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diff --git a/storage/mozStorageHelper.h b/storage/mozStorageHelper.h
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+++ b/storage/mozStorageHelper.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef MOZSTORAGEHELPER_H
+#define MOZSTORAGEHELPER_H
+
+#include "nsCOMPtr.h"
+#include "nsString.h"
+#include "mozilla/DebugOnly.h"
+#include "mozilla/ScopeExit.h"
+
+#include "mozilla/storage/SQLiteMutex.h"
+#include "mozIStorageConnection.h"
+#include "mozIStorageStatement.h"
+#include "mozIStoragePendingStatement.h"
+#include "mozilla/DebugOnly.h"
+#include "nsCOMPtr.h"
+#include "nsError.h"
+
+/**
+ * This class wraps a transaction inside a given C++ scope, guaranteeing that
+ * the transaction will be completed even if you have an exception or
+ * return early.
+ *
+ * A common use is to create an instance with aCommitOnComplete = false
+ * (rollback), then call Commit() on this object manually when your function
+ * completes successfully.
+ *
+ * @note nested transactions are not supported by Sqlite, only nested
+ * savepoints, so if a transaction is already in progress, this object creates
+ * a nested savepoint to the existing transaction which is considered as
+ * anonymous savepoint itself. However, aType and aAsyncCommit are ignored
+ * in the case of nested savepoints.
+ *
+ * @param aConnection
+ *        The connection to create the transaction on.
+ * @param aCommitOnComplete
+ *        Controls whether the transaction is committed or rolled back when
+ *        this object goes out of scope.
+ * @param aType [optional]
+ *        The transaction type, as defined in mozIStorageConnection. Uses the
+ *        default transaction behavior for the connection if unspecified.
+ * @param aAsyncCommit [optional]
+ *        Whether commit should be executed asynchronously on the helper thread.
+ *        This is a special option introduced as an interim solution to reduce
+ *        main-thread fsyncs in Places.  Can only be used on main-thread.
+ *
+ *        WARNING: YOU SHOULD _NOT_ WRITE NEW MAIN-THREAD CODE USING THIS!
+ *
+ *        Notice that async commit might cause synchronous statements to fail
+ *        with SQLITE_BUSY.  A possible mitigation strategy is to use
+ *        PRAGMA busy_timeout, but notice that might cause main-thread jank.
+ *        Finally, if the database is using WAL journaling mode, other
+ *        connections won't see the changes done in async committed transactions
+ *        until commit is complete.
+ *
+ *        For all of the above reasons, this should only be used as an interim
+ *        solution and avoided completely if possible.
+ */
+class mozStorageTransaction {
+  using SQLiteMutexAutoLock = mozilla::storage::SQLiteMutexAutoLock;
+
+ public:
+  mozStorageTransaction(
+      mozIStorageConnection* aConnection, bool aCommitOnComplete,
+      int32_t aType = mozIStorageConnection::TRANSACTION_DEFAULT,
+      bool aAsyncCommit = false)
+      : mConnection(aConnection),
+        mType(aType),
+        mNestingLevel(0),
+        mHasTransaction(false),
+        mCommitOnComplete(aCommitOnComplete),
+        mCompleted(false),
+        mAsyncCommit(aAsyncCommit) {}
+
+  ~mozStorageTransaction() {
+    if (mConnection && mHasTransaction && !mCompleted) {
+      if (mCommitOnComplete) {
+        mozilla::DebugOnly<nsresult> rv = Commit();
+        NS_WARNING_ASSERTION(NS_SUCCEEDED(rv),
+                             "A transaction didn't commit correctly");
+      } else {
+        mozilla::DebugOnly<nsresult> rv = Rollback();
+        NS_WARNING_ASSERTION(NS_SUCCEEDED(rv),
+                             "A transaction didn't rollback correctly");
+      }
+    }
+  }
+
+  /**
+   * Starts the transaction.
+   */
+  nsresult Start() {
+    // XXX We should probably get rid of mHasTransaction and use mConnection
+    // for checking if a transaction has been started. However, we need to
+    // first stop supporting null mConnection and also move aConnection from
+    // the constructor to Start.
+    MOZ_DIAGNOSTIC_ASSERT(!mHasTransaction);
+
+    // XXX We should probably stop supporting null mConnection.
+
+    // XXX We should probably get rid of mCompleted and allow to start the
+    // transaction again if it was already committed or rolled back.
