[sql] Scoped recovery framework.

sql/recovery.cc

Index: sql/recovery.cc
diff --git a/sql/recovery.cc b/sql/recovery.cc
new file mode 100644
index 0000000000000000000000000000000000000000..a8e5217d9baebdcf3585d8241fbcc153605d520d
--- /dev/null
+++ b/sql/recovery.cc
@@ -0,0 +1,189 @@
+// Copyright (c) 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 "sql/recovery.h"
+
+#include "base/files/file_path.h"
+#include "base/logging.h"
+#include "base/metrics/sparse_histogram.h"
+#include "sql/connection.h"
+#include "third_party/sqlite/sqlite3.h"
+
+namespace sql {
+
+// static
+scoped_ptr<Recovery> Recovery::Begin(
+    Connection* connection,
+    const base::FilePath& db_path) {
+  scoped_ptr<Recovery> r(new Recovery(connection));
+  if (!r->Init(db_path)) {
+    // TODO(shess): Should Init() failure result in Raze()?
+    r->Shutdown(POISON);
+    return scoped_ptr<Recovery>();
+  }
+
+  return r.Pass();
+}
+
+// static
+bool Recovery::Recovered(scoped_ptr<Recovery> r) {
+  return r->Backup();
+}
+
+// static
+void Recovery::Unrecoverable(scoped_ptr<Recovery> r) {
+  CHECK(r->db_);
+  // ~Recovery() will RAZE_AND_POISON.
+}
+
+Recovery::Recovery(Connection* connection)
+    : db_(connection),
+      recover_db_() {
+  // Result should keep the page size specified earlier.
+  if (db_->page_size_)
+    recover_db_.set_page_size(db_->page_size_);
+
+  // TODO(shess): This may not handle cases where the default page
+  // size is used, but the default has changed.  I do not think this
+  // has ever happened.  This could be handled by using "PRAGMA
+  // page_size", at the cost of potential additional failure cases.
+}
+
+Recovery::~Recovery() {
+  Shutdown(RAZE_AND_POISON);
+}
+
+bool Recovery::Init(const base::FilePath& db_path) {
+  // Prevent the possibility of re-entering this code due to errors
+  // which happen while executing this code.
+  DCHECK(!db_->has_error_callback());
+
+  // Break any outstanding transactions on the original database to
+  // prevent deadlocks reading through the attached version.
+  // TODO(shess): A client may legitimately wish to recover from
+  // within the transaction context, because it would potentially
+  // preserve any in-flight changes.  Unfortunately, any attach-based
+  // system could not handle that.  A system which manually queried
+  // one database and stored to the other possibly could, but would be
+  // more complicated.
+  db_->RollbackAllTransactions();
+
+  if (!recover_db_.OpenTemporary())
+    return false;
+
+  // Turn on |SQLITE_RecoveryMode| for the handle, which allows
+  // reading certain broken databases.
+  if (!recover_db_.Execute("PRAGMA writable_schema=1"))
+    return false;
+
+  if (!recover_db_.AttachDatabase(db_path, "corrupt"))
+    return false;
+
+  return true;
+}
+
+bool Recovery::Backup() {
+  CHECK(db_);
+  CHECK(recover_db_.is_open());
+
+  // TODO(shess): Some of the failure cases here may need further
+  // exploration.  Just as elsewhere, persistent problems probably
+  // need to be razed, while anything which might succeed on a future
+  // run probably should be allowed to try.  But since Raze() uses the
+  // same approach, even that wouldn't work when this code fails.
+  //
+  // The documentation for the backup system indicate a relatively
+  // small number of errors are expected:
+  // SQLITE_BUSY - cannot lock the destination database.  This should
+  //               only happen if someone has another handle to the
+  //               database, Chromium generally doesn't do that.
+  // SQLITE_LOCKED - someone locked the source database.  Should be
+  //                 impossible (perhaps anti-virus could?).
+  // SQLITE_READONLY - destination is read-only.
+  // SQLITE_IOERR - since source database is temporary, probably
+  //                indicates that the destination contains blocks
+  //                throwing errors, or gross filesystem errors.
+  // SQLITE_NOMEM - out of memory, should be transient.
+  //
+  // AFAICT, SQLITE_BUSY and SQLITE_NOMEM could perhaps be considered
+  // transient, with SQLITE_LOCKED being unclear.
+  //
+  // SQLITE_READONLY and SQLITE_IOERR are probably persistent, with a
+  // strong chance that Raze() would not resolve them.  If Delete()
+  // deletes the database file, the code could then re-open the file
+  // and attempt the backup again.
+  //
+  // For now, this code attempts a best effort and records histograms
+  // to inform future development.
+
+  // Backup the original db from the recovered db.
+  const char* kMain = "main";
+  sqlite3_backup* backup = sqlite3_backup_init(db_->db_, kMain,
+                                               recover_db_.db_, kMain);
+  if (!backup) {
+    // Error code is in the destination database handle.
+    int err = sqlite3_errcode(db_->db_);
+    UMA_HISTOGRAM_SPARSE_SLOWLY("Sqlite.RecoveryHandle", err);
+    LOG(ERROR) << "sqlite3_backup_init() failed: "
+               << sqlite3_errmsg(db_->db_);
+    return false;
+  }
+
+  // -1 backs up the entire database.
+  int rc = sqlite3_backup_step(backup, -1);
+  int pages = sqlite3_backup_pagecount(backup);
+  // TODO(shess): sqlite3_backup_finish() appears to allow returning a
+  // different value from sqlite3_backup_step().  Circle back and
+  // figure out if that can usefully inform the decision of whether to
+  // retry or not.
+  sqlite3_backup_finish(backup);
+  DCHECK_GT(pages, 0);
+
+  if (rc != SQLITE_DONE) {
+    UMA_HISTOGRAM_SPARSE_SLOWLY("Sqlite.RecoveryStep", rc);
+    LOG(ERROR) << "sqlite3_backup_step() failed: "
+               << sqlite3_errmsg(db_->db_);
+  }
+
+  // The destination database was locked.  Give up, but leave the data
+  // in place.  Maybe it won't be locked next time.
+  if (rc == SQLITE_BUSY || rc == SQLITE_LOCKED) {
+    Shutdown(POISON);
+    return false;
+  }
+
+  // Running out of memory should be transient, retry later.
+  if (rc == SQLITE_NOMEM) {
+    Shutdown(POISON);
+    return false;
+  }
+
+  // TODO(shess): For now, leave the original database alone, pending
+  // results from Sqlite.RecoveryStep.  Some errors should probably
+  // route to RAZE_AND_POISON.
+  if (rc != SQLITE_DONE) {
+    Shutdown(POISON);
+    return false;
+  }
+
+  // Clean up the recovery db, and terminate the main database
+  // connection.
+  Shutdown(POISON);
+  return true;
+}
+
+void Recovery::Shutdown(Recovery::Disposition raze) {
+  if (!db_)
+    return;
+
+  recover_db_.Close();
+  if (raze == RAZE_AND_POISON) {
+    db_->RazeAndClose();
+  } else if (raze == POISON) {
+    db_->Poison();
+  }
+  db_ = NULL;
+}
+
+}  // namespace sql