lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp - llvm-project/clang - Git at Google

 //===- DependencyScanningFilesystem.cpp - clang-scan-deps fs --------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//

 #include "clang/Tooling/DependencyScanning/DependencyScanningFilesystem.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/SmallVectorMemoryBuffer.h"
 #include "llvm/Support/Threading.h"
 #include <optional>

 using namespace clang;
 using namespace tooling;
 using namespace dependencies;

 llvm::ErrorOr<DependencyScanningWorkerFilesystem::TentativeEntry>
 DependencyScanningWorkerFilesystem::readFile(StringRef Filename) {
   // Load the file and its content from the file system.
   auto MaybeFile = getUnderlyingFS().openFileForRead(Filename);
   if (!MaybeFile)
     return MaybeFile.getError();
   auto File = std::move(*MaybeFile);

   auto MaybeStat = File->status();
   if (!MaybeStat)
     return MaybeStat.getError();
   auto Stat = std::move(*MaybeStat);

   auto MaybeBuffer = File->getBuffer(Stat.getName());
   if (!MaybeBuffer)
     return MaybeBuffer.getError();
   auto Buffer = std::move(*MaybeBuffer);

   // If the file size changed between read and stat, pretend it didn't.
   if (Stat.getSize() != Buffer->getBufferSize())
     Stat = llvm::vfs::Status::copyWithNewSize(Stat, Buffer->getBufferSize());

   return TentativeEntry(Stat, std::move(Buffer));
 }

 bool DependencyScanningWorkerFilesystem::ensureDirectiveTokensArePopulated(
     EntryRef Ref) {
   auto &Entry = Ref.Entry;

   if (Entry.isError() || Entry.isDirectory())
     return false;

   CachedFileContents *Contents = Entry.getCachedContents();
   assert(Contents && "contents not initialized");

   // Double-checked locking.
   if (Contents->DepDirectives.load())
     return true;

   std::lock_guard<std::mutex> GuardLock(Contents->ValueLock);

   // Double-checked locking.
   if (Contents->DepDirectives.load())
     return true;

   SmallVector<dependency_directives_scan::Directive, 64> Directives;
   // Scan the file for preprocessor directives that might affect the
   // dependencies.
   if (scanSourceForDependencyDirectives(Contents->Original->getBuffer(),
                                         Contents->DepDirectiveTokens,
                                         Directives)) {
     Contents->DepDirectiveTokens.clear();
     // FIXME: Propagate the diagnostic if desired by the client.
     Contents->DepDirectives.store(new std::optional<DependencyDirectivesTy>());
     return false;
   }

   // This function performed double-checked locking using `DepDirectives`.
   // Assigning it must be the last thing this function does, otherwise other
   // threads may skip the critical section (`DepDirectives != nullptr`), leading
   // to a data race.
   Contents->DepDirectives.store(
       new std::optional<DependencyDirectivesTy>(std::move(Directives)));
   return true;
 }

 DependencyScanningFilesystemSharedCache::
     DependencyScanningFilesystemSharedCache() {
   // This heuristic was chosen using a empirical testing on a
   // reasonably high core machine (iMacPro 18 cores / 36 threads). The cache
   // sharding gives a performance edge by reducing the lock contention.
   // FIXME: A better heuristic might also consider the OS to account for
   // the different cost of lock contention on different OSes.
   NumShards =
       std::max(2u, llvm::hardware_concurrency().compute_thread_count() / 4);
   CacheShards = std::make_unique<CacheShard[]>(NumShards);
 }

 DependencyScanningFilesystemSharedCache::CacheShard &
 DependencyScanningFilesystemSharedCache::getShardForFilename(
     StringRef Filename) const {
   assert(llvm::sys::path::is_absolute_gnu(Filename));
   return CacheShards[llvm::hash_value(Filename) % NumShards];
 }

 DependencyScanningFilesystemSharedCache::CacheShard &
 DependencyScanningFilesystemSharedCache::getShardForUID(
     llvm::sys::fs::UniqueID UID) const {
   auto Hash = llvm::hash_combine(UID.getDevice(), UID.getFile());
   return CacheShards[Hash % NumShards];
 }

 const CachedFileSystemEntry *
 DependencyScanningFilesystemSharedCache::CacheShard::findEntryByFilename(
     StringRef Filename) const {
   assert(llvm::sys::path::is_absolute_gnu(Filename));
   std::lock_guard<std::mutex> LockGuard(CacheLock);
   auto It = CacheByFilename.find(Filename);
   return It == CacheByFilename.end() ? nullptr : It->getValue().first;
 }

 const CachedFileSystemEntry *
 DependencyScanningFilesystemSharedCache::CacheShard::findEntryByUID(
     llvm::sys::fs::UniqueID UID) const {
   std::lock_guard<std::mutex> LockGuard(CacheLock);
   auto It = EntriesByUID.find(UID);
   return It == EntriesByUID.end() ? nullptr : It->getSecond();
 }

