Move download filename determintion into a separate class.

chrome/browser/download/download_target_determiner.cc

+// Copyright 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 "chrome/browser/download/download_target_determiner.h"
+
+#include "base/prefs/pref_service.h"
+#include "base/rand_util.h"
+#include "base/stringprintf.h"
+#include "base/time.h"
+#include "chrome/browser/download/chrome_download_manager_delegate.h"
+#include "chrome/browser/download/download_crx_util.h"
+#include "chrome/browser/download/download_extensions.h"
+#include "chrome/browser/download/download_prefs.h"
+#include "chrome/browser/download/download_util.h"
+#include "chrome/browser/extensions/webstore_installer.h"
+#include "chrome/browser/history/history_service.h"
+#include "chrome/browser/history/history_service_factory.h"
+#include "chrome/browser/profiles/profile.h"
+#include "chrome/common/extensions/extension.h"
+#include "chrome/common/extensions/feature_switch.h"
+#include "chrome/common/pref_names.h"
+#include "content/public/browser/browser_context.h"
+#include "content/public/browser/browser_thread.h"
+#include "grit/generated_resources.h"
+#include "net/base/net_util.h"
+#include "ui/base/l10n/l10n_util.h"
+
+#if defined(OS_CHROMEOS)
+#include "chrome/browser/chromeos/drive/drive_download_handler.h"
+#include "chrome/browser/chromeos/drive/drive_file_system_util.h"
+#endif
+
+using content::BrowserThread;
+using content::DownloadItem;
+
+namespace {
+
+// Condenses the results from HistoryService::GetVisibleVisitCountToHost() to a
+// single bool. A host is considered visited before if prior visible visits were
+// found in history and the first such visit was earlier than the most recent
+// midnight.
+void VisitCountsToVisitedBefore(
+    const base::Callback<void(bool)>& callback,
+    HistoryService::Handle unused_handle,
+    bool found_visits,
+    int count,
+    base::Time first_visit) {
+  callback.Run(
+      found_visits && count > 0 &&
+      (first_visit.LocalMidnight() < base::Time::Now().LocalMidnight()));
+}
+
+} // namespace
+
+DownloadTargetDeterminerDelegate::~DownloadTargetDeterminerDelegate() {
+}
+
+DownloadTargetDeterminer::DownloadTargetDeterminer(
+    DownloadItem* download,
+    DownloadPrefs* download_prefs,
+    const base::FilePath& last_selected_directory,
+    DownloadTargetDeterminerDelegate* delegate,
+    const content::DownloadTargetCallback& callback)
+    : next_state_(STATE_GENERATE_TARGET_PATH),
+      should_prompt_(false),
+      conflict_action_(download->GetForcedFilePath().empty() ?
+                       DownloadPathReservationTracker::UNIQUIFY :
+                       DownloadPathReservationTracker::OVERWRITE),
+      danger_type_(download->GetDangerType()),
+      download_(download),
+      download_prefs_(download_prefs),
+      delegate_(delegate),
+      last_selected_directory_(last_selected_directory),
+      completion_callback_(callback),
+      ALLOW_THIS_IN_INITIALIZER_LIST(weak_ptr_factory_(this)) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(download_);
+  DCHECK(delegate);
+  download_->AddObserver(this);
+
+  DoLoop();
+}
+
+DownloadTargetDeterminer::~DownloadTargetDeterminer() {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(download_);
+  DCHECK(completion_callback_.is_null());
+  download_->RemoveObserver(this);
+}
+
+void DownloadTargetDeterminer::DoLoop() {
+  Result result = CONTINUE;
+  do {
+    State current_state = next_state_;
+    next_state_ = STATE_NONE;
+
+    switch (current_state) {
+      case STATE_GENERATE_TARGET_PATH:
+        result = DoGenerateTargetPath();
+        break;
+      case STATE_NOTIFY_EXTENSIONS:
+        result = DoNotifyExtensions();
+        break;
+      case STATE_RESERVE_VIRTUAL_PATH:
+        result = DoReserveVirtualPath();
+        break;
+      case STATE_PROMPT_USER_FOR_DOWNLOAD_PATH:
+        result = DoPromptUserForDownloadPath();
+        break;
+      case STATE_DETERMINE_LOCAL_PATH:
+        result = DoDetermineLocalPath();
+        break;
+      case STATE_CHECK_DOWNLOAD_URL:
+        result = DoCheckDownloadUrl();
+        break;
+      case STATE_DETERMINE_INTERMEDIATE_PATH:
+        result = DoDetermineIntermediatePath();
+        break;
+      case STATE_CHECK_VISITED_REFERRER_BEFORE:
+        result = DoCheckVisitedReferrerBefore();
+        break;
+      case STATE_NONE:
+        NOTREACHED();
+        return;
+    }
+  } while (result == CONTINUE);
+  // Note that if a callback completes synchronously, the handler will still
+  // return QUIT_DOLOOP. In this case, an inner DoLoop() may complete the target
+  // determination and delete |this|.
