Rename ServerBoundCert => ChannelID to reflect the current name

chrome/browser/net/sqlite_server_bound_cert_store.cc

Index: chrome/browser/net/sqlite_server_bound_cert_store.cc
diff --git a/chrome/browser/net/sqlite_server_bound_cert_store.cc b/chrome/browser/net/sqlite_server_bound_cert_store.cc
deleted file mode 100644
index fbac4c36d72405ba4183f7446f913c31cc91ff69..0000000000000000000000000000000000000000
--- a/chrome/browser/net/sqlite_server_bound_cert_store.cc
+++ /dev/null
@@ -1,639 +0,0 @@
-// Copyright (c) 2012 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_server_bound_cert_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 SQLiteServerBoundCertStore::Backend
-    : public base::RefCountedThreadSafe<SQLiteServerBoundCertStore::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 server bound cert addition.
-  void AddServerBoundCert(
-      const net::DefaultServerBoundCertStore::ServerBoundCert& cert);
-
-  // Batch a server bound cert deletion.
-  void DeleteServerBoundCert(
-      const net::DefaultServerBoundCertStore::ServerBoundCert& cert);
-
-  // 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::DefaultServerBoundCertStore::ServerBoundCert>* certs);
-
-  friend class base::RefCountedThreadSafe<SQLiteServerBoundCertStore::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 {
-      CERT_ADD,
-      CERT_DELETE
-    } OperationType;
-
-    PendingOperation(
-        OperationType op,
-        const net::DefaultServerBoundCertStore::ServerBoundCert& cert)
-        : op_(op), cert_(cert) {}
-
-    OperationType op() const { return op_; }
-    const net::DefaultServerBoundCertStore::ServerBoundCert& cert() const {
-        return cert_;
-    }
-
-   private:
-    OperationType op_;
-    net::DefaultServerBoundCertStore::ServerBoundCert cert_;
-  };
-
- private:
-  // Batch a server bound cert operation (add or delete).
-  void BatchOperation(
-      PendingOperation::OperationType op,
-      const net::DefaultServerBoundCertStore::ServerBoundCert& cert);
-  // 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 certificates stored for.
-  std::set<std::string> cert_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 SQLiteServerBoundCertStore.  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 SQLiteServerBoundCertStore::Backend::Load(
-    const LoadedCallback& loaded_callback) {
-  // This function should be called only once per instance.
-  DCHECK(!db_.get());
-  scoped_ptr<ScopedVector<net::DefaultServerBoundCertStore::ServerBoundCert> >
-      certs(new ScopedVector<net::DefaultServerBoundCertStore::ServerBoundCert>(
-          ));
-  ScopedVector<net::DefaultServerBoundCertStore::ServerBoundCert>* certs_ptr =
-      certs.get();
-
-  background_task_runner_->PostTaskAndReply(
-      FROM_HERE,
-      base::Bind(&Backend::LoadOnDBThread, this, certs_ptr),
-      base::Bind(loaded_callback, base::Passed(&certs)));
-}
-
-void SQLiteServerBoundCertStore::Backend::LoadOnDBThread(
-    ScopedVector<net::DefaultServerBoundCertStore::ServerBoundCert>* certs) {
-  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(&SQLiteServerBoundCertStore::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::DefaultServerBoundCertStore::ServerBoundCert> cert(
-        new net::DefaultServerBoundCertStore::ServerBoundCert(
-            smt.ColumnString(0),  // origin
-            base::Time::FromInternalValue(smt.ColumnInt64(5)),
-            base::Time::FromInternalValue(smt.ColumnInt64(4)),
-            private_key_from_db,
-            cert_from_db));
-    cert_origins_.insert(cert->server_identifier());
-    certs->push_back(cert.release());
-  }
-
-  UMA_HISTOGRAM_COUNTS_10000("DomainBoundCerts.DBLoadedCount", certs->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 " << certs->size() << " in " << load_time.InMilliseconds()
-           << " ms";
-}
-
-bool SQLiteServerBoundCertStore::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 SQLiteServerBoundCertStore::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 SQLiteServerBoundCertStore::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 SQLiteServerBoundCertStore::Backend::AddServerBoundCert(
-    const net::DefaultServerBoundCertStore::ServerBoundCert& cert) {
-  BatchOperation(PendingOperation::CERT_ADD, cert);
-}
-
-void SQLiteServerBoundCertStore::Backend::DeleteServerBoundCert(
-    const net::DefaultServerBoundCertStore::ServerBoundCert& cert) {
-  BatchOperation(PendingOperation::CERT_DELETE, cert);
-}
-
-void SQLiteServerBoundCertStore::Backend::BatchOperation(
-    PendingOperation::OperationType op,
-    const net::DefaultServerBoundCertStore::ServerBoundCert& cert) {
-  // 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, cert));
-
-  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 SQLiteServerBoundCertStore::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::CERT_ADD: {
-        cert_origins_.insert(po->cert().server_identifier());
-        add_smt.Reset(true);
-        add_smt.BindString(0, po->cert().server_identifier());
-        const std::string& private_key = po->cert().private_key();
-        add_smt.BindBlob(1, private_key.data(), private_key.size());
-        const std::string& cert = po->cert().cert();
-        add_smt.BindBlob(2, cert.data(), cert.size());
-        add_smt.BindInt(3, net::CLIENT_CERT_ECDSA_SIGN);
-        add_smt.BindInt64(4, po->cert().expiration_time().ToInternalValue());
-        add_smt.BindInt64(5, po->cert().creation_time().ToInternalValue());
-        if (!add_smt.Run())
-          NOTREACHED() << "Could not add a server bound cert to the DB.";
-        break;
-      }
-      case PendingOperation::CERT_DELETE:
-        cert_origins_.erase(po->cert().server_identifier());
-        del_smt.Reset(true);
-        del_smt.BindString(0, po->cert().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 SQLiteServerBoundCertStore::Backend::Close() {
-  // Must close the backend on the background thread.
-  background_task_runner_->PostTask(
-      FROM_HERE, base::Bind(&Backend::InternalBackgroundClose, this));
-}
-
-void SQLiteServerBoundCertStore::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 SQLiteServerBoundCertStore::Backend::DeleteCertificatesOnShutdown() {
-  DCHECK(background_task_runner_->RunsTasksOnCurrentThread());
-
-  if (!db_.get())
-    return;
-
-  if (cert_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 = cert_origins_.begin();
-       it != cert_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 SQLiteServerBoundCertStore::Backend::SetForceKeepSessionState() {
-  base::AutoLock locked(lock_);
-  force_keep_session_state_ = true;
-}
-
-SQLiteServerBoundCertStore::SQLiteServerBoundCertStore(
-    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 SQLiteServerBoundCertStore::Load(
-    const LoadedCallback& loaded_callback) {
-  backend_->Load(loaded_callback);
-}
-
-void SQLiteServerBoundCertStore::AddServerBoundCert(
-    const net::DefaultServerBoundCertStore::ServerBoundCert& cert) {
-  backend_->AddServerBoundCert(cert);
-}
-
-void SQLiteServerBoundCertStore::DeleteServerBoundCert(
-    const net::DefaultServerBoundCertStore::ServerBoundCert& cert) {
-  backend_->DeleteServerBoundCert(cert);
-}
-
-void SQLiteServerBoundCertStore::SetForceKeepSessionState() {
-  backend_->SetForceKeepSessionState();
-}
-
-SQLiteServerBoundCertStore::~SQLiteServerBoundCertStore() {
-  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.
-}