 const CachedFileSystemEntry &
 DependencyScanningFilesystemSharedCache::CacheShard::
     getOrEmplaceEntryForFilename(StringRef Filename,
                                  llvm::ErrorOr<llvm::vfs::Status> Stat) {
   std::lock_guard<std::mutex> LockGuard(CacheLock);
   auto [It, Inserted] = CacheByFilename.insert({Filename, {nullptr, nullptr}});
   auto &[CachedEntry, CachedRealPath] = It->getValue();
   if (!CachedEntry) {
     // The entry is not present in the shared cache. Either the cache doesn't
     // know about the file at all, or it only knows about its real path.
     assert((Inserted || CachedRealPath) && "existing file with empty pair");
     CachedEntry =
         new (EntryStorage.Allocate()) CachedFileSystemEntry(std::move(Stat));
   }
   return *CachedEntry;
 }

 const CachedFileSystemEntry &
 DependencyScanningFilesystemSharedCache::CacheShard::getOrEmplaceEntryForUID(
     llvm::sys::fs::UniqueID UID, llvm::vfs::Status Stat,
     std::unique_ptr<llvm::MemoryBuffer> Contents) {
   std::lock_guard<std::mutex> LockGuard(CacheLock);
   auto [It, Inserted] = EntriesByUID.insert({UID, nullptr});
   auto &CachedEntry = It->getSecond();
   if (Inserted) {
     CachedFileContents *StoredContents = nullptr;
     if (Contents)
       StoredContents = new (ContentsStorage.Allocate())
           CachedFileContents(std::move(Contents));
     CachedEntry = new (EntryStorage.Allocate())
         CachedFileSystemEntry(std::move(Stat), StoredContents);
   }
   return *CachedEntry;
 }

 const CachedFileSystemEntry &
 DependencyScanningFilesystemSharedCache::CacheShard::
     getOrInsertEntryForFilename(StringRef Filename,
                                 const CachedFileSystemEntry &Entry) {
   std::lock_guard<std::mutex> LockGuard(CacheLock);
   auto [It, Inserted] = CacheByFilename.insert({Filename, {&Entry, nullptr}});
   auto &[CachedEntry, CachedRealPath] = It->getValue();
   if (!Inserted || !CachedEntry)
     CachedEntry = &Entry;
   return *CachedEntry;
 }

 const CachedRealPath *
 DependencyScanningFilesystemSharedCache::CacheShard::findRealPathByFilename(
     StringRef Filename) const {
   assert(llvm::sys::path::is_absolute_gnu(Filename));
   std::lock_guard<std::mutex> LockGuard(CacheLock);
   auto It = CacheByFilename.find(Filename);
   return It == CacheByFilename.end() ? nullptr : It->getValue().second;
 }

 const CachedRealPath &DependencyScanningFilesystemSharedCache::CacheShard::
     getOrEmplaceRealPathForFilename(StringRef Filename,
                                     llvm::ErrorOr<llvm::StringRef> RealPath) {
   std::lock_guard<std::mutex> LockGuard(CacheLock);

   const CachedRealPath *&StoredRealPath = CacheByFilename[Filename].second;
   if (!StoredRealPath) {
     auto OwnedRealPath = [&]() -> CachedRealPath {
       if (!RealPath)
         return RealPath.getError();
       return RealPath->str();
     }();

     StoredRealPath = new (RealPathStorage.Allocate())
         CachedRealPath(std::move(OwnedRealPath));
   }

   return *StoredRealPath;
 }

 static bool shouldCacheStatFailures(StringRef Filename) {
   StringRef Ext = llvm::sys::path::extension(Filename);
   if (Ext.empty())
     return false; // This may be the module cache directory.
   return true;
 }

 DependencyScanningWorkerFilesystem::DependencyScanningWorkerFilesystem(
     DependencyScanningFilesystemSharedCache &SharedCache,
     IntrusiveRefCntPtr<llvm::vfs::FileSystem> FS)
     : llvm::RTTIExtends<DependencyScanningWorkerFilesystem,
                         llvm::vfs::ProxyFileSystem>(std::move(FS)),
       SharedCache(SharedCache),
       WorkingDirForCacheLookup(llvm::errc::invalid_argument) {
   updateWorkingDirForCacheLookup();
 }

 const CachedFileSystemEntry &
 DependencyScanningWorkerFilesystem::getOrEmplaceSharedEntryForUID(
     TentativeEntry TEntry) {
   auto &Shard = SharedCache.getShardForUID(TEntry.Status.getUniqueID());
   return Shard.getOrEmplaceEntryForUID(TEntry.Status.getUniqueID(),
                                        std::move(TEntry.Status),
                                        std::move(TEntry.Contents));
 }

 const CachedFileSystemEntry *
 DependencyScanningWorkerFilesystem::findEntryByFilenameWithWriteThrough(
     StringRef Filename) {
   if (const auto *Entry = LocalCache.findEntryByFilename(Filename))
     return Entry;
   auto &Shard = SharedCache.getShardForFilename(Filename);
   if (const auto *Entry = Shard.findEntryByFilename(Filename))
     return &LocalCache.insertEntryForFilename(Filename, *Entry);
   return nullptr;
 }

