UnifiedOnDiskCache.cpp

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//===----------------------------------------------------------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
/// \file
/// Encapsulates \p OnDiskGraphDB and \p OnDiskKeyValueDB instances within one
/// directory while also restricting storage growth with a scheme of chaining
/// the two most recent directories (primary & upstream), where the primary
/// "faults-in" data from the upstream one. When the primary (most recent)
/// directory exceeds its intended limit a new empty directory becomes the
/// primary one.
///
/// Within the top-level directory (the path that \p UnifiedOnDiskCache::open
/// receives) there are directories named like this:
///
/// 'v<version>.<x>'
/// 'v<version>.<x+1>'
/// 'v<version>.<x+2>'
/// ...
///
/// 'version' is the version integer for this \p UnifiedOnDiskCache's scheme and
/// the part after the dot is an increasing integer. The primary directory is
/// the one with the highest integer and the upstream one is the directory
/// before it. For example, if the sub-directories contained are:
///
/// 'v1.5', 'v1.6', 'v1.7', 'v1.8'
///
/// Then the primary one is 'v1.8', the upstream one is 'v1.7', and the rest are
/// unused directories that can be safely deleted at any time and by any
/// process.
///
/// Contained within the top-level directory is a file named "lock" which is
/// used for processes to take shared or exclusive locks for the contents of the
/// top directory. While a \p UnifiedOnDiskCache is open it keeps a shared lock
/// for the top-level directory; when it closes, if the primary sub-directory
/// exceeded its limit, it attempts to get an exclusive lock in order to create
/// a new empty primary directory; if it can't get the exclusive lock it gives
/// up and lets the next \p UnifiedOnDiskCache instance that closes to attempt
/// again.
///
/// The downside of this scheme is that while \p UnifiedOnDiskCache is open on a
/// directory, by any process, the storage size in that directory will keep
/// growing unrestricted. But the major benefit is that garbage-collection can
/// be triggered on a directory concurrently, at any time and by any process,
/// without affecting any active readers/writers in the same process or other
/// processes.
///
/// The \c UnifiedOnDiskCache also provides validation and recovery on top of
/// the underlying on-disk storage. The low-level storage is designed to remain
/// coherent across regular process crashes, but may be invalid after power loss
/// or similar system failures. \c UnifiedOnDiskCache::validateIfNeeded allows
/// validating the contents once per boot and can recover by marking invalid
/// data for garbage collection.
///
/// The data recovery described above requires exclusive access to the CAS, and
/// it is an error to attempt recovery if the CAS is open in any process/thread.
/// In order to maximize backwards compatibility with tools that do not perform
/// validation before opening the CAS, we do not attempt to get exclusive access
/// until recovery is actually performed, meaning as long as the data is valid
/// it will not conflict with concurrent use.
//
//===----------------------------------------------------------------------===//
 
#include "llvm/CAS/UnifiedOnDiskCache.h"
#include "BuiltinCAS.h"
#include "OnDiskCommon.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/ScopeExit.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/CAS/ActionCache.h"
#include "llvm/CAS/OnDiskGraphDB.h"
#include "llvm/CAS/OnDiskKeyValueDB.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Errc.h"
#include "llvm/Support/Error.h"
#include "llvm/Support/FileSystem.h"
#include "llvm/Support/FileUtilities.h"
#include "llvm/Support/MemoryBuffer.h"
#include "llvm/Support/Path.h"
#include "llvm/Support/Program.h"
#include "llvm/Support/raw_ostream.h"
#include <optional>
 
using namespace llvm;
using namespace llvm::cas;
using namespace llvm::cas::ondisk;
 
/// FIXME: When the version of \p DBDirPrefix is bumped up we need to figure out
/// how to handle the leftover sub-directories of the previous version, within
/// the \p UnifiedOnDiskCache::collectGarbage function.
static constexpr StringLiteral DBDirPrefix = "v1.";
 
static constexpr StringLiteral ValidationFilename = "v1.validation";
static constexpr StringLiteral CorruptPrefix = "corrupt.";
 
ObjectID UnifiedOnDiskCache::getObjectIDFromValue(ArrayRef<char> Value) {
  // little endian encoded.
  assert(Value.size() == sizeof(uint64_t));
  return ObjectID::fromOpaqueData(support::endian::read64le(Value.data()));
}
 
