root/chrome/browser/history/top_sites_database.cc

DEFINITIONS

1. InitTables
2. GetRedirects
3. SetRedirects
4. RecordRecoveryEvent
5. FixThumbnailsTable
6. RecoverDatabaseOrRaze
7. DatabaseErrorCallback
8. Init
9. InitImpl
10. UpgradeToVersion3
11. GetPageThumbnails
12. SetPageThumbnail
13. UpdatePageThumbnail
14. AddPageThumbnail
15. UpdatePageRank
16. UpdatePageRankNoTransaction
17. GetPageThumbnail
18. GetURLRank
19. RemoveURL
20. CreateDB

// 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 "base/file_util.h"
#include "base/memory/ref_counted.h"
#include "base/metrics/histogram.h"
#include "base/strings/string_split.h"
#include "base/strings/string_util.h"
#include "chrome/browser/history/history_types.h"
#include "chrome/browser/history/top_sites.h"
#include "chrome/browser/history/top_sites_database.h"
#include "chrome/common/thumbnail_score.h"
#include "sql/connection.h"
#include "sql/recovery.h"
#include "sql/statement.h"
#include "sql/transaction.h"
#include "third_party/sqlite/sqlite3.h"

// Description of database table:
//
// thumbnails
//   url              URL of the sites for which we have a thumbnail.
//   url_rank         Index of the URL in that thumbnail, 0-based. The thumbnail
//                    with the highest rank will be the next one evicted. Forced
//                    thumbnails have a rank of -1.
//   title            The title to display under that thumbnail.
//   redirects        A space separated list of URLs that are known to redirect
//                    to this url.
//   boring_score     How "boring" that thumbnail is. See ThumbnailScore.
//   good_clipping    True if the thumbnail was clipped from the bottom, keeping
//                    the entire width of the window. See ThumbnailScore.
//   at_top           True if the thumbnail was captured at the top of the
//                    website.
//   last_updated     The time at which this thumbnail was last updated.
//   load_completed   True if the thumbnail was captured after the page load was
//                    completed.
//   last_forced      If this is a forced thumbnail, records the last time it
//                    was forced. If it's not a forced thumbnail, 0.

namespace {

// For this database, schema migrations are deprecated after two
// years.  This means that the oldest non-deprecated version should be
// two years old or greater (thus the migrations to get there are
// older).  Databases containing deprecated versions will be cleared
// at startup.  Since this database is a cache, losing old data is not
// fatal (in fact, very old data may be expired immediately at startup
// anyhow).

// Version 3: b6d6a783/r231648 by beaudoin@chromium.org on 2013-10-29
// Version 2: eb0b24e6/r87284 by satorux@chromium.org on 2011-05-31
// Version 1: 809cc4d8/r64072 by sky@chromium.org on 2010-10-27 (deprecated)

// NOTE(shess): When changing the version, add a new golden file for
// the new version and a test to verify that Init() works with it.
// NOTE(shess): RecoverDatabaseOrRaze() depends on the specific
// version number.  The code is subtle and in development, contact me
// if the necessary changes are not obvious.
static const int kVersionNumber = 3;
static const int kDeprecatedVersionNumber = 1;  // and earlier.

bool InitTables(sql::Connection* db) {
  const char kThumbnailsSql[] =
      "CREATE TABLE IF NOT EXISTS thumbnails ("
      "url LONGVARCHAR PRIMARY KEY,"
      "url_rank INTEGER,"
      "title LONGVARCHAR,"
      "thumbnail BLOB,"
      "redirects LONGVARCHAR,"
      "boring_score DOUBLE DEFAULT 1.0,"
      "good_clipping INTEGER DEFAULT 0,"
      "at_top INTEGER DEFAULT 0,"
      "last_updated INTEGER DEFAULT 0,"
      "load_completed INTEGER DEFAULT 0,"
      "last_forced INTEGER DEFAULT 0)";
  return db->Execute(kThumbnailsSql);
}

// Encodes redirects into a string.
std::string GetRedirects(const history::MostVisitedURL& url) {
  std::vector<std::string> redirects;
  for (size_t i = 0; i < url.redirects.size(); i++)
    redirects.push_back(url.redirects[i].spec());
  return JoinString(redirects, ' ');
}