 llvm::ErrorOr<const CachedFileSystemEntry &>
 DependencyScanningWorkerFilesystem::computeAndStoreResult(
     StringRef OriginalFilename, StringRef FilenameForLookup) {
   llvm::ErrorOr<llvm::vfs::Status> Stat =
       getUnderlyingFS().status(OriginalFilename);
   if (!Stat) {
     if (!shouldCacheStatFailures(OriginalFilename))
       return Stat.getError();
     const auto &Entry =
         getOrEmplaceSharedEntryForFilename(FilenameForLookup, Stat.getError());
     return insertLocalEntryForFilename(FilenameForLookup, Entry);
   }

   if (const auto *Entry = findSharedEntryByUID(*Stat))
     return insertLocalEntryForFilename(FilenameForLookup, *Entry);

   auto TEntry =
       Stat->isDirectory() ? TentativeEntry(*Stat) : readFile(OriginalFilename);

   const CachedFileSystemEntry *SharedEntry = [&]() {
     if (TEntry) {
       const auto &UIDEntry = getOrEmplaceSharedEntryForUID(std::move(*TEntry));
       return &getOrInsertSharedEntryForFilename(FilenameForLookup, UIDEntry);
     }
     return &getOrEmplaceSharedEntryForFilename(FilenameForLookup,
                                                TEntry.getError());
   }();

   return insertLocalEntryForFilename(FilenameForLookup, *SharedEntry);
 }

 llvm::ErrorOr<EntryRef>
 DependencyScanningWorkerFilesystem::getOrCreateFileSystemEntry(
     StringRef OriginalFilename) {
   SmallString<256> PathBuf;
   auto FilenameForLookup = tryGetFilenameForLookup(OriginalFilename, PathBuf);
   if (!FilenameForLookup)
     return FilenameForLookup.getError();

   if (const auto *Entry =
           findEntryByFilenameWithWriteThrough(*FilenameForLookup))
     return EntryRef(OriginalFilename, *Entry).unwrapError();
   auto MaybeEntry = computeAndStoreResult(OriginalFilename, *FilenameForLookup);
   if (!MaybeEntry)
     return MaybeEntry.getError();
   return EntryRef(OriginalFilename, *MaybeEntry).unwrapError();
 }

 llvm::ErrorOr<llvm::vfs::Status>
 DependencyScanningWorkerFilesystem::status(const Twine &Path) {
   SmallString<256> OwnedFilename;
   StringRef Filename = Path.toStringRef(OwnedFilename);

   if (Filename.ends_with(".pcm"))
     return getUnderlyingFS().status(Path);

   llvm::ErrorOr<EntryRef> Result = getOrCreateFileSystemEntry(Filename);
   if (!Result)
     return Result.getError();
   return Result->getStatus();
 }

 bool DependencyScanningWorkerFilesystem::exists(const Twine &Path) {
   // While some VFS overlay filesystems may implement more-efficient
   // mechanisms for `exists` queries, `DependencyScanningWorkerFilesystem`
   // typically wraps `RealFileSystem` which does not specialize `exists`,
   // so it is not likely to benefit from such optimizations. Instead,
   // it is more-valuable to have this query go through the
   // cached-`status` code-path of the `DependencyScanningWorkerFilesystem`.
   llvm::ErrorOr<llvm::vfs::Status> Status = status(Path);
   return Status && Status->exists();
 }

 namespace {

 /// The VFS that is used by clang consumes the \c CachedFileSystemEntry using
 /// this subclass.
 class DepScanFile final : public llvm::vfs::File {
 public:
   DepScanFile(std::unique_ptr<llvm::MemoryBuffer> Buffer,
               llvm::vfs::Status Stat)
       : Buffer(std::move(Buffer)), Stat(std::move(Stat)) {}

   static llvm::ErrorOr<std::unique_ptr<llvm::vfs::File>> create(EntryRef Entry);

   llvm::ErrorOr<llvm::vfs::Status> status() override { return Stat; }

   llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>>
   getBuffer(const Twine &Name, int64_t FileSize, bool RequiresNullTerminator,
             bool IsVolatile) override {
     return std::move(Buffer);
   }

   std::error_code close() override { return {}; }

 private:
   std::unique_ptr<llvm::MemoryBuffer> Buffer;
   llvm::vfs::Status Stat;
 };