UnifiedOnDiskCache::ValueBytes
UnifiedOnDiskCache::getValueFromObjectID(ObjectID ID) {
  // little endian encoded.
  UnifiedOnDiskCache::ValueBytes ValBytes;
  static_assert(ValBytes.size() == sizeof(ID.getOpaqueData()));
  support::endian::write64le(ValBytes.data(), ID.getOpaqueData());
  return ValBytes;
}
 
Expected<std::optional<ArrayRef<char>>>
UnifiedOnDiskCache::faultInFromUpstreamKV(ArrayRef<uint8_t> Key) {
  assert(UpstreamGraphDB);
  assert(UpstreamKVDB);
 
  std::optional<ArrayRef<char>> UpstreamValue;
  if (Error E = UpstreamKVDB->get(Key).moveInto(UpstreamValue))
    return std::move(E);
  if (!UpstreamValue)
    return std::nullopt;
 
  // The value is the \p ObjectID in the context of the upstream
  // \p OnDiskGraphDB instance. Translate it to the context of the primary
  // \p OnDiskGraphDB instance.
  ObjectID UpstreamID = getObjectIDFromValue(*UpstreamValue);
  auto PrimaryID =
      PrimaryGraphDB->getReference(UpstreamGraphDB->getDigest(UpstreamID));
  if (LLVM_UNLIKELY(!PrimaryID))
    return PrimaryID.takeError();
  return PrimaryKVDB->put(Key, getValueFromObjectID(*PrimaryID));
}
 
/// \returns all the 'v<version>.<x>' names of sub-directories, sorted with
/// ascending order of the integer after the dot. Corrupt directories, if
/// included, will come first.
static Expected<SmallVector<std::string, 4>>
getAllDBDirs(StringRef Path, bool IncludeCorrupt = false) {
  struct DBDir {
    uint64_t Order;
    std::string Name;
  };
  SmallVector<DBDir> FoundDBDirs;
 
  std::error_code EC;
  for (sys::fs::directory_iterator DirI(Path, EC), DirE; !EC && DirI != DirE;
       DirI.increment(EC)) {
    if (DirI->type() != sys::fs::file_type::directory_file)
      continue;
    StringRef SubDir = sys::path::filename(DirI->path());
    if (IncludeCorrupt && SubDir.starts_with(CorruptPrefix)) {
      FoundDBDirs.push_back({0, std::string(SubDir)});
      continue;
    }
    if (!SubDir.starts_with(DBDirPrefix))
      continue;
    uint64_t Order;
    if (SubDir.substr(DBDirPrefix.size()).getAsInteger(10, Order))
      return createStringError(inconvertibleErrorCode(),
                               "unexpected directory " + DirI->path());
    FoundDBDirs.push_back({Order, std::string(SubDir)});
  }
  if (EC)
    return createFileError(Path, EC);
 
  llvm::sort(FoundDBDirs, [](const DBDir &LHS, const DBDir &RHS) -> bool {
    return LHS.Order < RHS.Order;
  });
 
  SmallVector<std::string, 4> DBDirs;
  for (DBDir &Dir : FoundDBDirs)
    DBDirs.push_back(std::move(Dir.Name));
  return DBDirs;
}
 
static Expected<SmallVector<std::string, 4>> getAllGarbageDirs(StringRef Path) {
  auto DBDirs = getAllDBDirs(Path, /*IncludeCorrupt=*/true);
  if (!DBDirs)
    return DBDirs.takeError();
 
  // FIXME: When the version of \p DBDirPrefix is bumped up we need to figure
  // out how to handle the leftover sub-directories of the previous version.
 
  for (unsigned Keep = 2; Keep > 0 && !DBDirs->empty(); --Keep) {
    StringRef Back(DBDirs->back());
    if (Back.starts_with(CorruptPrefix))
      break;
    DBDirs->pop_back();
  }
  return *DBDirs;
}
 
