Index: clangd/index/BackgroundIndexStorage.cpp
===================================================================
--- clangd/index/BackgroundIndexStorage.cpp
+++ clangd/index/BackgroundIndexStorage.cpp
@@ -9,6 +9,8 @@
 
 #include "Logger.h"
 #include "index/Background.h"
+#include "llvm/ADT/ScopeExit.h"
+#include "llvm/Support/Error.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/Path.h"
@@ -33,6 +35,29 @@
   return ShardRootSS.str();
 }
 
+template <typename WriterFunction>
+llvm::Error writeAtomically(llvm::StringRef OutPath, WriterFunction &&Writer) {
+  // Write to a temporary file first.
+  llvm::Twine TempPath(OutPath, ".tmp.");
+  TempPath.concat(std::to_string(rand()));
+  std::error_code EC;
+  llvm::raw_fd_ostream OS(TempPath.str(), EC);
+  if (EC)
+    return llvm::errorCodeToError(EC);
+  // Make sure temp file is destroyed at exit.
+  auto _ =
+      llvm::make_scope_exit([TempPath] { llvm::sys::fs::remove(TempPath); });
+  Writer(OS);
+  OS.close();
+  if (OS.has_error())
+    return llvm::errorCodeToError(OS.error());
+  // Then move to real location.
+  EC = llvm::sys::fs::rename(TempPath, OutPath);
+  if (EC)
+    return llvm::errorCodeToError(EC);
+  return llvm::ErrorSuccess();
+}
+
 // Uses disk as a storage for index shards. Creates a directory called
 // ".clangd-index/" under the path provided during construction.
 class DiskBackedIndexStorage : public BackgroundIndexStorage {
@@ -70,14 +95,9 @@
 
   llvm::Error storeShard(llvm::StringRef ShardIdentifier,
                          IndexFileOut Shard) const override {
-    auto ShardPath = getShardPathFromFilePath(DiskShardRoot, ShardIdentifier);
-    std::error_code EC;
-    llvm::raw_fd_ostream OS(ShardPath, EC);
-    if (EC)
-      return llvm::errorCodeToError(EC);
-    OS << Shard;
-    OS.close();
-    return llvm::errorCodeToError(OS.error());
+    return writeAtomically(
+        getShardPathFromFilePath(DiskShardRoot, ShardIdentifier),
+        [&Shard](llvm::raw_ostream &OS) { OS << Shard; });
   }
 };