+
+  if (result == COMPLETE)
+    ScheduleCallbackAndDeleteSelf();
+}
+
+DownloadTargetDeterminer::Result
+    DownloadTargetDeterminer::DoGenerateTargetPath() {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(virtual_path_.empty());
+  DCHECK(local_path_.empty());
+  bool is_forced_path = !download_->GetForcedFilePath().empty();
+
+  next_state_ = STATE_NOTIFY_EXTENSIONS;
+
+  // If we don't have a forced path, we should construct a path for the
+  // download. Forced paths are only specified for programmatic downloads
+  // (WebStore, Drag&Drop). Treat the path as a virtual path. We will eventually
+  // determine whether this is a local path and if not, figure out a local path.
+  if (!is_forced_path) {
+   std::string default_filename(
+        l10n_util::GetStringUTF8(IDS_DEFAULT_DOWNLOAD_FILENAME));
+    base::FilePath generated_filename = net::GenerateFileName(
+        download_->GetURL(),
+        download_->GetContentDisposition(),
+        GetProfile()->GetPrefs()->GetString(prefs::kDefaultCharset),
+        download_->GetSuggestedFilename(),
+        download_->GetMimeType(),
+        default_filename);
+    should_prompt_ = ShouldPromptForDownload(generated_filename);
+    base::FilePath target_directory;
+    if (should_prompt_ && !last_selected_directory_.empty()) {
+      DCHECK(!download_prefs_->IsDownloadPathManaged());
+      // If the user is going to be prompted and the user has been prompted
+      // before, then always prefer the last directory that the user selected.
+      target_directory = last_selected_directory_;
+    } else {
+      target_directory = download_prefs_->DownloadPath();
+    }
+    virtual_path_ = target_directory.Append(generated_filename);
+  } else {
+    DCHECK(!should_prompt_);
+    virtual_path_ = download_->GetForcedFilePath();
+  }
+  DCHECK(virtual_path_.IsAbsolute());
+  DVLOG(20) << "Generated virtual path: " << virtual_path_.AsUTF8Unsafe();
+
+  // If the download is DOA, don't bother going any further. This would be the
+  // case for a download that failed to initialize (e.g. the initial temporary
+  // file couldn't be created because both the downloads directory and the
+  // temporary directory are unwriteable).
+  if (!download_->IsInProgress())
+    return COMPLETE;
+  return CONTINUE;
+}
+
+DownloadTargetDeterminer::Result
+    DownloadTargetDeterminer::DoNotifyExtensions() {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!virtual_path_.empty());
+
+  next_state_ = STATE_RESERVE_VIRTUAL_PATH;
+
+  // If the target path is forced or if we don't have an extensions event
+  // router, then proceed with the original path.