/// \returns Given a sub-directory named 'v<version>.<x>', it outputs the
/// 'v<version>.<x+1>' name.
static void getNextDBDirName(StringRef DBDir, llvm::raw_ostream &OS) {
  assert(DBDir.starts_with(DBDirPrefix));
  uint64_t Count;
  bool Failed = DBDir.substr(DBDirPrefix.size()).getAsInteger(10, Count);
  assert(!Failed);
  (void)Failed;
  OS << DBDirPrefix << Count + 1;
}
 
static Error validateOutOfProcess(StringRef LLVMCasBinary, StringRef RootPath,
                                  bool CheckHash) {
  SmallVector<StringRef> Args{LLVMCasBinary, "-cas", RootPath, "-validate"};
  if (CheckHash)
    Args.push_back("-check-hash");
 
  llvm::SmallString<128> StdErrPath;
  int StdErrFD = -1;
  if (std::error_code EC = sys::fs::createTemporaryFile(
          "llvm-cas-validate-stderr", "txt", StdErrFD, StdErrPath,
          llvm::sys::fs::OF_Text))
    return createStringError(EC, "failed to create temporary file");
  FileRemover OutputRemover(StdErrPath.c_str());
 
  std::optional<llvm::StringRef> Redirects[] = {
      {""}, // stdin = /dev/null
      {""}, // stdout = /dev/null
      StdErrPath.str(),
  };
 
  std::string ErrMsg;
  int Result =
      sys::ExecuteAndWait(LLVMCasBinary, Args, /*Env=*/std::nullopt, Redirects,
                          /*SecondsToWait=*/120, /*MemoryLimit=*/0, &ErrMsg);
 
  if (Result == -1)
    return createStringError("failed to exec " + join(Args, " ") + ": " +
                             ErrMsg);
  if (Result != 0) {
    llvm::SmallString<64> Err("cas contents invalid");
    if (!ErrMsg.empty()) {
      Err += ": ";
      Err += ErrMsg;
    }
    auto StdErrBuf = MemoryBuffer::getFile(StdErrPath.c_str());
    if (StdErrBuf && !(*StdErrBuf)->getBuffer().empty()) {
      Err += ": ";
      Err += (*StdErrBuf)->getBuffer();
    }
    return createStringError(Err);
  }
  return Error::success();
}
 
static Error validateInProcess(StringRef RootPath, StringRef HashName,
                               unsigned HashByteSize, bool CheckHash) {
  std::shared_ptr<UnifiedOnDiskCache> UniDB;
  if (Error E = UnifiedOnDiskCache::open(RootPath, std::nullopt, HashName,
                                         HashByteSize)
                    .moveInto(UniDB))
    return E;
  auto CAS = builtin::createObjectStoreFromUnifiedOnDiskCache(UniDB);
  if (Error E = CAS->validate(CheckHash))
    return E;
  auto Cache = builtin::createActionCacheFromUnifiedOnDiskCache(UniDB);
  if (Error E = Cache->validate())
    return E;
  return Error::success();
}
 
Expected<ValidationResult> UnifiedOnDiskCache::validateIfNeeded(
    StringRef RootPath, StringRef HashName, unsigned HashByteSize,
    bool CheckHash, bool AllowRecovery, bool ForceValidation,
    std::optional<StringRef> LLVMCasBinaryPath) {
  if (std::error_code EC = sys::fs::create_directories(RootPath))
    return createFileError(RootPath, EC);
 
  SmallString<256> PathBuf(RootPath);
  sys::path::append(PathBuf, ValidationFilename);
  int FD = -1;
  if (std::error_code EC = sys::fs::openFileForReadWrite(
          PathBuf, FD, sys::fs::CD_OpenAlways, sys::fs::OF_None))
    return createFileError(PathBuf, EC);
  assert(FD != -1);
 
  sys::fs::file_t File = sys::fs::convertFDToNativeFile(FD);
  auto CloseFile = make_scope_exit([&]() { sys::fs::closeFile(File); });
 
  if (std::error_code EC = lockFileThreadSafe(FD, sys::fs::LockKind::Exclusive))
    return createFileError(PathBuf, EC);
  auto UnlockFD = make_scope_exit([&]() { unlockFileThreadSafe(FD); });
 
  SmallString<8> Bytes;
  if (Error E = sys::fs::readNativeFileToEOF(File, Bytes))
    return createFileError(PathBuf, std::move(E));
 
  uint64_t ValidationBootTime = 0;
  if (!Bytes.empty() &&
      StringRef(Bytes).trim().getAsInteger(10, ValidationBootTime))
    return createFileError(PathBuf, errc::illegal_byte_sequence,
                           "expected integer");
 
  static uint64_t BootTime = 0;
  if (BootTime == 0)
    if (Error E = getBootTime().moveInto(BootTime))
      return std::move(E);
 
  if (ValidationBootTime == BootTime && !ForceValidation)
    return ValidationResult::Skipped;
 