 } // end anonymous namespace

 llvm::ErrorOr<std::unique_ptr<llvm::vfs::File>>
 DepScanFile::create(EntryRef Entry) {
   assert(!Entry.isError() && "error");

   if (Entry.isDirectory())
     return std::make_error_code(std::errc::is_a_directory);

   auto Result = std::make_unique<DepScanFile>(
       llvm::MemoryBuffer::getMemBuffer(Entry.getContents(),
                                        Entry.getStatus().getName(),
                                        /*RequiresNullTerminator=*/false),
       Entry.getStatus());

   return llvm::ErrorOr<std::unique_ptr<llvm::vfs::File>>(
       std::unique_ptr<llvm::vfs::File>(std::move(Result)));
 }

 llvm::ErrorOr<std::unique_ptr<llvm::vfs::File>>
 DependencyScanningWorkerFilesystem::openFileForRead(const Twine &Path) {
   SmallString<256> OwnedFilename;
   StringRef Filename = Path.toStringRef(OwnedFilename);

   if (Filename.ends_with(".pcm"))
     return getUnderlyingFS().openFileForRead(Path);

   llvm::ErrorOr<EntryRef> Result = getOrCreateFileSystemEntry(Filename);
   if (!Result)
     return Result.getError();
   return DepScanFile::create(Result.get());
 }

 std::error_code
 DependencyScanningWorkerFilesystem::getRealPath(const Twine &Path,
                                                 SmallVectorImpl<char> &Output) {
   SmallString<256> OwnedFilename;
   StringRef OriginalFilename = Path.toStringRef(OwnedFilename);

   SmallString<256> PathBuf;
   auto FilenameForLookup = tryGetFilenameForLookup(OriginalFilename, PathBuf);
   if (!FilenameForLookup)
     return FilenameForLookup.getError();

   auto HandleCachedRealPath =
       [&Output](const CachedRealPath &RealPath) -> std::error_code {
     if (!RealPath)
       return RealPath.getError();
     Output.assign(RealPath->begin(), RealPath->end());
     return {};
   };

   // If we already have the result in local cache, no work required.
   if (const auto *RealPath =
           LocalCache.findRealPathByFilename(*FilenameForLookup))
     return HandleCachedRealPath(*RealPath);

   // If we have the result in the shared cache, cache it locally.
   auto &Shard = SharedCache.getShardForFilename(*FilenameForLookup);
   if (const auto *ShardRealPath =
           Shard.findRealPathByFilename(*FilenameForLookup)) {
     const auto &RealPath = LocalCache.insertRealPathForFilename(
         *FilenameForLookup, *ShardRealPath);
     return HandleCachedRealPath(RealPath);
   }

   // If we don't know the real path, compute it...
   std::error_code EC = getUnderlyingFS().getRealPath(OriginalFilename, Output);
   llvm::ErrorOr<llvm::StringRef> ComputedRealPath = EC;
   if (!EC)
     ComputedRealPath = StringRef{Output.data(), Output.size()};

   // ...and try to write it into the shared cache. In case some other thread won
   // this race and already wrote its own result there, just adopt it. Write
   // whatever is in the shared cache into the local one.
   const auto &RealPath = Shard.getOrEmplaceRealPathForFilename(
       *FilenameForLookup, ComputedRealPath);
   return HandleCachedRealPath(
       LocalCache.insertRealPathForFilename(*FilenameForLookup, RealPath));
 }

 std::error_code DependencyScanningWorkerFilesystem::setCurrentWorkingDirectory(
     const Twine &Path) {
   std::error_code EC = ProxyFileSystem::setCurrentWorkingDirectory(Path);
   updateWorkingDirForCacheLookup();
   return EC;
 }

 void DependencyScanningWorkerFilesystem::updateWorkingDirForCacheLookup() {
   llvm::ErrorOr<std::string> CWD =
       getUnderlyingFS().getCurrentWorkingDirectory();
   if (!CWD) {
     WorkingDirForCacheLookup = CWD.getError();
   } else if (!llvm::sys::path::is_absolute_gnu(*CWD)) {
     WorkingDirForCacheLookup = llvm::errc::invalid_argument;
   } else {
     WorkingDirForCacheLookup = *CWD;
   }
   assert(!WorkingDirForCacheLookup ||
          llvm::sys::path::is_absolute_gnu(*WorkingDirForCacheLookup));
 }

 llvm::ErrorOr<StringRef>
 DependencyScanningWorkerFilesystem::tryGetFilenameForLookup(
     StringRef OriginalFilename, llvm::SmallVectorImpl<char> &PathBuf) const {
   StringRef FilenameForLookup;
   if (llvm::sys::path::is_absolute_gnu(OriginalFilename)) {
     FilenameForLookup = OriginalFilename;
   } else if (!WorkingDirForCacheLookup) {
     return WorkingDirForCacheLookup.getError();
   } else {
     StringRef RelFilename = OriginalFilename;
     RelFilename.consume_front("./");
     PathBuf.assign(WorkingDirForCacheLookup->begin(),
                    WorkingDirForCacheLookup->end());
     llvm::sys::path::append(PathBuf, RelFilename);
     FilenameForLookup = StringRef{PathBuf.begin(), PathBuf.size()};
   }
   assert(llvm::sys::path::is_absolute_gnu(FilenameForLookup));
   return FilenameForLookup;
 }

 const char DependencyScanningWorkerFilesystem::ID = 0;
	//===- DependencyScanningFilesystem.cpp - clang-scan-deps fs --------------===//
	//
	// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
	// See https://llvm.org/LICENSE.txt for license information.
	// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
	//
	//===----------------------------------------------------------------------===//

