Implement sql::Connection::Raze() in terms of sqlite3_backup API.

Wraps up the notion of reseting a database in a way which respects
SQLite locking constraints and doesn't require closing the database.

A similar outcome could be managed using filesystem operations, which
requires coordination between clients of the database to make sure
that no corruption occurs due to incorrect handling of -journal files.
Also, Windows pins files until the last handle closes, making that
approach challenging in some cases.

BUG=none
TEST=none

Review URL: https://chromiumcodereview.appspot.com/9768006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@131167 0039d316-1c4b-4281-b951-d872f2087c98

3 files changed, 232 insertions, 1 deletions

diff --git a/sql/connection.cc b/sql/connection.cc
index 58bfb11..b2274f1 100644
--- a/sql/connection.cc
+++ b/sql/connection.cc
@@ -168,6 +168,85 @@ void Connection::Preload() {
 #endif
 }
 
+// Create an in-memory database with the existing database's page
+// size, then backup that database over the existing database.
+bool Connection::Raze() {
+  if (!db_) {
+    DLOG(FATAL) << "Cannot raze null db";
+    return false;
+  }
+
+  if (transaction_nesting_ > 0) {
+    DLOG(FATAL) << "Cannot raze within a transaction";
+    return false;
+  }
+
+  sql::Connection null_db;
+  if (!null_db.OpenInMemory()) {
+    DLOG(FATAL) << "Unable to open in-memory database.";
+    return false;
+  }
+
+  // Get the page size from the current connection, then propagate it
+  // to the null database.
+  Statement s(GetUniqueStatement("PRAGMA page_size"));
+  if (!s.Step())
+    return false;
+  const std::string sql = StringPrintf("PRAGMA page_size=%d", s.ColumnInt(0));
+  if (!null_db.Execute(sql.c_str()))
+    return false;
+
+  // The page size doesn't take effect until a database has pages, and
+  // at this point the null database has none.  Changing the schema
+  // version will create the first page.  This will not affect the
+  // schema version in the resulting database, as SQLite's backup
+  // implementation propagates the schema version from the original
+  // connection to the new version of the database, incremented by one
+  // so that other readers see the schema change and act accordingly.
+  if (!null_db.Execute("PRAGMA schema_version = 1"))
+    return false;
+
+  sqlite3_backup* backup = sqlite3_backup_init(db_, "main",
+                                               null_db.db_, "main");
+  if (!backup) {
+    DLOG(FATAL) << "Unable to start sqlite3_backup().";
+    return false;
+  }
+
+  // -1 backs up the entire database.
+  int rc = sqlite3_backup_step(backup, -1);
+  int pages = sqlite3_backup_pagecount(backup);
+  sqlite3_backup_finish(backup);
+
+  // The destination database was locked.
+  if (rc == SQLITE_BUSY) {
+    return false;
+  }
+
+  // The entire database should have been backed up.
+  if (rc != SQLITE_DONE) {
+    DLOG(FATAL) << "Unable to copy entire null database.";
+    return false;
+  }
+
+  // Exactly one page should have been backed up.  If this breaks,
+  // check this function to make sure assumptions aren't being broken.
+  DCHECK_EQ(pages, 1);
+
+  return true;
+}
+
+bool Connection::RazeWithTimout(base::TimeDelta timeout) {
+  if (!db_) {
+    DLOG(FATAL) << "Cannot raze null db";
+    return false;
+  }
+
+  ScopedBusyTimeout busy_timeout(db_);
+  busy_timeout.SetTimeout(timeout);
+  return Raze();
+}
+
 bool Connection::BeginTransaction() {
   if (needs_rollback_) {
     DCHECK_GT(transaction_nesting_, 0);
diff --git a/sql/connection.h b/sql/connection.h
index 9b81804b..1e3414f 100644
--- a/sql/connection.h
+++ b/sql/connection.h
@@ -181,6 +181,26 @@ class SQL_EXPORT Connection {
   // generally exist either.
   void Preload();
 