  // Validate!
  bool NeedsRecovery = false;
  if (Error E =
          LLVMCasBinaryPath
              ? validateOutOfProcess(*LLVMCasBinaryPath, RootPath, CheckHash)
              : validateInProcess(RootPath, HashName, HashByteSize,
                                  CheckHash)) {
    if (AllowRecovery) {
      consumeError(std::move(E));
      NeedsRecovery = true;
    } else {
      return std::move(E);
    }
  }
 
  if (NeedsRecovery) {
    sys::path::remove_filename(PathBuf);
    sys::path::append(PathBuf, "lock");
 
    int LockFD = -1;
    if (std::error_code EC = sys::fs::openFileForReadWrite(
            PathBuf, LockFD, sys::fs::CD_OpenAlways, sys::fs::OF_None))
      return createFileError(PathBuf, EC);
    sys::fs::file_t LockFile = sys::fs::convertFDToNativeFile(LockFD);
    auto CloseLock = make_scope_exit([&]() { sys::fs::closeFile(LockFile); });
    if (std::error_code EC = tryLockFileThreadSafe(LockFD)) {
      if (EC == std::errc::no_lock_available)
        return createFileError(
            PathBuf, EC,
            "CAS validation requires exclusive access but CAS was in use");
      return createFileError(PathBuf, EC);
    }
    auto UnlockFD = make_scope_exit([&]() { unlockFileThreadSafe(LockFD); });
 
    auto DBDirs = getAllDBDirs(RootPath);
    if (!DBDirs)
      return DBDirs.takeError();
 
    for (StringRef DBDir : *DBDirs) {
      sys::path::remove_filename(PathBuf);
      sys::path::append(PathBuf, DBDir);
      std::error_code EC;
      int Attempt = 0, MaxAttempts = 100;
      SmallString<128> GCPath;
      for (; Attempt < MaxAttempts; ++Attempt) {
        GCPath.assign(RootPath);
        sys::path::append(GCPath, CorruptPrefix + std::to_string(Attempt) +
                                      "." + DBDir);
        EC = sys::fs::rename(PathBuf, GCPath);
        // Darwin uses ENOTEMPTY. Linux may return either ENOTEMPTY or EEXIST.
        if (EC != errc::directory_not_empty && EC != errc::file_exists)
          break;
      }
      if (Attempt == MaxAttempts)
        return createStringError(
            EC, "rename " + PathBuf +
                    " failed: too many CAS directories awaiting pruning");
      if (EC)
        return createStringError(EC, "rename " + PathBuf + " to " + GCPath +
                                         " failed: " + EC.message());
    }
  }
 
  if (ValidationBootTime != BootTime) {
    // Fix filename in case we have error to report.
    sys::path::remove_filename(PathBuf);
    sys::path::append(PathBuf, ValidationFilename);
    if (std::error_code EC = sys::fs::resize_file(FD, 0))
      return createFileError(PathBuf, EC);
    raw_fd_ostream OS(FD, /*shouldClose=*/false);
    OS.seek(0); // resize does not reset position
    OS << BootTime << '\n';
    if (OS.has_error())
      return createFileError(PathBuf, OS.error());
  }
 
  return NeedsRecovery ? ValidationResult::Recovered : ValidationResult::Valid;
}
 
Expected<std::unique_ptr<UnifiedOnDiskCache>>
UnifiedOnDiskCache::open(StringRef RootPath, std::optional<uint64_t> SizeLimit,
                         StringRef HashName, unsigned HashByteSize,
                         OnDiskGraphDB::FaultInPolicy FaultInPolicy) {
  if (std::error_code EC = sys::fs::create_directories(RootPath))
    return createFileError(RootPath, EC);
 