// Decodes redirects from a string and sets them for the url.
void SetRedirects(const std::string& redirects, history::MostVisitedURL* url) {
  std::vector<std::string> redirects_vector;
  base::SplitStringAlongWhitespace(redirects, &redirects_vector);
  for (size_t i = 0; i < redirects_vector.size(); ++i)
    url->redirects.push_back(GURL(redirects_vector[i]));
}

// Track various failure (and success) cases in recovery code.
//
// TODO(shess): The recovery code is complete, but by nature runs in challenging
// circumstances, so initially the default error response is to leave the
// existing database in place.  This histogram is intended to expose the
// failures seen in the fleet.  Frequent failure cases can be explored more
// deeply to see if the complexity to fix them is warranted.  Infrequent failure
// cases can be resolved by marking the database unrecoverable (which will
// delete the data).
//
// Based on the thumbnail_database.cc recovery code, FAILED_SCOPER should
// dominate, followed distantly by FAILED_META, with few or no other failures.
enum RecoveryEventType {
  // Database successfully recovered.
  RECOVERY_EVENT_RECOVERED = 0,

  // Database successfully deprecated.
  RECOVERY_EVENT_DEPRECATED,

  // Sqlite.RecoveryEvent can usually be used to get more detail about the
  // specific failure (see sql/recovery.cc).
  RECOVERY_EVENT_FAILED_SCOPER,
  RECOVERY_EVENT_FAILED_META_VERSION,
  RECOVERY_EVENT_FAILED_META_WRONG_VERSION,
  RECOVERY_EVENT_FAILED_META_INIT,
  RECOVERY_EVENT_FAILED_SCHEMA_INIT,
  RECOVERY_EVENT_FAILED_AUTORECOVER_THUMBNAILS,
  RECOVERY_EVENT_FAILED_COMMIT,

  // Track invariants resolved by FixThumbnailsTable().
  RECOVERY_EVENT_INVARIANT_RANK,
  RECOVERY_EVENT_INVARIANT_REDIRECT,
  RECOVERY_EVENT_INVARIANT_CONTIGUOUS,

  // Always keep this at the end.
  RECOVERY_EVENT_MAX,
};

void RecordRecoveryEvent(RecoveryEventType recovery_event) {
  UMA_HISTOGRAM_ENUMERATION("History.TopSitesRecovery",
                            recovery_event, RECOVERY_EVENT_MAX);
}

// Most corruption comes down to atomic updates between pages being broken
// somehow.  This can result in either missing data, or overlapping data,
// depending on the operation broken.  This table has large rows, which will use
// overflow pages, so it is possible (though unlikely) that a chain could fit
// together and yield a row with errors.
void FixThumbnailsTable(sql::Connection* db) {
  // Enforce invariant separating forced and non-forced thumbnails.
  const char kFixRankSql[] =
      "DELETE FROM thumbnails "
      "WHERE (url_rank = -1 AND last_forced = 0) "
      "OR (url_rank <> -1 AND last_forced <> 0)";
  ignore_result(db->Execute(kFixRankSql));
  if (db->GetLastChangeCount() > 0)
    RecordRecoveryEvent(RECOVERY_EVENT_INVARIANT_RANK);

  // Enforce invariant that url is in its own redirects.
  const char kFixRedirectsSql[] =
      "DELETE FROM thumbnails "
      "WHERE url <> substr(redirects, -length(url), length(url))";
  ignore_result(db->Execute(kFixRedirectsSql));
  if (db->GetLastChangeCount() > 0)
    RecordRecoveryEvent(RECOVERY_EVENT_INVARIANT_REDIRECT);

  // Enforce invariant that url_rank>=0 forms a contiguous series.
  // TODO(shess): I have not found an UPDATE+SUBSELECT method of managing this.
  // It can be done with a temporary table and a subselect, but doing it
  // manually is easier to follow.  Another option would be to somehow integrate
  // the renumbering into the table recovery code.
  const char kByRankSql[] =
      "SELECT url_rank, rowid FROM thumbnails WHERE url_rank <> -1 "
      "ORDER BY url_rank";
  sql::Statement select_statement(db->GetUniqueStatement(kByRankSql));

  const char kAdjustRankSql[] =
      "UPDATE thumbnails SET url_rank = ? WHERE rowid = ?";
  sql::Statement update_statement(db->GetUniqueStatement(kAdjustRankSql));