+    if (!mConnection || mCompleted) {
+      return NS_OK;
+    }
+
+    SQLiteMutexAutoLock lock(mConnection->GetSharedDBMutex());
+
+    // We nee to speculatively set the nesting level to be able to decide
+    // if this is a top level transaction and to be able to generate the
+    // savepoint name.
+    TransactionStarted(lock);
+
+    // If there's a failure we need to revert the speculatively set nesting
+    // level on the connection.
+    auto autoFinishTransaction =
+        mozilla::MakeScopeExit([&] { TransactionFinished(lock); });
+
+    nsAutoCString query;
+
+    if (TopLevelTransaction(lock)) {
+      query.Assign("BEGIN");
+      int32_t type = mType;
+      if (type == mozIStorageConnection::TRANSACTION_DEFAULT) {
+        MOZ_ALWAYS_SUCCEEDS(mConnection->GetDefaultTransactionType(&type));
+      }
+      switch (type) {
+        case mozIStorageConnection::TRANSACTION_IMMEDIATE:
+          query.AppendLiteral(" IMMEDIATE");
+          break;
+        case mozIStorageConnection::TRANSACTION_EXCLUSIVE:
+          query.AppendLiteral(" EXCLUSIVE");
+          break;
+        case mozIStorageConnection::TRANSACTION_DEFERRED:
+          query.AppendLiteral(" DEFERRED");
+          break;
+        default:
+          MOZ_ASSERT(false, "Unknown transaction type");
+      }
+    } else {
+      query.Assign("SAVEPOINT sp"_ns + IntToCString(mNestingLevel));
+    }
+
+    nsresult rv = mConnection->ExecuteSimpleSQL(query);
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    autoFinishTransaction.release();
+
+    return NS_OK;
+  }
+
+  /**
+   * Commits the transaction if one is in progress. If one is not in progress,
+   * this is a NOP since the actual owner of the transaction outside of our
+   * scope is in charge of finally committing or rolling back the transaction.
+   */
+  nsresult Commit() {
+    // XXX Assert instead of returning NS_OK if the transaction hasn't been
+    // started.
+    if (!mConnection || mCompleted || !mHasTransaction) return NS_OK;
+
+    SQLiteMutexAutoLock lock(mConnection->GetSharedDBMutex());
+
+#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+    MOZ_DIAGNOSTIC_ASSERT(CurrentTransaction(lock));
+#else
+    if (!CurrentTransaction(lock)) {
+      return NS_ERROR_NOT_AVAILABLE;
+    }
+#endif
+
+    mCompleted = true;
+
+    nsresult rv;
+
+    if (TopLevelTransaction(lock)) {
+      // TODO (bug 559659): this might fail with SQLITE_BUSY, but we don't
+      // handle it, thus the transaction might stay open until the next COMMIT.
+      if (mAsyncCommit) {
+        nsCOMPtr<mozIStoragePendingStatement> ps;
+        rv = mConnection->ExecuteSimpleSQLAsync("COMMIT"_ns, nullptr,
+                                                getter_AddRefs(ps));
+      } else {
+        rv = mConnection->ExecuteSimpleSQL("COMMIT"_ns);
+      }
+    } else {
+      rv = mConnection->ExecuteSimpleSQL("RELEASE sp"_ns +
+                                         IntToCString(mNestingLevel));
+    }
+
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    TransactionFinished(lock);
+
+    return NS_OK;
+  }
+
+  /**
+   * Rolls back the transaction if one is in progress. If one is not in
+   * progress, this is a NOP since the actual owner of the transaction outside
+   * of our scope is in charge of finally rolling back the transaction.
+   */
+  nsresult Rollback() {
+    // XXX Assert instead of returning NS_OK if the transaction hasn't been
+    // started.
+    if (!mConnection || mCompleted || !mHasTransaction) return NS_OK;
+
+    SQLiteMutexAutoLock lock(mConnection->GetSharedDBMutex());
+
+#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
+    MOZ_DIAGNOSTIC_ASSERT(CurrentTransaction(lock));
+#else
+    if (!CurrentTransaction(lock)) {
+      return NS_ERROR_NOT_AVAILABLE;
+    }
+#endif
+
+    mCompleted = true;
+
+    nsresult rv;
+
+    if (TopLevelTransaction(lock)) {
+      // TODO (bug 1062823): from Sqlite 3.7.11 on, rollback won't ever return
+      // a busy error, so this handling can be removed.
