set an explicit memory fence between stored series inserts and count …

…increments (morganstanley#267)

lib/hobbes/db/series.C

@@ -1,5 +1,6 @@
 
 #include <hobbes/db/series.H>
+#include <atomic>
 
 namespace hobbes {
 
@@ -328,6 +329,9 @@ void RawStoredSeries::record(const void* v, bool signal) {
   // store this data at the stream head
   this->storeFn(this->outputFile, v, this->batchHead);
 
+  // fence data storage and count increment
+  std::atomic_thread_fence(std::memory_order_release);
+
   // then advance the stream head
   //  (allocate a new batch cell if necessary)
   this->batchHead += this->storageSize;
@@ -686,6 +690,10 @@ const MonoTypePtr& CompressedStoredSeries::storageType() const {
 // (assumes that all such values are passed by reference)
 void CompressedStoredSeries::record(const void* x, bool signal) {
   this->writeFn(&this->w, this->dynModel, x);
+
+  // fence data storage and count increment
+  std::atomic_thread_fence(std::memory_order_release);
+
   if (this->w.step()) {
     this->prepMFn(&this->w, this->dynModel);
   }

lib/hobbes/db/signals.C

@@ -6,6 +6,7 @@
 #include <hobbes/hobbes.H>
 #include <vector>
 #include <set>
+#include <atomic>
 #include <string.h>
 #include <errno.h>
 
@@ -51,6 +52,10 @@ struct FileWatch {
 };
 
 void sweepFileWatch(FileWatch& fw) {
+  // if we get a signal to read, we should be able to read everything up to the signal
+  std::atomic_thread_fence(std::memory_order_acquire);
+
+  // check whether any value updates signal changes
   for (auto brwi = fw.byteRangeWatches.begin(); brwi != fw.byteRangeWatches.end();) {
     auto&           brwp = *brwi;
     ByteRangeWatch& brw  = brwp.second;