+  if (!download_->GetForcedFilePath().empty())
+    return CONTINUE;
+
+  delegate_->NotifyExtensions(download_, virtual_path_,
+      base::Bind(&DownloadTargetDeterminer::NotifyExtensionsDone,
+                 weak_ptr_factory_.GetWeakPtr()));
+  return QUIT_DOLOOP;
+}
+
+void DownloadTargetDeterminer::NotifyExtensionsDone(
+    const base::FilePath& suggested_path,
+    DownloadPathReservationTracker::FilenameConflictAction conflict_action) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DVLOG(20) << "Extension suggested path: " << suggested_path.AsUTF8Unsafe();
+
+  if (!suggested_path.empty()) {
+    // If an extension overrides the filename, then the target directory will be
+    // forced to download_prefs_->DownloadPath() since extensions cannot place
+    // downloaded files anywhere except there. This prevents subdirectories from
+    // accumulating: if an extension is allowed to say that a file should go in
+    // last_download_path/music/foo.mp3, then last_download_path will accumulate
+    // the subdirectory /music/ so that the next download may end up in
+    // Downloads/music/music/music/bar.mp3.
+    base::FilePath new_path(download_prefs_->DownloadPath().Append(
+        suggested_path).NormalizePathSeparators());
+    // Do not pass a mime type to GenerateSafeFileName so that it does not force
+    // the filename to have an extension if the (Chrome) extension does not
+    // suggest it.
+    net::GenerateSafeFileName(std::string(), false, &new_path);
+    virtual_path_ = new_path;
+    conflict_action_ = conflict_action;
+  }
+
+  DoLoop();
+}
+
+DownloadTargetDeterminer::Result
+    DownloadTargetDeterminer::DoReserveVirtualPath() {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!virtual_path_.empty());
+
+  next_state_ = STATE_PROMPT_USER_FOR_DOWNLOAD_PATH;
+
+  delegate_->ReserveVirtualPath(
+      download_, virtual_path_, conflict_action_,
+      base::Bind(&DownloadTargetDeterminer::ReserveVirtualPathDone,
+                 weak_ptr_factory_.GetWeakPtr()));
+  return QUIT_DOLOOP;
+}
+
+void DownloadTargetDeterminer::ReserveVirtualPathDone(
+    const base::FilePath& path, bool verified) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DVLOG(20) << "Reserved path: " << path.AsUTF8Unsafe()
+            << " Verified:" << verified;
+  should_prompt_ = (should_prompt_ || !verified);
+  if (verified)
+    virtual_path_ = path;
+  DoLoop();
+}
+
+DownloadTargetDeterminer::Result
+    DownloadTargetDeterminer::DoPromptUserForDownloadPath() {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!virtual_path_.empty());
+
+  next_state_ = STATE_DETERMINE_LOCAL_PATH;
+
+  if (should_prompt_) {
+    delegate_->PromptUserForDownloadPath(
+        download_,
+        virtual_path_,
+        base::Bind(&DownloadTargetDeterminer::PromptUserForDownloadPathDone,
+                   weak_ptr_factory_.GetWeakPtr()));
+    return QUIT_DOLOOP;
+  }
+  return CONTINUE;
+}
+
+void DownloadTargetDeterminer::PromptUserForDownloadPathDone(
+    const base::FilePath& virtual_path) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DVLOG(20) << "User selected path:" << virtual_path.AsUTF8Unsafe();
+  if (virtual_path.empty()) {
+    CancelOnFailureAndDeleteSelf();
+    return;
+  }
+  virtual_path_ = virtual_path;
+  DoLoop();
+}
+
+DownloadTargetDeterminer::Result
+    DownloadTargetDeterminer::DoDetermineLocalPath() {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!virtual_path_.empty());
+  DCHECK(local_path_.empty());
+
+  next_state_ = STATE_CHECK_DOWNLOAD_URL;
+
+  delegate_->DetermineLocalPath(
+      download_,
+      virtual_path_,
+      base::Bind(&DownloadTargetDeterminer::DetermineLocalPathDone,
+                 weak_ptr_factory_.GetWeakPtr()));
+  return QUIT_DOLOOP;
+}
+
+void DownloadTargetDeterminer::DetermineLocalPathDone(
+    const base::FilePath& local_path) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DVLOG(20) << "Local path: " << local_path.AsUTF8Unsafe();
+  if (local_path.empty()) {
+    // Path subsitution failed.