	#include "clang/Tooling/DependencyScanning/DependencyScanningFilesystem.h"
	#include "llvm/Support/MemoryBuffer.h"
	#include "llvm/Support/SmallVectorMemoryBuffer.h"
	#include "llvm/Support/Threading.h"
	#include <optional>

	using namespace clang;
	using namespace tooling;
	using namespace dependencies;

	llvm::ErrorOr<DependencyScanningWorkerFilesystem::TentativeEntry>
	DependencyScanningWorkerFilesystem::readFile(StringRef Filename) {
	// Load the file and its content from the file system.
	auto MaybeFile = getUnderlyingFS().openFileForRead(Filename);
	if (!MaybeFile)
	return MaybeFile.getError();
	auto File = std::move(*MaybeFile);

	auto MaybeStat = File->status();
	if (!MaybeStat)
	return MaybeStat.getError();
	auto Stat = std::move(*MaybeStat);

	auto MaybeBuffer = File->getBuffer(Stat.getName());
	if (!MaybeBuffer)
	return MaybeBuffer.getError();
	auto Buffer = std::move(*MaybeBuffer);

	// If the file size changed between read and stat, pretend it didn't.
	if (Stat.getSize() != Buffer->getBufferSize())
	Stat = llvm::vfs::Status::copyWithNewSize(Stat, Buffer->getBufferSize());

	return TentativeEntry(Stat, std::move(Buffer));
	}

	bool DependencyScanningWorkerFilesystem::ensureDirectiveTokensArePopulated(
	EntryRef Ref) {
	auto &Entry = Ref.Entry;

	if (Entry.isError() \|\| Entry.isDirectory())
	return false;

	CachedFileContents *Contents = Entry.getCachedContents();
	assert(Contents && "contents not initialized");

	// Double-checked locking.
	if (Contents->DepDirectives.load())
	return true;

	std::lock_guard<std::mutex> GuardLock(Contents->ValueLock);

	// Double-checked locking.
	if (Contents->DepDirectives.load())
	return true;

	SmallVector<dependency_directives_scan::Directive, 64> Directives;
	// Scan the file for preprocessor directives that might affect the
	// dependencies.
	if (scanSourceForDependencyDirectives(Contents->Original->getBuffer(),
	Contents->DepDirectiveTokens,
	Directives)) {
	Contents->DepDirectiveTokens.clear();
	// FIXME: Propagate the diagnostic if desired by the client.
	Contents->DepDirectives.store(new std::optional<DependencyDirectivesTy>());
	return false;
	}

	// This function performed double-checked locking using `DepDirectives`.
	// Assigning it must be the last thing this function does, otherwise other
	// threads may skip the critical section (`DepDirectives != nullptr`), leading
	// to a data race.
	Contents->DepDirectives.store(
	new std::optional<DependencyDirectivesTy>(std::move(Directives)));
	return true;
	}

	DependencyScanningFilesystemSharedCache::
	DependencyScanningFilesystemSharedCache() {
	// This heuristic was chosen using a empirical testing on a
	// reasonably high core machine (iMacPro 18 cores / 36 threads). The cache
	// sharding gives a performance edge by reducing the lock contention.
	// FIXME: A better heuristic might also consider the OS to account for
	// the different cost of lock contention on different OSes.
	NumShards =
	std::max(2u, llvm::hardware_concurrency().compute_thread_count() / 4);
	CacheShards = std::make_unique<CacheShard[]>(NumShards);
	}

	DependencyScanningFilesystemSharedCache::CacheShard &
	DependencyScanningFilesystemSharedCache::getShardForFilename(
	StringRef Filename) const {
	assert(llvm::sys::path::is_absolute_gnu(Filename));
	return CacheShards[llvm::hash_value(Filename) % NumShards];
	}

	DependencyScanningFilesystemSharedCache::CacheShard &
	DependencyScanningFilesystemSharedCache::getShardForUID(
	llvm::sys::fs::UniqueID UID) const {
	auto Hash = llvm::hash_combine(UID.getDevice(), UID.getFile());
	return CacheShards[Hash % NumShards];
	}

	const CachedFileSystemEntry *
	DependencyScanningFilesystemSharedCache::CacheShard::findEntryByFilename(
	StringRef Filename) const {
	assert(llvm::sys::path::is_absolute_gnu(Filename));
	std::lock_guard<std::mutex> LockGuard(CacheLock);
	auto It = CacheByFilename.find(Filename);
	return It == CacheByFilename.end() ? nullptr : It->getValue().first;
	}

	const CachedFileSystemEntry *
	DependencyScanningFilesystemSharedCache::CacheShard::findEntryByUID(
	llvm::sys::fs::UniqueID UID) const {
	std::lock_guard<std::mutex> LockGuard(CacheLock);
	auto It = EntriesByUID.find(UID);
	return It == EntriesByUID.end() ? nullptr : It->getSecond();
	}