+  // Raze the database to the ground.  This approximates creating a
+  // fresh database from scratch, within the constraints of SQLite's
+  // locking protocol (locks and open handles can make doing this with
+  // filesystem operations problematic).  Returns true if the database
+  // was razed.
+  //
+  // false is returned if the database is locked by some other
+  // process.  RazeWithTimeout() may be used if appropriate.
+  //
+  // NOTE(shess): Raze() will DCHECK in the following situations:
+  // - database is not open.
+  // - the connection has a transaction open.
+  // - a SQLite issue occurs which is structural in nature (like the
+  //   statements used are broken).
+  // Since Raze() is expected to be called in unexpected situations,
+  // these all return false, since it is unlikely that the caller
+  // could fix them.
+  bool Raze();
+  bool RazeWithTimout(base::TimeDelta timeout);
+
   // Transactions --------------------------------------------------------------
 
   // Transaction management. We maintain a virtual transaction stack to emulate
diff --git a/sql/connection_unittest.cc b/sql/connection_unittest.cc
index 58734ac..96cc25b 100644
--- a/sql/connection_unittest.cc
+++ b/sql/connection_unittest.cc
@@ -15,7 +15,7 @@ class SQLConnectionTest : public testing::Test {
 
   void SetUp() {
     ASSERT_TRUE(temp_dir_.CreateUniqueTempDir());
-    ASSERT_TRUE(db_.Open(temp_dir_.path().AppendASCII("SQLConnectionTest.db")));
+    ASSERT_TRUE(db_.Open(db_path()));
   }
 
   void TearDown() {
@@ -24,6 +24,10 @@ class SQLConnectionTest : public testing::Test {
 