  // Update any rows where |next_rank| doesn't match |url_rank|.
  int next_rank = 0;
  bool adjusted = false;
  while (select_statement.Step()) {
    const int url_rank = select_statement.ColumnInt(0);
    if (url_rank != next_rank) {
      adjusted = true;
      update_statement.Reset(true);
      update_statement.BindInt(0, next_rank);
      update_statement.BindInt64(1, select_statement.ColumnInt64(1));
      update_statement.Run();
    }
    ++next_rank;
  }
  if (adjusted)
    RecordRecoveryEvent(RECOVERY_EVENT_INVARIANT_CONTIGUOUS);
}

// Recover the database to the extent possible, razing it if recovery is not
// possible.
void RecoverDatabaseOrRaze(sql::Connection* db, const base::FilePath& db_path) {
  // NOTE(shess): If the version changes, review this code.
  DCHECK_EQ(3, kVersionNumber);

  // It is almost certain that some operation against |db| will fail, prevent
  // reentry.
  db->reset_error_callback();

  // For generating histogram stats.
  size_t thumbnails_recovered = 0;
  int64 original_size = 0;
  base::GetFileSize(db_path, &original_size);

  scoped_ptr<sql::Recovery> recovery = sql::Recovery::Begin(db, db_path);
  if (!recovery) {
    RecordRecoveryEvent(RECOVERY_EVENT_FAILED_SCOPER);
    return;
  }

  // Setup the meta recovery table and fetch the version number from the corrupt
  // database.
  int version = 0;
  if (!recovery->SetupMeta() || !recovery->GetMetaVersionNumber(&version)) {
    // TODO(shess): Prior histograms indicate all failures are in creating the
    // recover virtual table for corrupt.meta.  The table may not exist, or the
    // database may be too far gone.  Either way, unclear how to resolve.
    sql::Recovery::Rollback(recovery.Pass());
    RecordRecoveryEvent(RECOVERY_EVENT_FAILED_META_VERSION);
    return;
  }

  // This code runs in a context which may be able to read version information
  // that the regular deprecation path cannot.  The effect of this code will be
  // to raze the database.
  if (version <= kDeprecatedVersionNumber) {
    sql::Recovery::Unrecoverable(recovery.Pass());
    RecordRecoveryEvent(RECOVERY_EVENT_DEPRECATED);
    return;
  }

  // TODO(shess): Earlier versions have been deprecated, later versions should
  // be impossible.  Unrecoverable() seems like a feasible response if this is
  // infrequent enough.
  if (version != 2 && version != 3) {
    RecordRecoveryEvent(RECOVERY_EVENT_FAILED_META_WRONG_VERSION);
    sql::Recovery::Rollback(recovery.Pass());
    return;
  }

  // Both v2 and v3 recover to current schema version.
  sql::MetaTable recover_meta_table;
  if (!recover_meta_table.Init(recovery->db(), kVersionNumber,
                               kVersionNumber)) {
    sql::Recovery::Rollback(recovery.Pass());
    RecordRecoveryEvent(RECOVERY_EVENT_FAILED_META_INIT);
    return;
  }

  // Create a fresh version of the schema.  The recovery code uses
  // conflict-resolution to handle duplicates, so any indices are necessary.
  if (!InitTables(recovery->db())) {
    // TODO(shess): Unable to create the new schema in the new database.  The
    // new database should be a temporary file, so being unable to work with it
    // is pretty unclear.
    //
    // What are the potential responses, even?  The recovery database could be
    // opened as in-memory.  If the temp database had a filesystem problem and
    // the temp filesystem differs from the main database, then that could fix
    // it.
    sql::Recovery::Rollback(recovery.Pass());
    RecordRecoveryEvent(RECOVERY_EVENT_FAILED_SCHEMA_INIT);
    return;
  }

  // The |1| is because v2 [thumbnails] has one less column than v3 did.  In the
  // v2 case the column will get default values.
  if (!recovery->AutoRecoverTable("thumbnails", 1, &thumbnails_recovered)) {
    sql::Recovery::Rollback(recovery.Pass());
    RecordRecoveryEvent(RECOVERY_EVENT_FAILED_AUTORECOVER_THUMBNAILS);
    return;
  }

  // TODO(shess): Inline this?
  FixThumbnailsTable(recovery->db());

  if (!sql::Recovery::Recovered(recovery.Pass())) {
    // TODO(shess): Very unclear what this failure would actually mean, and what
    // should be done.  Add histograms to Recovered() implementation to get some
    // insight.
    RecordRecoveryEvent(RECOVERY_EVENT_FAILED_COMMIT);
    return;
  }