	const CachedFileSystemEntry &
	DependencyScanningFilesystemSharedCache::CacheShard::
	getOrEmplaceEntryForFilename(StringRef Filename,
	llvm::ErrorOr<llvm::vfs::Status> Stat) {
	std::lock_guard<std::mutex> LockGuard(CacheLock);
	auto [It, Inserted] = CacheByFilename.insert({Filename, {nullptr, nullptr}});
	auto &[CachedEntry, CachedRealPath] = It->getValue();
	if (!CachedEntry) {
	// The entry is not present in the shared cache. Either the cache doesn't
	// know about the file at all, or it only knows about its real path.
	assert((Inserted \|\| CachedRealPath) && "existing file with empty pair");
	CachedEntry =
	new (EntryStorage.Allocate()) CachedFileSystemEntry(std::move(Stat));
	}
	return *CachedEntry;
	}

	const CachedFileSystemEntry &
	DependencyScanningFilesystemSharedCache::CacheShard::getOrEmplaceEntryForUID(
	llvm::sys::fs::UniqueID UID, llvm::vfs::Status Stat,
	std::unique_ptr<llvm::MemoryBuffer> Contents) {
	std::lock_guard<std::mutex> LockGuard(CacheLock);
	auto [It, Inserted] = EntriesByUID.insert({UID, nullptr});
	auto &CachedEntry = It->getSecond();
	if (Inserted) {
	CachedFileContents *StoredContents = nullptr;
	if (Contents)
	StoredContents = new (ContentsStorage.Allocate())
	CachedFileContents(std::move(Contents));
	CachedEntry = new (EntryStorage.Allocate())
	CachedFileSystemEntry(std::move(Stat), StoredContents);
	}
	return *CachedEntry;
	}

	const CachedFileSystemEntry &
	DependencyScanningFilesystemSharedCache::CacheShard::
	getOrInsertEntryForFilename(StringRef Filename,
	const CachedFileSystemEntry &Entry) {
	std::lock_guard<std::mutex> LockGuard(CacheLock);
	auto [It, Inserted] = CacheByFilename.insert({Filename, {&Entry, nullptr}});
	auto &[CachedEntry, CachedRealPath] = It->getValue();
	if (!Inserted \|\| !CachedEntry)
	CachedEntry = &Entry;
	return *CachedEntry;
	}

	const CachedRealPath *
	DependencyScanningFilesystemSharedCache::CacheShard::findRealPathByFilename(
	StringRef Filename) const {
	assert(llvm::sys::path::is_absolute_gnu(Filename));
	std::lock_guard<std::mutex> LockGuard(CacheLock);
	auto It = CacheByFilename.find(Filename);
	return It == CacheByFilename.end() ? nullptr : It->getValue().second;
	}

	const CachedRealPath &DependencyScanningFilesystemSharedCache::CacheShard::
	getOrEmplaceRealPathForFilename(StringRef Filename,
	llvm::ErrorOr<llvm::StringRef> RealPath) {
	std::lock_guard<std::mutex> LockGuard(CacheLock);

	const CachedRealPath *&StoredRealPath = CacheByFilename[Filename].second;
	if (!StoredRealPath) {
	auto OwnedRealPath = [&]() -> CachedRealPath {
	if (!RealPath)
	return RealPath.getError();
	return RealPath->str();
	}();

	StoredRealPath = new (RealPathStorage.Allocate())
	CachedRealPath(std::move(OwnedRealPath));
	}

	return *StoredRealPath;
	}

	static bool shouldCacheStatFailures(StringRef Filename) {
	StringRef Ext = llvm::sys::path::extension(Filename);
	if (Ext.empty())
	return false; // This may be the module cache directory.
	return true;
	}

	DependencyScanningWorkerFilesystem::DependencyScanningWorkerFilesystem(
	DependencyScanningFilesystemSharedCache &SharedCache,
	IntrusiveRefCntPtr<llvm::vfs::FileSystem> FS)
	: llvm::RTTIExtends<DependencyScanningWorkerFilesystem,
	llvm::vfs::ProxyFileSystem>(std::move(FS)),
	SharedCache(SharedCache),
	WorkingDirForCacheLookup(llvm::errc::invalid_argument) {
	updateWorkingDirForCacheLookup();
	}

	const CachedFileSystemEntry &
	DependencyScanningWorkerFilesystem::getOrEmplaceSharedEntryForUID(
	TentativeEntry TEntry) {
	auto &Shard = SharedCache.getShardForUID(TEntry.Status.getUniqueID());
	return Shard.getOrEmplaceEntryForUID(TEntry.Status.getUniqueID(),
	std::move(TEntry.Status),
	std::move(TEntry.Contents));
	}

