[clangd] Change diskbackedstorage to be atomic
Summary:
There was a chance that multiple clangd instances could try to write
same shard, in which case we would get a malformed file most likely. This patch
changes the writing mechanism to first write to a temporary file and then rename
it to fit real destination. Which is guaranteed to be atomic by POSIX.
Reviewers: ilya-biryukov
Subscribers: ioeric, MaskRay, jkorous, arphaman, jfb, cfe-commits
Differential Revision: https://reviews.llvm.org/D55417
git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@349348 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/clangd/index/BackgroundIndexStorage.cpp b/clangd/index/BackgroundIndexStorage.cpp
index 4325ded..a83bec6 100644
--- a/clangd/index/BackgroundIndexStorage.cpp
+++ b/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 +35,34 @@
return ShardRootSS.str();
}
+llvm::Error
+writeAtomically(llvm::StringRef OutPath,
+ llvm::function_ref<void(llvm::raw_ostream &)> Writer) {
+ // Write to a temporary file first.
+ llvm::SmallString<128> TempPath;
+ int FD;
+ auto EC =
+ llvm::sys::fs::createUniqueFile(OutPath + ".tmp.%%%%%%%%", FD, TempPath);
+ if (EC)
+ return llvm::errorCodeToError(EC);
+ // Make sure temp file is destroyed on failure.
+ auto RemoveOnFail =
+ llvm::make_scope_exit([TempPath] { llvm::sys::fs::remove(TempPath); });
+ llvm::raw_fd_ostream OS(FD, /*shouldClose=*/true);
+ 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);
+ // If everything went well, we already moved the file to another name. So
+ // don't delete the file, as the name might be taken by another file.
+ RemoveOnFail.release();
+ 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 +100,9 @@
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; });
}
};
