Gradient checker multithreading bugfix.

This is a follow-up on c/7470. GradientCheckingCostFunction calls
callback_->SetGradientErrorDetected() in its Evaluate method,
which will run in multiple threads simultaneously when enabling
this option in the solver. Thus, the string append operation
inside that method has to be protected by a mutex.

Change-Id: I314ef1df2be52595370d9af05851bf6da39bb45e

internal/ceres/gradient_checking_cost_function.cc

diff --git a/internal/ceres/gradient_checking_cost_function.cc b/internal/ceres/gradient_checking_cost_function.cc
index 6f0c59b..f2c7336 100644
--- a/internal/ceres/gradient_checking_cost_function.cc
+++ b/internal/ceres/gradient_checking_cost_function.cc
@@ -148,8 +148,10 @@
 }
 void GradientCheckingIterationCallback::SetGradientErrorDetected(
     std::string& error_log) {
+  mutex_.Lock();
   gradient_error_detected_ = true;
   error_log_ += "\n" + error_log;
+  mutex_.Unlock();
 }
 
 CostFunction* CreateGradientCheckingCostFunction(

internal/ceres/gradient_checking_cost_function.h

diff --git a/internal/ceres/gradient_checking_cost_function.h b/internal/ceres/gradient_checking_cost_function.h
index d8bbfed..f137349 100644
--- a/internal/ceres/gradient_checking_cost_function.h
+++ b/internal/ceres/gradient_checking_cost_function.h
@@ -37,6 +37,7 @@
 #include "ceres/cost_function.h"
 #include "ceres/iteration_callback.h"
 #include "ceres/local_parameterization.h"
+#include "ceres/mutex.h"
 
 namespace ceres {
 namespace internal {
@@ -53,14 +54,17 @@
   // then return SOLVER_ABORT.
   virtual CallbackReturnType operator()(const IterationSummary& summary);
 
-  // Notify this that a gradient error has occured.
+  // Notify this that a gradient error has occured (thread safe).
   void SetGradientErrorDetected(std::string& error_log);
 
+  // Retrieve error status (not thread safe).
   bool gradient_error_detected() const { return gradient_error_detected_; }
   const std::string& error_log() const { return error_log_; }
  private:
   bool gradient_error_detected_;
   std::string error_log_;
+  // Mutex protecting member variables.
+  ceres::internal::Mutex mutex_;
 };
 
 // Creates a CostFunction that checks the Jacobians that cost_function computes