	const CachedFileSystemEntry *
	DependencyScanningWorkerFilesystem::findEntryByFilenameWithWriteThrough(
	StringRef Filename) {
	if (const auto *Entry = LocalCache.findEntryByFilename(Filename))
	return Entry;
	auto &Shard = SharedCache.getShardForFilename(Filename);
	if (const auto *Entry = Shard.findEntryByFilename(Filename))
	return &LocalCache.insertEntryForFilename(Filename, *Entry);
	return nullptr;
	}

	llvm::ErrorOr<const CachedFileSystemEntry &>
	DependencyScanningWorkerFilesystem::computeAndStoreResult(
	StringRef OriginalFilename, StringRef FilenameForLookup) {
	llvm::ErrorOr<llvm::vfs::Status> Stat =
	getUnderlyingFS().status(OriginalFilename);
	if (!Stat) {
	if (!shouldCacheStatFailures(OriginalFilename))
	return Stat.getError();
	const auto &Entry =
	getOrEmplaceSharedEntryForFilename(FilenameForLookup, Stat.getError());
	return insertLocalEntryForFilename(FilenameForLookup, Entry);
	}

	if (const auto Entry = findSharedEntryByUID(Stat))
	return insertLocalEntryForFilename(FilenameForLookup, *Entry);

	auto TEntry =
	Stat->isDirectory() ? TentativeEntry(*Stat) : readFile(OriginalFilename);

	const CachedFileSystemEntry *SharedEntry = [&]() {
	if (TEntry) {
	const auto &UIDEntry = getOrEmplaceSharedEntryForUID(std::move(*TEntry));
	return &getOrInsertSharedEntryForFilename(FilenameForLookup, UIDEntry);
	}
	return &getOrEmplaceSharedEntryForFilename(FilenameForLookup,
	TEntry.getError());
	}();

	return insertLocalEntryForFilename(FilenameForLookup, *SharedEntry);
	}

	llvm::ErrorOr<EntryRef>
	DependencyScanningWorkerFilesystem::getOrCreateFileSystemEntry(
	StringRef OriginalFilename) {
	SmallString<256> PathBuf;
	auto FilenameForLookup = tryGetFilenameForLookup(OriginalFilename, PathBuf);
	if (!FilenameForLookup)
	return FilenameForLookup.getError();

	if (const auto *Entry =
	findEntryByFilenameWithWriteThrough(*FilenameForLookup))
	return EntryRef(OriginalFilename, *Entry).unwrapError();
	auto MaybeEntry = computeAndStoreResult(OriginalFilename, *FilenameForLookup);
	if (!MaybeEntry)
	return MaybeEntry.getError();
	return EntryRef(OriginalFilename, *MaybeEntry).unwrapError();
	}

	llvm::ErrorOr<llvm::vfs::Status>
	DependencyScanningWorkerFilesystem::status(const Twine &Path) {
	SmallString<256> OwnedFilename;
	StringRef Filename = Path.toStringRef(OwnedFilename);

	if (Filename.ends_with(".pcm"))
	return getUnderlyingFS().status(Path);

	llvm::ErrorOr<EntryRef> Result = getOrCreateFileSystemEntry(Filename);
	if (!Result)
	return Result.getError();
	return Result->getStatus();
	}

	bool DependencyScanningWorkerFilesystem::exists(const Twine &Path) {
	// While some VFS overlay filesystems may implement more-efficient
	// mechanisms for `exists` queries, `DependencyScanningWorkerFilesystem`
	// typically wraps `RealFileSystem` which does not specialize `exists`,
	// so it is not likely to benefit from such optimizations. Instead,
	// it is more-valuable to have this query go through the
	// cached-`status` code-path of the `DependencyScanningWorkerFilesystem`.
	llvm::ErrorOr<llvm::vfs::Status> Status = status(Path);
	return Status && Status->exists();
	}

	namespace {

	/// The VFS that is used by clang consumes the \c CachedFileSystemEntry using
	/// this subclass.
	class DepScanFile final : public llvm::vfs::File {
	public:
	DepScanFile(std::unique_ptr<llvm::MemoryBuffer> Buffer,
	llvm::vfs::Status Stat)
	: Buffer(std::move(Buffer)), Stat(std::move(Stat)) {}

	static llvm::ErrorOr<std::unique_ptr<llvm::vfs::File>> create(EntryRef Entry);

	llvm::ErrorOr<llvm::vfs::Status> status() override { return Stat; }

	llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>>
	getBuffer(const Twine &Name, int64_t FileSize, bool RequiresNullTerminator,
	bool IsVolatile) override {
	return std::move(Buffer);
	}

	std::error_code close() override { return {}; }

	private:
	std::unique_ptr<llvm::MemoryBuffer> Buffer;
	llvm::vfs::Status Stat;
	};

