SERVER-22244 Restart initial sync on sync source rollback

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diff --git a/src/mongo/db/repl/rollback_checker.h b/src/mongo/db/repl/rollback_checker.h
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+/**
+ *    Copyright (C) 2015 MongoDB Inc.
+ *
+ *    This program is free software: you can redistribute it and/or  modify
+ *    it under the terms of the GNU Affero General Public License, version 3,
+ *    as published by the Free Software Foundation.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    GNU Affero General Public License for more details.
+ *
+ *    You should have received a copy of the GNU Affero General Public License
+ *    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the GNU Affero General Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#pragma once
+
+#include "mongo/base/status_with.h"
+#include "mongo/executor/task_executor.h"
+
+namespace mongo {
+namespace repl {
+
+class Mutex;
+
+/**
+ * The RollbackChecker maintains a sync source and its baseline rollback ID (rbid). It
+ * contains methods to check if a rollback occurred by checking if the rbid has changed since
+ * the baseline was set.
+ *
+ * To use the RollbackChecker:
+ * 1) Create a RollbackChecker by passing in an executor, and a sync source to use. If the
+ *    sync source changes, make a new RollbackChecker.
+ * 2) Call reset(), either synchronously or asynchronously, so that the RollbackChecker retrieves
+ *    the state it needs from the sync source to check for rollbacks.
+ * 3) Call checkForRollback(), passing in the next action to check if there was a rollback at the
+ *    sync source. If there is a rollback or another error, the error function provided will be
+ *    called. This error could be an UnrecoverableRollbackError, when there is a rollback.
+ *    Alternatively, call hasHadRollback() which synchonously checks for a rollback and returns
+ *    true if any error occurs. Checking for a rollback does not reset the baseline rbid.
+ * 4) Repeat steps 2 and 3 as needed.
+ *
+ */
+class RollbackChecker {
+    MONGO_DISALLOW_COPYING(RollbackChecker);
+
+public:
+    using CallbackFn = stdx::function<void(const Status& status)>;
+    using RemoteCommandCallbackFn = executor::TaskExecutor::RemoteCommandCallbackFn;
+    using CallbackHandle = executor::TaskExecutor::CallbackHandle;
+
+    RollbackChecker(executor::TaskExecutor* executor, HostAndPort syncSource);
+
+    virtual ~RollbackChecker();
+
+    // Checks whether the the sync source has had a rollback since the last time reset was called,
+    // and then calls the nextAction with a status specifying what should occur next. The status
+    // will either be OK if there was no rollback and no error, UnrecoverableRollbackError if there
+    // was a rollback, or another status if the command failed. The nextAction should account for
+    // each of these cases.
+    CallbackHandle checkForRollback(const CallbackFn& nextAction);
+
+    // Synchronously checks if there has been a rollback and returns a boolean specifying if one
+    // has occurred. If any error occurs this will return true.
+    bool hasHadRollback();
+
+    // Resets the state used to decide if a rollback occurs, and then calls the nextAction with a
+    // status specifying what should occur next. The status will either be OK if there was no
+    // error or another status if the command failed. The nextAction should account for
+    // each of these cases.
+    CallbackHandle reset(const CallbackFn& nextAction);
+
+    // Synchronously calls reset and returns the Status of the command.
+    Status reset_sync();
+
+    // ================== Test Support API ===================
+
+    // Returns the current baseline rbid.
+    int getBaseRBID_forTest();
+
+    // Returns the last rbid seen.
+    int getLastRBID_forTest();
+
+private:
+    // Assumes a lock has been taken. Returns if a rollback has occurred by comparing the remoteRBID
+    // provided and the stored baseline rbid. Sets _lastRBID to the remoteRBID provided.
+    bool _checkForRollback_inlock(int remoteRBID);
+
+    // Schedules a remote command to get the rbid at the sync source and then calls the nextAction.
+    // If there is an error scheduling the call, it calls the errorFn with the status.
+    CallbackHandle _scheduleGetRollbackId(const RemoteCommandCallbackFn& nextAction,
+                                          const CallbackFn& errorFn);
+
+    // Assumes a lock has been taken. Sets the current rbid used as the baseline for rollbacks.
+    void _setRBID_inlock(int rbid);
+
+    // Not owned by us.
+    executor::TaskExecutor* const _executor;
+
+    // Protects member data of this RollbackChecker.
+    mutable stdx::mutex _mutex;
+
+    // The sync source to check for rollbacks against.
+    HostAndPort _syncSource;
+
+    // The baseline rbid of the sync source to use when deciding if a rollback should occur.
+    int _baseRBID;
+
+    // The last rbid that we saw.
+    int _lastRBID;
+};
+
+}  // namespace repl
+}  // namespace mongo