  SmallString<256> PathBuf(RootPath);
  sys::path::append(PathBuf, "lock");
  int LockFD = -1;
  if (std::error_code EC = sys::fs::openFileForReadWrite(
          PathBuf, LockFD, sys::fs::CD_OpenAlways, sys::fs::OF_None))
    return createFileError(PathBuf, EC);
  assert(LockFD != -1);
  // Locking the directory using shared lock, which will prevent other processes
  // from creating a new chain (essentially while a \p UnifiedOnDiskCache
  // instance holds a shared lock the storage for the primary directory will
  // grow unrestricted).
  if (std::error_code EC =
          lockFileThreadSafe(LockFD, sys::fs::LockKind::Shared))
    return createFileError(PathBuf, EC);
 
  auto DBDirs = getAllDBDirs(RootPath);
  if (!DBDirs)
    return DBDirs.takeError();
  if (DBDirs->empty())
    DBDirs->push_back((Twine(DBDirPrefix) + "1").str());
 
  assert(!DBDirs->empty());
 
  /// If there is only one directory open databases on it. If there are 2 or
  /// more directories, get the most recent directories and chain them, with the
  /// most recent being the primary one. The remaining directories are unused
  /// data than can be garbage-collected.
  auto UniDB = std::unique_ptr<UnifiedOnDiskCache>(new UnifiedOnDiskCache());
  std::unique_ptr<OnDiskGraphDB> UpstreamGraphDB;
  std::unique_ptr<OnDiskKeyValueDB> UpstreamKVDB;
  if (DBDirs->size() > 1) {
    StringRef UpstreamDir = *(DBDirs->end() - 2);
    PathBuf = RootPath;
    sys::path::append(PathBuf, UpstreamDir);
    if (Error E = OnDiskGraphDB::open(PathBuf, HashName, HashByteSize,
                                      /*UpstreamDB=*/nullptr, FaultInPolicy)
                      .moveInto(UpstreamGraphDB))
      return std::move(E);
    if (Error E = OnDiskKeyValueDB::open(PathBuf, HashName, HashByteSize,
                                         /*ValueName=*/"objectid",
                                         /*ValueSize=*/sizeof(uint64_t))
                      .moveInto(UpstreamKVDB))
      return std::move(E);
  }
 
  StringRef PrimaryDir = *(DBDirs->end() - 1);
  PathBuf = RootPath;
  sys::path::append(PathBuf, PrimaryDir);
  std::unique_ptr<OnDiskGraphDB> PrimaryGraphDB;
  if (Error E = OnDiskGraphDB::open(PathBuf, HashName, HashByteSize,
                                    UpstreamGraphDB.get(), FaultInPolicy)
                    .moveInto(PrimaryGraphDB))
    return std::move(E);
  std::unique_ptr<OnDiskKeyValueDB> PrimaryKVDB;
  // \p UnifiedOnDiskCache does manual chaining for key-value requests,
  // including an extra translation step of the value during fault-in.
  if (Error E =
          OnDiskKeyValueDB::open(PathBuf, HashName, HashByteSize,
                                 /*ValueName=*/"objectid",
                                 /*ValueSize=*/sizeof(uint64_t), UniDB.get())
              .moveInto(PrimaryKVDB))
    return std::move(E);
 
  UniDB->RootPath = RootPath;
  UniDB->SizeLimit = SizeLimit.value_or(0);
  UniDB->LockFD = LockFD;
  UniDB->NeedsGarbageCollection = DBDirs->size() > 2;
  UniDB->PrimaryDBDir = PrimaryDir;
  UniDB->UpstreamGraphDB = std::move(UpstreamGraphDB);
  UniDB->PrimaryGraphDB = std::move(PrimaryGraphDB);
  UniDB->UpstreamKVDB = std::move(UpstreamKVDB);
  UniDB->PrimaryKVDB = std::move(PrimaryKVDB);
 
  return std::move(UniDB);
}
 
void UnifiedOnDiskCache::setSizeLimit(std::optional<uint64_t> SizeLimit) {
  this->SizeLimit = SizeLimit.value_or(0);
}
 
uint64_t UnifiedOnDiskCache::getStorageSize() const {
  uint64_t TotalSize = getPrimaryStorageSize();
  if (UpstreamGraphDB)
    TotalSize += UpstreamGraphDB->getStorageSize();
  if (UpstreamKVDB)
    TotalSize += UpstreamKVDB->getStorageSize();
  return TotalSize;
}
 
uint64_t UnifiedOnDiskCache::getPrimaryStorageSize() const {
  return PrimaryGraphDB->getStorageSize() + PrimaryKVDB->getStorageSize();
}
 
bool UnifiedOnDiskCache::hasExceededSizeLimit() const {
  uint64_t CurSizeLimit = SizeLimit;
  if (!CurSizeLimit)
    return false;
 
