. We could still have a
+// pending commit timer that will be holding a reference on us, but if/when
+// this fires we will already have been cleaned up and it will be ignored.
+void SQLiteChannelIDStore::Backend::Close() {
+  // Must close the backend on the background thread.
+  background_task_runner_->PostTask(
+      FROM_HERE, base::Bind(&Backend::InternalBackgroundClose, this));
+}
+
+void SQLiteChannelIDStore::Backend::InternalBackgroundClose() {
+  DCHECK(background_task_runner_->RunsTasksOnCurrentThread());
+  // Commit any pending operations
+  Commit();
+
+  if (!force_keep_session_state_ &&
+      special_storage_policy_.get() &&
+      special_storage_policy_->HasSessionOnlyOrigins()) {
+    DeleteCertificatesOnShutdown();
+  }
+
+  db_.reset();
+}
+
+void SQLiteChannelIDStore::Backend::DeleteCertificatesOnShutdown() {
+  DCHECK(background_task_runner_->RunsTasksOnCurrentThread());
+
+  if (!db_.get())
+    return;
+
+  if (channel_id_origins_.empty())
+    return;
+
+  if (!special_storage_policy_.get())
+    return;
+
+  sql::Statement del_smt(db_->GetCachedStatement(
+      SQL_FROM_HERE, "DELETE FROM origin_bound_certs WHERE origin=?"));
+  if (!del_smt.is_valid()) {
+    LOG(WARNING) << "Unable to delete certificates on shutdown.";
+    return;
+  }
+
+  sql::Transaction transaction(db_.get());
+  if (!transaction.Begin()) {
+    LOG(WARNING) << "Unable to delete certificates on shutdown.";
+    return;
+  }
+
+  for (std::set<std::string>::iterator it = channel_id_origins_.begin();
+       it != channel_id_origins_.end(); ++it) {
+    const GURL url(net::cookie_util::CookieOriginToURL(*it, true));
+    if (!url.is_valid() || !special_storage_policy_->IsStorageSessionOnly(url))
+      continue;
+    del_smt.Reset(true);
+    del_smt.BindString(0, *it);
+    if (!del_smt.Run())
+      NOTREACHED() << "Could not delete a certificate from the DB.";
+  }
+
+  if (!transaction.Commit())
+    LOG(WARNING) << "Unable to delete certificates on shutdown.";
+}
+
+void SQLiteChannelIDStore::Backend::SetForceKeepSessionState() {
+  base::AutoLock locked(lock_);
+  force_keep_session_state_ = true;
+}
+
+SQLiteChannelIDStore::SQLiteChannelIDStore(
+    const base::FilePath& path,
+    const scoped_refptr<base::SequencedTaskRunner>& background_task_runner,
+    quota::SpecialStoragePolicy* special_storage_policy)
+    : backend_(new Backend(path,
+                           background_task_runner,
+                           special_storage_policy)) {}
+
+void SQLiteChannelIDStore::Load(
+    const LoadedCallback& loaded_callback) {
+  backend_->Load(loaded_callback);
+}
+
+void SQLiteChannelIDStore::AddChannelID(
+    const net::DefaultChannelIDStore::ChannelID& channel_id) {
+  backend_->AddChannelID(channel_id);
+}
+
+void SQLiteChannelIDStore::DeleteChannelID(
+    const net::DefaultChannelIDStore::ChannelID& channel_id) {
+  backend_->DeleteChannelID(channel_id);
+}
+
+void SQLiteChannelIDStore::SetForceKeepSessionState() {
+  backend_->SetForceKeepSessionState();
+}
+
+SQLiteChannelIDStore::~SQLiteChannelIDStore() {
+  backend_->Close();
+  // We release our reference to the Backend, though it will probably still have
+  // a reference if the background thread has not run Close() yet.
+}