+    CancelOnFailureAndDeleteSelf();
+    return;
+  }
+  local_path_ = local_path;
+  DoLoop();
+}
+
+DownloadTargetDeterminer::Result
+    DownloadTargetDeterminer::DoCheckDownloadUrl() {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!virtual_path_.empty());
+  next_state_ = STATE_CHECK_VISITED_REFERRER_BEFORE;
+  delegate_->CheckDownloadUrl(
+      download_,
+      virtual_path_,
+      base::Bind(&DownloadTargetDeterminer::CheckDownloadUrlDone,
+                 weak_ptr_factory_.GetWeakPtr()));
+  return QUIT_DOLOOP;
+}
+
+void DownloadTargetDeterminer::CheckDownloadUrlDone(
+    content::DownloadDangerType danger_type) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DVLOG(20) << "URL Check Result:" << danger_type;
+  danger_type_ = danger_type;
+  DoLoop();
+}
+
+DownloadTargetDeterminer::Result
+    DownloadTargetDeterminer::DoCheckVisitedReferrerBefore() {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+
+  next_state_ = STATE_DETERMINE_INTERMEDIATE_PATH;
+
+  // Checking if there are prior visits to the referrer is only necessary if the
+  // danger level of the download depends on the file type. This excludes cases
+  // where the download has already been deemed dangerous, or where the user is
+  // going to be prompted or where this is a programmatic download.
+  if (danger_type_ != content::DOWNLOAD_DANGER_TYPE_NOT_DANGEROUS ||
+      should_prompt_ ||
+      !download_->GetForcedFilePath().empty()) {
+    return CONTINUE;
+  }
+
+  // Assume that:
+  // IsDangerousFile(VISITED_REFERRER) => IsDangerousFile(NO_VISITS_...)
+  // I.e. having visited a referrer only lowers a file's danger level.
+  if (IsDangerousFile(NO_VISITS_TO_REFERRER)) {
+    // Only need to ping the history DB if the download would be considered safe
+    // if there are prior visits and is considered dangerous otherwise.
+    if (!IsDangerousFile(VISITED_REFERRER)) {
+      // HistoryServiceFactory redirects incognito profiles to on-record
+      // profiles.  There's no history for on-record profiles in unit_tests.
+      HistoryService* history_service = HistoryServiceFactory::GetForProfile(
+          GetProfile(), Profile::EXPLICIT_ACCESS);
+
+      if (history_service && download_->GetReferrerUrl().is_valid()) {
+        history_service->GetVisibleVisitCountToHost(
+            download_->GetReferrerUrl(), &history_consumer_,
+            base::Bind(&VisitCountsToVisitedBefore, base::Bind(
+                &DownloadTargetDeterminer::CheckVisitedReferrerBeforeDone,
+                weak_ptr_factory_.GetWeakPtr())));
+        return QUIT_DOLOOP;
+      }
+    }
+
+    // If the danger level doesn't depend on having visited the refererrer URL
+    // or if original profile doesn't have a HistoryService or the referrer url
+    // is invalid, then assume the referrer has not been visited before.
+    danger_type_ = content::DOWNLOAD_DANGER_TYPE_DANGEROUS_FILE;
+  }
+  return CONTINUE;
+}
+
+void DownloadTargetDeterminer::CheckVisitedReferrerBeforeDone(
+    bool visited_referrer_before) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  if (IsDangerousFile(
+          visited_referrer_before ? VISITED_REFERRER : NO_VISITS_TO_REFERRER))
+    danger_type_ = content::DOWNLOAD_DANGER_TYPE_DANGEROUS_FILE;
+  DoLoop();
+}
+
+DownloadTargetDeterminer::Result
+    DownloadTargetDeterminer::DoDetermineIntermediatePath() {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK(!virtual_path_.empty());
+  DCHECK(!local_path_.empty());
+  DCHECK(intermediate_path_.empty());
+
+  next_state_ = STATE_NONE;
+
+  // Note that the intermediate filename is always uniquified (i.e. if a file by
+  // the same name exists, it is never overwritten). Therefore the code below
+  // does not attempt to find a name that doesn't conflict with an existing
+  // file.