  // If the hard limit is beyond 85%, declare above limit and request clean up.
  unsigned CurrentPercent =
      std::max(PrimaryGraphDB->getHardStorageLimitUtilization(),
               PrimaryKVDB->getHardStorageLimitUtilization());
  if (CurrentPercent > 85)
    return true;
 
  // We allow each of the directories in the chain to reach up to half the
  // intended size limit. Check whether the primary directory has exceeded half
  // the limit or not, in order to decide whether we need to start a new chain.
  //
  // We could check the size limit against the sum of sizes of both the primary
  // and upstream directories but then if the upstream is significantly larger
  // than the intended limit, it would trigger a new chain to be created before
  // the primary has reached its own limit. Essentially in such situation we
  // prefer reclaiming the storage later in order to have more consistent cache
  // hits behavior.
  return (CurSizeLimit / 2) < getPrimaryStorageSize();
}
 
Error UnifiedOnDiskCache::close(bool CheckSizeLimit) {
  if (LockFD == -1)
    return Error::success(); // already closed.
  auto CloseLock = make_scope_exit([&]() {
    assert(LockFD >= 0);
    sys::fs::file_t LockFile = sys::fs::convertFDToNativeFile(LockFD);
    sys::fs::closeFile(LockFile);
    LockFD = -1;
  });
 
  bool ExceededSizeLimit = CheckSizeLimit ? hasExceededSizeLimit() : false;
  UpstreamKVDB.reset();
  PrimaryKVDB.reset();
  UpstreamGraphDB.reset();
  PrimaryGraphDB.reset();
  if (std::error_code EC = unlockFileThreadSafe(LockFD))
    return createFileError(RootPath, EC);
 
  if (!ExceededSizeLimit)
    return Error::success();
 
  // The primary directory exceeded its intended size limit. Try to get an
  // exclusive lock in order to create a new primary directory for next time
  // this \p UnifiedOnDiskCache path is opened.
 
  if (std::error_code EC = tryLockFileThreadSafe(
          LockFD, std::chrono::milliseconds(0), sys::fs::LockKind::Exclusive)) {
    if (EC == errc::no_lock_available)
      return Error::success(); // couldn't get exclusive lock, give up.
    return createFileError(RootPath, EC);
  }
  auto UnlockFile = make_scope_exit([&]() { unlockFileThreadSafe(LockFD); });
 
  // Managed to get an exclusive lock which means there are no other open
  // \p UnifiedOnDiskCache instances for the same path, so we can safely start a
  // new primary directory. To start a new primary directory we just have to
  // create a new empty directory with the next consecutive index; since this is
  // an atomic operation we will leave the top-level directory in a consistent
  // state even if the process dies during this code-path.
 
  SmallString<256> PathBuf(RootPath);
  raw_svector_ostream OS(PathBuf);
  OS << sys::path::get_separator();
  getNextDBDirName(PrimaryDBDir, OS);
  if (std::error_code EC = sys::fs::create_directory(PathBuf))
    return createFileError(PathBuf, EC);
 
  NeedsGarbageCollection = true;
  return Error::success();
}
 
UnifiedOnDiskCache::UnifiedOnDiskCache() = default;
 
UnifiedOnDiskCache::~UnifiedOnDiskCache() { consumeError(close()); }
 
Error UnifiedOnDiskCache::collectGarbage(StringRef Path) {
  auto DBDirs = getAllGarbageDirs(Path);
  if (!DBDirs)
    return DBDirs.takeError();
 
  SmallString<256> PathBuf(Path);
  for (StringRef UnusedSubDir : *DBDirs) {
    sys::path::append(PathBuf, UnusedSubDir);
    if (std::error_code EC = sys::fs::remove_directories(PathBuf))
      return createFileError(PathBuf, EC);
    sys::path::remove_filename(PathBuf);
  }
  return Error::success();
}
 
Error UnifiedOnDiskCache::collectGarbage() { return collectGarbage(RootPath); }