   sql::Connection& db() { return db_; }
 
+  FilePath db_path() {
+    return temp_dir_.path().AppendASCII("SQLConnectionTest.db");
+  }
+
  private:
   ScopedTempDir temp_dir_;
   sql::Connection db_;
@@ -129,3 +133,131 @@ TEST_F(SQLConnectionTest, Rollback) {
   EXPECT_FALSE(db().CommitTransaction());
   EXPECT_TRUE(db().BeginTransaction());
 }
+
+// Test that sql::Connection::Raze() results in a database without the
+// tables from the original database.
+TEST_F(SQLConnectionTest, Raze) {
+  const char* kCreateSql = "CREATE TABLE foo (id INTEGER PRIMARY KEY, value)";
+  ASSERT_TRUE(db().Execute(kCreateSql));
+  ASSERT_TRUE(db().Execute("INSERT INTO foo (value) VALUES (12)"));
+
+  {
+    sql::Statement s(db().GetUniqueStatement("PRAGMA page_count"));
+    ASSERT_TRUE(s.Step());
+    EXPECT_EQ(2, s.ColumnInt(0));
+  }
+
+  {
+    sql::Statement s(db().GetUniqueStatement("SELECT * FROM sqlite_master"));
+    ASSERT_TRUE(s.Step());
+    EXPECT_EQ("table", s.ColumnString(0));
+    EXPECT_EQ("foo", s.ColumnString(1));
+    EXPECT_EQ("foo", s.ColumnString(2));
+    EXPECT_EQ(2, s.ColumnInt(3));
+    EXPECT_EQ(kCreateSql, s.ColumnString(4));
+  }
+
+  ASSERT_TRUE(db().Raze());
+
+  {
+    sql::Statement s(db().GetUniqueStatement("PRAGMA page_count"));
+    ASSERT_TRUE(s.Step());
+    EXPECT_EQ(1, s.ColumnInt(0));
+  }
+
+  {
+    sql::Statement s(db().GetUniqueStatement("SELECT * FROM sqlite_master"));
+    ASSERT_FALSE(s.Step());
+  }
+}
+
+// Test that Raze() maintains page_size.
+TEST_F(SQLConnectionTest, RazePageSize) {
+  const int kPageSize = 4096;
+
+  // Make sure that the default size isn't already |kPageSize|.
+  // Scoped to release statement before Close().
+  {
+    sql::Statement s(db().GetUniqueStatement("PRAGMA page_size"));
+    ASSERT_TRUE(s.Step());
+    ASSERT_NE(kPageSize, s.ColumnInt(0));
+  }
+
+  // Re-open the database to allow setting the page size.
+  db().Close();
+  db().set_page_size(kPageSize);
+  ASSERT_TRUE(db().Open(db_path()));
+
+  // page_size should match the indicated value.
+  sql::Statement s(db().GetUniqueStatement("PRAGMA page_size"));
+  ASSERT_TRUE(s.Step());
+  ASSERT_EQ(kPageSize, s.ColumnInt(0));
+
+  // After raze, page_size should still match the indicated value.
+  ASSERT_TRUE(db().Raze());
+  s.Reset();
+  ASSERT_TRUE(s.Step());
+  ASSERT_EQ(kPageSize, s.ColumnInt(0));
+}
+
+// Test that Raze() results are seen in other connections.
+TEST_F(SQLConnectionTest, RazeMultiple) {
+  const char* kCreateSql = "CREATE TABLE foo (id INTEGER PRIMARY KEY, value)";
+  ASSERT_TRUE(db().Execute(kCreateSql));
+
+  sql::Connection other_db;
+  ASSERT_TRUE(other_db.Open(db_path()));
+
+  // Check that the second connection sees the table.
+  const char *kTablesQuery = "SELECT COUNT(*) FROM sqlite_master";
+  sql::Statement s(other_db.GetUniqueStatement(kTablesQuery));
+  ASSERT_TRUE(s.Step());
+  ASSERT_EQ(1, s.ColumnInt(0));
+  ASSERT_FALSE(s.Step());  // Releases the shared lock.
+
+  ASSERT_TRUE(db().Raze());
+
+  // The second connection sees the updated database.
+  s.Reset();
+  ASSERT_TRUE(s.Step());
+  ASSERT_EQ(0, s.ColumnInt(0));
+}
+
+TEST_F(SQLConnectionTest, RazeLocked) {
+  const char* kCreateSql = "CREATE TABLE foo (id INTEGER PRIMARY KEY, value)";
+  ASSERT_TRUE(db().Execute(kCreateSql));
+
+  // Open a transaction and write some data in a second connection.
+  // This will acquire a PENDING or EXCLUSIVE transaction, which will
+  // cause the raze to fail.
+  sql::Connection other_db;
+  ASSERT_TRUE(other_db.Open(db_path()));
+  ASSERT_TRUE(other_db.BeginTransaction());
+  const char* kInsertSql = "INSERT INTO foo VALUES (1, 'data')";
+  ASSERT_TRUE(other_db.Execute(kInsertSql));
+
+  ASSERT_FALSE(db().Raze());
+
+  // Works after COMMIT.
+  ASSERT_TRUE(other_db.CommitTransaction());
+  ASSERT_TRUE(db().Raze());
+
+  // Re-create the database.
+  ASSERT_TRUE(db().Execute(kCreateSql));
+  ASSERT_TRUE(db().Execute(kInsertSql));
+
+  // An unfinished read transaction in the other connection also
+  // blocks raze.
+  const char *kQuery = "SELECT COUNT(*) FROM foo";
+  sql::Statement s(other_db.GetUniqueStatement(kQuery));
+  ASSERT_TRUE(s.Step());
+  ASSERT_FALSE(db().Raze());
+
+  // Complete the statement unlocks the database.
+  ASSERT_FALSE(s.Step());
+  ASSERT_TRUE(db().Raze());
+}
+
+// TODO(shess): Spin up a background thread to hold other_db, to more
+// closely match real life.  That would also allow testing
+// RazeWithTimeout().