+
+  // If the actual target of the download is a virtual path, then the local path
+  // is considered to point to a temporary path. A separate intermediate path is
+  // unnecessary since the local path already serves that purpose.
+  if (virtual_path_.BaseName() != local_path_.BaseName()) {
+    intermediate_path_ = local_path_;
+    return COMPLETE;
+  }
+
+  // If the download has a forced path and is safe, then just use the
+  // target path. In practice the temporary download file that was created prior
+  // to download filename determination is already named
+  // download_->GetForcedFilePath().
+  if (danger_type_ == content::DOWNLOAD_DANGER_TYPE_NOT_DANGEROUS &&
+      !download_->GetForcedFilePath().empty()) {
+    DCHECK_EQ(download_->GetForcedFilePath().value(), local_path_.value());
+    intermediate_path_ = local_path_;
+    return COMPLETE;
+  }
+
+  // Other safe downloads get a .crdownload suffix for their intermediate name.
+  if (danger_type_ == content::DOWNLOAD_DANGER_TYPE_NOT_DANGEROUS) {
+    intermediate_path_ = download_util::GetCrDownloadPath(local_path_);
+    return COMPLETE;
+  }
+
+  // Dangerous downloads receive a random intermediate name that looks like:
+  // 'Unconfirmed <random>.crdownload'.
+  const base::FilePath::CharType kUnconfirmedFormatSuffix[] =
+      FILE_PATH_LITERAL(" %d.crdownload");
+  // Range of the <random> uniquifier.
+  const int kUnconfirmedUniquifierRange = 1000000;
+#if defined(OS_WIN)
+  string16 unconfirmed_format =
+      l10n_util::GetStringUTF16(IDS_DOWNLOAD_UNCONFIRMED_PREFIX);
+#else
+  std::string unconfirmed_format =
+      l10n_util::GetStringUTF8(IDS_DOWNLOAD_UNCONFIRMED_PREFIX);
+#endif
+  unconfirmed_format.append(kUnconfirmedFormatSuffix);
+
+  base::FilePath::StringType file_name = base::StringPrintf(
+      unconfirmed_format.c_str(),
+      base::RandInt(0, kUnconfirmedUniquifierRange));
+  intermediate_path_ = local_path_.DirName().Append(file_name);
+  return COMPLETE;
+}
+
+void DownloadTargetDeterminer::ScheduleCallbackAndDeleteSelf() {
+  DCHECK(download_);
+  DVLOG(20) << "Scheduling callback. Virtual:" << virtual_path_.AsUTF8Unsafe()
+            << " Local:" << local_path_.AsUTF8Unsafe()
+            << " Intermediate:" << intermediate_path_.AsUTF8Unsafe()
+            << " Should prompt:" << should_prompt_
+            << " Danger type:" << danger_type_;
+  MessageLoop::current()->PostTask(
+      FROM_HERE,
+      base::Bind(completion_callback_,
+                 local_path_,
+                 (should_prompt_ ? DownloadItem::TARGET_DISPOSITION_PROMPT :
+                                   DownloadItem::TARGET_DISPOSITION_OVERWRITE),
+                 danger_type_,
+                 intermediate_path_));
+  completion_callback_.Reset();
+  delete this;
+}
+
+void DownloadTargetDeterminer::CancelOnFailureAndDeleteSelf() {
+  // Path substitution failed.
+  virtual_path_.clear();
+  local_path_.clear();
+  intermediate_path_.clear();
+  ScheduleCallbackAndDeleteSelf();
+}
+
+Profile* DownloadTargetDeterminer::GetProfile() {
+  DCHECK(download_->GetBrowserContext());
+  return Profile::FromBrowserContext(download_->GetBrowserContext());
+}
+
+bool DownloadTargetDeterminer::ShouldPromptForDownload(
+    const base::FilePath& filename) {
+  // If the download path is forced, don't prompt.
+  if (!download_->GetForcedFilePath().empty()) {
+    // 'Save As' downloads shouldn't have a forced path.