  // Track the size of the recovered database relative to the size of the input
  // database.  The size should almost always be smaller, unless the input
  // database was empty to start with.  If the percentage results are very low,
  // something is awry.
  int64 final_size = 0;
  if (original_size > 0 &&
      base::GetFileSize(db_path, &final_size) &&
      final_size > 0) {
    UMA_HISTOGRAM_PERCENTAGE("History.TopSitesRecoveredPercentage",
                             final_size * 100 / original_size);
  }

  // Using 10,000 because these cases mostly care about "none recovered" and
  // "lots recovered".  More than 10,000 rows recovered probably means there's
  // something wrong with the profile.
  UMA_HISTOGRAM_COUNTS_10000("History.TopSitesRecoveredRowsThumbnails",
                             thumbnails_recovered);

  RecordRecoveryEvent(RECOVERY_EVENT_RECOVERED);
}

void DatabaseErrorCallback(sql::Connection* db,
                           const base::FilePath& db_path,
                           int extended_error,
                           sql::Statement* stmt) {
  // TODO(shess): Assert that this is running on a safe thread.  AFAICT, should
  // be the history thread, but at this level I can't see how to reach that.

  // Attempt to recover corrupt databases.
  int error = (extended_error & 0xFF);
  if (error == SQLITE_CORRUPT ||
      error == SQLITE_CANTOPEN ||
      error == SQLITE_NOTADB) {
    RecoverDatabaseOrRaze(db, db_path);
  }

  // TODO(shess): This database's error histograms look like:
  // 84% SQLITE_CORRUPT, SQLITE_CANTOPEN, SQLITE_NOTADB
  //  7% SQLITE_ERROR
  //  6% SQLITE_IOERR variants
  //  2% SQLITE_READONLY
  // .4% SQLITE_FULL
  // nominal SQLITE_TOBIG, SQLITE_AUTH, and SQLITE_BUSY.  In the case of
  // thumbnail_database.cc, as soon as the recovery code landed, SQLITE_IOERR
  // shot to leadership.  If the I/O error is system-level, there is probably no
  // hope, but if it is restricted to something about the database file, it is
  // possible that the recovery code could be brought to bear.  In fact, it is
  // possible that running recovery would be a reasonable default when errors
  // are seen.

  // The default handling is to assert on debug and to ignore on release.
  if (!sql::Connection::ShouldIgnoreSqliteError(extended_error))
    DLOG(FATAL) << db->GetErrorMessage();
}

}  // namespace

namespace history {

// static
const int TopSitesDatabase::kRankOfForcedURL = -1;

// static
const int TopSitesDatabase::kRankOfNonExistingURL = -2;

TopSitesDatabase::TopSitesDatabase() {
}

TopSitesDatabase::~TopSitesDatabase() {
}

bool TopSitesDatabase::Init(const base::FilePath& db_name) {
  // Retry failed InitImpl() in case the recovery system fixed things.
  // TODO(shess): Instrument to figure out if there are any persistent failure
  // cases which do not resolve themselves.
  const size_t kAttempts = 2;

  for (size_t i = 0; i < kAttempts; ++i) {
    if (InitImpl(db_name))
      return true;

    meta_table_.Reset();
    db_.reset();
  }
  return false;
}

bool TopSitesDatabase::InitImpl(const base::FilePath& db_name) {
  const bool file_existed = base::PathExists(db_name);

  db_.reset(CreateDB(db_name));
  if (!db_)
    return false;

  // An older version had data with no meta table.  Deprecate by razing.
  // TODO(shess): Just have RazeIfDeprecated() handle this case.
  const bool does_meta_exist = sql::MetaTable::DoesTableExist(db_.get());
  if (!does_meta_exist && file_existed) {
    if (!db_->Raze())
      return false;
  }

  // Clear databases which are too old to process.
  DCHECK_LT(kDeprecatedVersionNumber, kVersionNumber);
  sql::MetaTable::RazeIfDeprecated(db_.get(), kDeprecatedVersionNumber);

  // Scope initialization in a transaction so we can't be partially
  // initialized.
  sql::Transaction transaction(db_.get());
  // TODO(shess): Failure to open transaction is bad, address it.
  if (!transaction.Begin())
    return false;

  if (!meta_table_.Init(db_.get(), kVersionNumber, kVersionNumber))
    return false;

  if (!InitTables(db_.get()))
    return false;

  if (meta_table_.GetVersionNumber() == 2) {
    if (!UpgradeToVersion3()) {
      LOG(WARNING) << "Unable to upgrade top sites database to version 3.";
      return false;
    }
  }