+      do {
+        rv = mConnection->ExecuteSimpleSQL("ROLLBACK"_ns);
+        if (rv == NS_ERROR_STORAGE_BUSY) (void)PR_Sleep(PR_INTERVAL_NO_WAIT);
+      } while (rv == NS_ERROR_STORAGE_BUSY);
+    } else {
+      const auto nestingLevelCString = IntToCString(mNestingLevel);
+      rv = mConnection->ExecuteSimpleSQL(
+          "ROLLBACK TO sp"_ns + nestingLevelCString + "; RELEASE sp"_ns +
+          nestingLevelCString);
+    }
+
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    TransactionFinished(lock);
+
+    return NS_OK;
+  }
+
+ protected:
+  void TransactionStarted(const SQLiteMutexAutoLock& aProofOfLock) {
+    MOZ_ASSERT(mConnection);
+    MOZ_ASSERT(!mHasTransaction);
+    MOZ_ASSERT(mNestingLevel == 0);
+    mHasTransaction = true;
+    mNestingLevel = mConnection->IncreaseTransactionNestingLevel(aProofOfLock);
+  }
+
+  bool CurrentTransaction(const SQLiteMutexAutoLock& aProofOfLock) const {
+    MOZ_ASSERT(mConnection);
+    MOZ_ASSERT(mHasTransaction);
+    MOZ_ASSERT(mNestingLevel > 0);
+    return mNestingLevel ==
+           mConnection->GetTransactionNestingLevel(aProofOfLock);
+  }
+
+  bool TopLevelTransaction(const SQLiteMutexAutoLock& aProofOfLock) const {
+    MOZ_ASSERT(mConnection);
+    MOZ_ASSERT(mHasTransaction);
+    MOZ_ASSERT(mNestingLevel > 0);
+    MOZ_ASSERT(CurrentTransaction(aProofOfLock));
+    return mNestingLevel == 1;
+  }
+
+  void TransactionFinished(const SQLiteMutexAutoLock& aProofOfLock) {
+    MOZ_ASSERT(mConnection);
+    MOZ_ASSERT(mHasTransaction);
+    MOZ_ASSERT(mNestingLevel > 0);
+    MOZ_ASSERT(CurrentTransaction(aProofOfLock));
+    mConnection->DecreaseTransactionNestingLevel(aProofOfLock);
+    mNestingLevel = 0;
+    mHasTransaction = false;
+  }
+
+  nsCOMPtr<mozIStorageConnection> mConnection;
+  int32_t mType;
+  uint32_t mNestingLevel;
+  bool mHasTransaction;
+  bool mCommitOnComplete;
+  bool mCompleted;
+  bool mAsyncCommit;
+};
+
+/**
+ * This class wraps a statement so that it is guaraneed to be reset when
+ * this object goes out of scope.
+ *
+ * Note that this always just resets the statement. If the statement doesn't
+ * need resetting, the reset operation is inexpensive.
+ */
+class MOZ_STACK_CLASS mozStorageStatementScoper {
+ public:
+  explicit mozStorageStatementScoper(mozIStorageStatement* aStatement)
+      : mStatement(aStatement) {}
+  ~mozStorageStatementScoper() {
+    if (mStatement) mStatement->Reset();
+  }
+
+  mozStorageStatementScoper(mozStorageStatementScoper&&) = default;
+  mozStorageStatementScoper& operator=(mozStorageStatementScoper&&) = default;
+  mozStorageStatementScoper(const mozStorageStatementScoper&) = delete;
+  mozStorageStatementScoper& operator=(const mozStorageStatementScoper&) =
+      delete;
+
+  /**
+   * Call this to make the statement not reset. You might do this if you know
+   * that the statement has been reset.
+   */
+  void Abandon() { mStatement = nullptr; }
+
+ protected:
+  nsCOMPtr<mozIStorageStatement> mStatement;
+};
+
+// Use this to make queries uniquely identifiable in telemetry
+// statistics, especially PRAGMAs.  We don't include __LINE__ so that
+// queries are stable in the face of source code changes.
+#define MOZ_STORAGE_UNIQUIFY_QUERY_STR "/* " __FILE__ " */ "
+
+#endif /* MOZSTORAGEHELPER_H */