+    DCHECK_NE(DownloadItem::TARGET_DISPOSITION_PROMPT,
+              download_->GetTargetDisposition());
+    return false;
+  }
+
+  // Don't ask where to save if the download path is managed. Even if the user
+  // wanted to be prompted for "all" downloads, or if this was a 'Save As'
+  // download.
+  if (download_prefs_->IsDownloadPathManaged())
+    return false;
+
+  // Prompt if this is a 'Save As' download.
+  if (download_->GetTargetDisposition() ==
+      DownloadItem::TARGET_DISPOSITION_PROMPT)
+    return true;
+
+  // Check if the user has the "Always prompt for download location" preference
+  // set. If so we prompt for most downloads except for the following scenarios:
+  // 1) Extension installation. Note that we only care here about the case where
+  //    an extension is installed, not when one is downloaded with "save as...".
+  // 2) Filetypes marked "always open." If the user just wants this file opened,
+  //    don't bother asking where to keep it.
+  if (download_prefs_->PromptForDownload() &&
+      !download_crx_util::IsExtensionDownload(*download_) &&
+      !extensions::Extension::IsExtension(filename) &&
+      !download_prefs_->IsAutoOpenEnabledBasedOnExtension(filename))
+    return true;
+
+  // Otherwise, don't prompt. Note that the user might still be prompted if
+  // there are unresolved conflicts during path reservation (e.g. due to the
+  // target path being unwriteable or because there are too many conflicting
+  // files), or if an extension signals that the user be prompted on a filename
+  // conflict.
+  return false;
+}
+
+bool DownloadTargetDeterminer::IsDangerousFile(PriorVisitsToReferrer visits) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  const bool is_extension_download =
+      download_crx_util::IsExtensionDownload(*download_);
+
+  // User-initiated extension downloads from pref-whitelisted sources are not
+  // considered dangerous.
+  if (download_->HasUserGesture() &&
+      is_extension_download &&
+      download_crx_util::OffStoreInstallAllowedByPrefs(
+          GetProfile(), *download_)) {
+    return false;
+  }
+
+  // Extensions that are not from the gallery are considered dangerous.
+  // When off-store install is disabled we skip this, since in this case, we
+  // will not offer to install the extension.
+  if (extensions::FeatureSwitch::easy_off_store_install()->IsEnabled() &&
+      is_extension_download &&
+      !extensions::WebstoreInstaller::GetAssociatedApproval(*download_)) {
+    return true;
+  }
+
+  // Anything the user has marked auto-open is OK if it's user-initiated.
+  if (download_prefs_->IsAutoOpenEnabledBasedOnExtension(virtual_path_) &&
+      download_->HasUserGesture())
+    return false;
+
+  switch (download_util::GetFileDangerLevel(virtual_path_.BaseName())) {
+    case download_util::NotDangerous:
+      return false;
+
+    case download_util::AllowOnUserGesture:
+      // "Allow on user gesture" is OK when we have a user gesture and the
+      // hosting page has been visited before today.
+      if (download_->GetTransitionType() &
+          content::PAGE_TRANSITION_FROM_ADDRESS_BAR) {
+        return false;
+      }
+      return !download_->HasUserGesture() || visits == NO_VISITS_TO_REFERRER;
+
+    case download_util::Dangerous:
+      return true;
+  }
+  NOTREACHED();
+  return false;
+}
+
+void DownloadTargetDeterminer::OnDownloadDestroyed(
+    DownloadItem* download) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  DCHECK_EQ(download_, download);
+  CancelOnFailureAndDeleteSelf();
+}
+
+// static
+void DownloadTargetDeterminer::Start(
+    content::DownloadItem* download,
+    DownloadPrefs* download_prefs,
+    const base::FilePath& last_selected_directory,
+    DownloadTargetDeterminerDelegate* delegate,
+    const content::DownloadTargetCallback& callback) {
+  // DownloadTargetDeterminer owns itself and will self destruct when the job is
+  // complete or the download item is destroyed. The callback is always invoked
+  // asynchronously.
+  new DownloadTargetDeterminer(download, download_prefs,
+                               last_selected_directory, delegate, callback);
+}