  // Version check.
  if (meta_table_.GetVersionNumber() != kVersionNumber)
    return false;

  // Initialization is complete.
  if (!transaction.Commit())
    return false;

  return true;
}

bool TopSitesDatabase::UpgradeToVersion3() {
  // Add 'last_forced' column.
  if (!db_->Execute(
          "ALTER TABLE thumbnails ADD last_forced INTEGER DEFAULT 0")) {
    NOTREACHED();
    return false;
  }
  meta_table_.SetVersionNumber(3);
  return true;
}

void TopSitesDatabase::GetPageThumbnails(MostVisitedURLList* urls,
                                         URLToImagesMap* thumbnails) {
  sql::Statement statement(db_->GetCachedStatement(
      SQL_FROM_HERE,
      "SELECT url, url_rank, title, thumbnail, redirects, "
      "boring_score, good_clipping, at_top, last_updated, load_completed, "
      "last_forced FROM thumbnails ORDER BY url_rank, last_forced"));

  if (!statement.is_valid()) {
    LOG(WARNING) << db_->GetErrorMessage();
    return;
  }

  urls->clear();
  thumbnails->clear();

  while (statement.Step()) {
    // Results are sorted by url_rank. For forced thumbnails with url_rank = -1,
    // thumbnails are sorted by last_forced.
    MostVisitedURL url;
    GURL gurl(statement.ColumnString(0));
    url.url = gurl;
    url.title = statement.ColumnString16(2);
    url.last_forced_time =
        base::Time::FromInternalValue(statement.ColumnInt64(10));
    std::string redirects = statement.ColumnString(4);
    SetRedirects(redirects, &url);
    urls->push_back(url);

    std::vector<unsigned char> data;
    statement.ColumnBlobAsVector(3, &data);
    Images thumbnail;
    if (!data.empty())
      thumbnail.thumbnail = base::RefCountedBytes::TakeVector(&data);
    thumbnail.thumbnail_score.boring_score = statement.ColumnDouble(5);
    thumbnail.thumbnail_score.good_clipping = statement.ColumnBool(6);
    thumbnail.thumbnail_score.at_top = statement.ColumnBool(7);
    thumbnail.thumbnail_score.time_at_snapshot =
        base::Time::FromInternalValue(statement.ColumnInt64(8));
    thumbnail.thumbnail_score.load_completed = statement.ColumnBool(9);
    (*thumbnails)[gurl] = thumbnail;
  }
}

void TopSitesDatabase::SetPageThumbnail(const MostVisitedURL& url,
                                        int new_rank,
                                        const Images& thumbnail) {
  sql::Transaction transaction(db_.get());
  transaction.Begin();

  int rank = GetURLRank(url);
  if (rank == kRankOfNonExistingURL) {
    AddPageThumbnail(url, new_rank, thumbnail);
  } else {
    UpdatePageRankNoTransaction(url, new_rank);
    UpdatePageThumbnail(url, thumbnail);
  }

  transaction.Commit();
}

bool TopSitesDatabase::UpdatePageThumbnail(
    const MostVisitedURL& url, const Images& thumbnail) {
  sql::Statement statement(db_->GetCachedStatement(
      SQL_FROM_HERE,
      "UPDATE thumbnails SET "
      "title = ?, thumbnail = ?, redirects = ?, "
      "boring_score = ?, good_clipping = ?, at_top = ?, last_updated = ?, "
      "load_completed = ?, last_forced = ?"
      "WHERE url = ? "));
  statement.BindString16(0, url.title);
  if (thumbnail.thumbnail.get() && thumbnail.thumbnail->front()) {
    statement.BindBlob(1, thumbnail.thumbnail->front(),
                       static_cast<int>(thumbnail.thumbnail->size()));
  }
  statement.BindString(2, GetRedirects(url));
  const ThumbnailScore& score = thumbnail.thumbnail_score;
  statement.BindDouble(3, score.boring_score);
  statement.BindBool(4, score.good_clipping);
  statement.BindBool(5, score.at_top);
  statement.BindInt64(6, score.time_at_snapshot.ToInternalValue());
  statement.BindBool(7, score.load_completed);
  statement.BindInt64(8, url.last_forced_time.ToInternalValue());
  statement.BindString(9, url.url.spec());