	} // end anonymous namespace

	llvm::ErrorOr<std::unique_ptr<llvm::vfs::File>>
	DepScanFile::create(EntryRef Entry) {
	assert(!Entry.isError() && "error");

	if (Entry.isDirectory())
	return std::make_error_code(std::errc::is_a_directory);

	auto Result = std::make_unique<DepScanFile>(
	llvm::MemoryBuffer::getMemBuffer(Entry.getContents(),
	Entry.getStatus().getName(),
	/RequiresNullTerminator=/false),
	Entry.getStatus());

	return llvm::ErrorOr<std::unique_ptr<llvm::vfs::File>>(
	std::unique_ptr<llvm::vfs::File>(std::move(Result)));
	}

	llvm::ErrorOr<std::unique_ptr<llvm::vfs::File>>
	DependencyScanningWorkerFilesystem::openFileForRead(const Twine &Path) {
	SmallString<256> OwnedFilename;
	StringRef Filename = Path.toStringRef(OwnedFilename);

	if (Filename.ends_with(".pcm"))
	return getUnderlyingFS().openFileForRead(Path);

	llvm::ErrorOr<EntryRef> Result = getOrCreateFileSystemEntry(Filename);
	if (!Result)
	return Result.getError();
	return DepScanFile::create(Result.get());
	}

	std::error_code
	DependencyScanningWorkerFilesystem::getRealPath(const Twine &Path,
	SmallVectorImpl<char> &Output) {
	SmallString<256> OwnedFilename;
	StringRef OriginalFilename = Path.toStringRef(OwnedFilename);

	SmallString<256> PathBuf;
	auto FilenameForLookup = tryGetFilenameForLookup(OriginalFilename, PathBuf);
	if (!FilenameForLookup)
	return FilenameForLookup.getError();

	auto HandleCachedRealPath =
	[&Output](const CachedRealPath &RealPath) -> std::error_code {
	if (!RealPath)
	return RealPath.getError();
	Output.assign(RealPath->begin(), RealPath->end());
	return {};
	};

	// If we already have the result in local cache, no work required.
	if (const auto *RealPath =
	LocalCache.findRealPathByFilename(*FilenameForLookup))
	return HandleCachedRealPath(*RealPath);

	// If we have the result in the shared cache, cache it locally.
	auto &Shard = SharedCache.getShardForFilename(*FilenameForLookup);
	if (const auto *ShardRealPath =
	Shard.findRealPathByFilename(*FilenameForLookup)) {
	const auto &RealPath = LocalCache.insertRealPathForFilename(
	FilenameForLookup, ShardRealPath);
	return HandleCachedRealPath(RealPath);
	}

	// If we don't know the real path, compute it...
	std::error_code EC = getUnderlyingFS().getRealPath(OriginalFilename, Output);
	llvm::ErrorOr<llvm::StringRef> ComputedRealPath = EC;
	if (!EC)
	ComputedRealPath = StringRef{Output.data(), Output.size()};

	// ...and try to write it into the shared cache. In case some other thread won
	// this race and already wrote its own result there, just adopt it. Write
	// whatever is in the shared cache into the local one.
	const auto &RealPath = Shard.getOrEmplaceRealPathForFilename(
	*FilenameForLookup, ComputedRealPath);
	return HandleCachedRealPath(
	LocalCache.insertRealPathForFilename(*FilenameForLookup, RealPath));
	}

	std::error_code DependencyScanningWorkerFilesystem::setCurrentWorkingDirectory(
	const Twine &Path) {
	std::error_code EC = ProxyFileSystem::setCurrentWorkingDirectory(Path);
	updateWorkingDirForCacheLookup();
	return EC;
	}

	void DependencyScanningWorkerFilesystem::updateWorkingDirForCacheLookup() {
	llvm::ErrorOr<std::string> CWD =
	getUnderlyingFS().getCurrentWorkingDirectory();
	if (!CWD) {
	WorkingDirForCacheLookup = CWD.getError();
	} else if (!llvm::sys::path::is_absolute_gnu(*CWD)) {
	WorkingDirForCacheLookup = llvm::errc::invalid_argument;
	} else {
	WorkingDirForCacheLookup = *CWD;
	}
	assert(!WorkingDirForCacheLookup \|\|
	llvm::sys::path::is_absolute_gnu(*WorkingDirForCacheLookup));
	}

	llvm::ErrorOr<StringRef>
	DependencyScanningWorkerFilesystem::tryGetFilenameForLookup(
	StringRef OriginalFilename, llvm::SmallVectorImpl<char> &PathBuf) const {
	StringRef FilenameForLookup;
	if (llvm::sys::path::is_absolute_gnu(OriginalFilename)) {
	FilenameForLookup = OriginalFilename;
	} else if (!WorkingDirForCacheLookup) {
	return WorkingDirForCacheLookup.getError();
	} else {
	StringRef RelFilename = OriginalFilename;
	RelFilename.consume_front("./");
	PathBuf.assign(WorkingDirForCacheLookup->begin(),
	WorkingDirForCacheLookup->end());
	llvm::sys::path::append(PathBuf, RelFilename);
	FilenameForLookup = StringRef{PathBuf.begin(), PathBuf.size()};
	}
	assert(llvm::sys::path::is_absolute_gnu(FilenameForLookup));
	return FilenameForLookup;
	}

	const char DependencyScanningWorkerFilesystem::ID = 0;