[CAS] sync memory mapped file when closing CAS

To avoid unnecessary data lost due to power lost immediately after the
build, make sure the mapped files in the CAS are sync'ed back to the
file system when the last user of the CAS closed the file.

rdar://150379440
(cherry picked from commit a67d9ef)

Author: cachemeifyoucan

llvm/lib/CAS/MappedFileRegionBumpPtr.cpp

@@ -53,7 +53,6 @@
 
 #include "llvm/CAS/MappedFileRegionBumpPtr.h"
 #include "OnDiskCommon.h"
-#include "llvm/ADT/StringMap.h"
 #include "llvm/CAS/OnDiskCASLogger.h"
 
 using namespace llvm;
@@ -196,6 +195,8 @@ void MappedFileRegionBumpPtr::destroyImpl() {
       size_t Size = size();
       size_t Capacity = capacity();
       assert(Size < Capacity);
+      // sync to file system to make sure all contents are up-to-date.
+      (void)Region.sync();
       (void)sys::fs::resize_file(*FD, size());
       (void)unlockFileThreadSafe(*SharedLockFD);
 

llvm/lib/Support/Unix/Path.inc

@@ -876,6 +876,12 @@ void mapped_file_region::unmapImpl() {
     ::munmap(Mapping, Size);
 }
 
+std::error_code mapped_file_region::sync() const {
+  if (int Res = ::msync(Mapping, Size, MS_SYNC))
+    return std::error_code(Res, std::generic_category());
+  return std::error_code();
+}
+
 void mapped_file_region::dontNeedImpl() {
   assert(Mode == mapped_file_region::readonly);
   if (!Mapping)

llvm/lib/Support/Windows/Path.inc

@@ -1011,6 +1011,12 @@ void mapped_file_region::unmapImpl() {
 
 void mapped_file_region::dontNeedImpl() {}
 
+std::error_code mapped_file_region::sync() const {
+  if (::FlushViewOfFile(Mapping, 0))
+    return std::error_code();
+  return mapWindowsError(::GetLastError());
+}
+
 int mapped_file_region::alignment() {
   SYSTEM_INFO SysInfo;
   ::GetSystemInfo(&SysInfo);