  return statement.Run();
}

void TopSitesDatabase::AddPageThumbnail(const MostVisitedURL& url,
                                        int new_rank,
                                        const Images& thumbnail) {
  sql::Statement statement(db_->GetCachedStatement(
      SQL_FROM_HERE,
      "INSERT OR REPLACE INTO thumbnails "
      "(url, url_rank, title, thumbnail, redirects, "
      "boring_score, good_clipping, at_top, last_updated, load_completed, "
      "last_forced) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)"));
  statement.BindString(0, url.url.spec());
  statement.BindInt(1, kRankOfForcedURL);  // Fist make it a forced thumbnail.
  statement.BindString16(2, url.title);
  if (thumbnail.thumbnail.get() && thumbnail.thumbnail->front()) {
    statement.BindBlob(3, thumbnail.thumbnail->front(),
                       static_cast<int>(thumbnail.thumbnail->size()));
  }
  statement.BindString(4, GetRedirects(url));
  const ThumbnailScore& score = thumbnail.thumbnail_score;
  statement.BindDouble(5, score.boring_score);
  statement.BindBool(6, score.good_clipping);
  statement.BindBool(7, score.at_top);
  statement.BindInt64(8, score.time_at_snapshot.ToInternalValue());
  statement.BindBool(9, score.load_completed);
  int64 last_forced = url.last_forced_time.ToInternalValue();
  DCHECK((last_forced == 0) == (new_rank != kRankOfForcedURL))
      << "Thumbnail without a forced time stamp has a forced rank, or the "
      << "opposite.";
  statement.BindInt64(10, last_forced);
  if (!statement.Run())
    return;

  // Update rank if this is not a forced thumbnail.
  if (new_rank != kRankOfForcedURL)
    UpdatePageRankNoTransaction(url, new_rank);
}

void TopSitesDatabase::UpdatePageRank(const MostVisitedURL& url,
                                      int new_rank) {
  DCHECK((url.last_forced_time.ToInternalValue() == 0) ==
         (new_rank != kRankOfForcedURL))
      << "Thumbnail without a forced time stamp has a forced rank, or the "
      << "opposite.";
  sql::Transaction transaction(db_.get());
  transaction.Begin();
  UpdatePageRankNoTransaction(url, new_rank);
  transaction.Commit();
}

// Caller should have a transaction open.
void TopSitesDatabase::UpdatePageRankNoTransaction(
    const MostVisitedURL& url, int new_rank) {
  DCHECK_GT(db_->transaction_nesting(), 0);
  DCHECK((url.last_forced_time.is_null()) == (new_rank != kRankOfForcedURL))
      << "Thumbnail without a forced time stamp has a forced rank, or the "
      << "opposite.";

  int prev_rank = GetURLRank(url);
  if (prev_rank == kRankOfNonExistingURL) {
    LOG(WARNING) << "Updating rank of an unknown URL: " << url.url.spec();
    return;
  }

  // Shift the ranks.
  if (prev_rank > new_rank) {
    if (new_rank == kRankOfForcedURL) {
      // From non-forced to forced, shift down.
      // Example: 2 -> -1
      // -1, -1, -1, 0, 1, [2 -> -1], [3 -> 2], [4 -> 3]
      sql::Statement shift_statement(db_->GetCachedStatement(
          SQL_FROM_HERE,
          "UPDATE thumbnails "
          "SET url_rank = url_rank - 1 "
          "WHERE url_rank > ?"));
      shift_statement.BindInt(0, prev_rank);
      shift_statement.Run();
    } else {
      // From non-forced to non-forced, shift up.
      // Example: 3 -> 1
      // -1, -1, -1, 0, [1 -> 2], [2 -> 3], [3 -> 1], 4
      sql::Statement shift_statement(db_->GetCachedStatement(
          SQL_FROM_HERE,
          "UPDATE thumbnails "
          "SET url_rank = url_rank + 1 "
          "WHERE url_rank >= ? AND url_rank < ?"));
      shift_statement.BindInt(0, new_rank);
      shift_statement.BindInt(1, prev_rank);
      shift_statement.Run();
    }
  } else if (prev_rank < new_rank) {
    if (prev_rank == kRankOfForcedURL) {
      // From non-forced to forced, shift up.
      // Example: -1 -> 2
      // -1, [-1 -> 2], -1, 0, 1, [2 -> 3], [3 -> 4], [4 -> 5]
      sql::Statement shift_statement(db_->GetCachedStatement(
          SQL_FROM_HERE,
          "UPDATE thumbnails "
          "SET url_rank = url_rank + 1 "
          "WHERE url_rank >= ?"));
      shift_statement.BindInt(0, new_rank);
      shift_statement.Run();
    } else {
      // From non-forced to non-forced, shift down.
      // Example: 1 -> 3.
      // -1, -1, -1, 0, [1 -> 3], [2 -> 1], [3 -> 2], 4
      sql::Statement shift_statement(db_->GetCachedStatement(
          SQL_FROM_HERE,
          "UPDATE thumbnails "
          "SET url_rank = url_rank - 1 "
          "WHERE url_rank > ? AND url_rank <= ?"));
      shift_statement.BindInt(0, prev_rank);
      shift_statement.BindInt(1, new_rank);
      shift_statement.Run();
    }
  }

  // Set the url's rank and last_forced, since the latter changes when a URL
  // goes from forced to non-forced and vice-versa.
  sql::Statement set_statement(db_->GetCachedStatement(
      SQL_FROM_HERE,
      "UPDATE thumbnails "
      "SET url_rank = ?, last_forced = ? "
      "WHERE url == ?"));
  set_statement.BindInt(0, new_rank);
  set_statement.BindInt64(1, url.last_forced_time.ToInternalValue());
  set_statement.BindString(2, url.url.spec());
  set_statement.Run();
}

bool TopSitesDatabase::GetPageThumbnail(const GURL& url,
                                        Images* thumbnail) {
  sql::Statement statement(db_->GetCachedStatement(
      SQL_FROM_HERE,
      "SELECT thumbnail, boring_score, good_clipping, at_top, last_updated "
      "FROM thumbnails WHERE url=?"));
  statement.BindString(0, url.spec());
  if (!statement.Step())
    return false;

  std::vector<unsigned char> data;
  statement.ColumnBlobAsVector(0, &data);
  thumbnail->thumbnail = base::RefCountedBytes::TakeVector(&data);
  thumbnail->thumbnail_score.boring_score = statement.ColumnDouble(1);
  thumbnail->thumbnail_score.good_clipping = statement.ColumnBool(2);
  thumbnail->thumbnail_score.at_top = statement.ColumnBool(3);
  thumbnail->thumbnail_score.time_at_snapshot =
      base::Time::FromInternalValue(statement.ColumnInt64(4));
  return true;
}

int TopSitesDatabase::GetURLRank(const MostVisitedURL& url) {
  sql::Statement select_statement(db_->GetCachedStatement(
      SQL_FROM_HERE,
      "SELECT url_rank "
      "FROM thumbnails WHERE url=?"));
  select_statement.BindString(0, url.url.spec());
  if (select_statement.Step())
    return select_statement.ColumnInt(0);

  return kRankOfNonExistingURL;
}

// Remove the record for this URL. Returns true iff removed successfully.
bool TopSitesDatabase::RemoveURL(const MostVisitedURL& url) {
  int old_rank = GetURLRank(url);
  if (old_rank == kRankOfNonExistingURL)
    return false;

  sql::Transaction transaction(db_.get());
  transaction.Begin();
  if (old_rank != kRankOfForcedURL) {
    // Decrement all following ranks.
    sql::Statement shift_statement(db_->GetCachedStatement(
        SQL_FROM_HERE,
        "UPDATE thumbnails "
        "SET url_rank = url_rank - 1 "
        "WHERE url_rank > ?"));
    shift_statement.BindInt(0, old_rank);

    if (!shift_statement.Run())
      return false;
  }

  sql::Statement delete_statement(
      db_->GetCachedStatement(SQL_FROM_HERE,
                              "DELETE FROM thumbnails WHERE url = ?"));
  delete_statement.BindString(0, url.url.spec());

  if (!delete_statement.Run())
    return false;

  return transaction.Commit();
}

sql::Connection* TopSitesDatabase::CreateDB(const base::FilePath& db_name) {
  scoped_ptr<sql::Connection> db(new sql::Connection());
  // Settings copied from ThumbnailDatabase.
  db->set_histogram_tag("TopSites");
  db->set_error_callback(base::Bind(&DatabaseErrorCallback,
                                    db.get(), db_name));
  db->set_page_size(4096);
  db->set_cache_size(32);

  if (!db->Open(db_name))
    return NULL;
  return db.release();
}

}  // namespace history