lib/LTO/Caching.cpp

//===-Caching.cpp - LLVM Link Time Optimizer Cache Handling ---------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// This file implements the Caching for ThinLTO.
//
//===----------------------------------------------------------------------===//

#include "llvm/LTO/Caching.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/Support/Errc.h"
#include "llvm/Support/MemoryBuffer.h"
#include "llvm/Support/Path.h"
#include "llvm/Support/Process.h"
#include "llvm/Support/raw_ostream.h"

#if !defined(_MSC_VER) && !defined(__MINGW32__)
#include <unistd.h>
#else
#include <io.h>
#endif

using namespace llvm;
using namespace llvm::lto;

Expected<NativeObjectCache> lto::localCache(StringRef CacheDirectoryPath,
                                            AddBufferFn AddBuffer) {
  if (std::error_code EC = sys::fs::create_directories(CacheDirectoryPath))
    return errorCodeToError(EC);

  return [=](unsigned Task, StringRef Key) -> AddStreamFn {
    // This choice of file name allows the cache to be pruned (see pruneCache()
    // in include/llvm/Support/CachePruning.h).
    SmallString<64> EntryPath;
    sys::path::append(EntryPath, CacheDirectoryPath, "llvmcache-" + Key);
    // First, see if we have a cache hit.
    int FD;
    SmallString<64> ResultPath;
    std::error_code EC = sys::fs::openFileForRead(
        Twine(EntryPath), FD, sys::fs::OF_UpdateAtime, &ResultPath);
    if (!EC) {
      ErrorOr<std::unique_ptr<MemoryBuffer>> MBOrErr =
          MemoryBuffer::getOpenFile(FD, EntryPath,
                                    /*FileSize*/ -1,
                                    /*RequiresNullTerminator*/ false);
      close(FD);
      if (MBOrErr) {
        AddBuffer(Task, std::move(*MBOrErr));
        return AddStreamFn();
      }
      EC = MBOrErr.getError();
    }

    // On Windows we can fail to open a cache file with a permission denied
    // error. This generally means that another process has requested to delete
    // the file while it is still open, but it could also mean that another
    // process has opened the file without the sharing permissions we need.
    // Since the file is probably being deleted we handle it in the same way as
    // if the file did not exist at all.
    if (EC != errc::no_such_file_or_directory && EC != errc::permission_denied)
      report_fatal_error(Twine("Failed to open cache file ") + EntryPath +
                         ": " + EC.message() + "\n");

    // This native object stream is responsible for commiting the resulting
    // file to the cache and calling AddBuffer to add it to the link.
    struct CacheStream : NativeObjectStream {
      AddBufferFn AddBuffer;
      sys::fs::TempFile TempFile;
      std::string EntryPath;
      unsigned Task;

      CacheStream(std::unique_ptr<raw_pwrite_stream> OS, AddBufferFn AddBuffer,
                  sys::fs::TempFile TempFile, std::string EntryPath,
                  unsigned Task)
          : NativeObjectStream(std::move(OS)), AddBuffer(std::move(AddBuffer)),
            TempFile(std::move(TempFile)), EntryPath(std::move(EntryPath)),
            Task(Task) {}

      ~CacheStream() {
        // Make sure the stream is closed before committing it.
        OS.reset();

        // Open the file first to avoid racing with a cache pruner.
        ErrorOr<std::unique_ptr<MemoryBuffer>> MBOrErr =
            MemoryBuffer::getOpenFile(TempFile.FD, TempFile.TmpName,
                                      /*FileSize*/ -1,
                                      /*RequiresNullTerminator*/ false);
        if (!MBOrErr)
          report_fatal_error(Twine("Failed to open new cache file ") +
                             TempFile.TmpName + ": " +
                             MBOrErr.getError().message() + "\n");

        // On POSIX systems, this will atomically replace the destination if
        // it already exists. We try to emulate this on Windows, but this may
        // fail with a permission denied error (for example, if the destination
        // is currently opened by another process that does not give us the
        // sharing permissions we need). Since the existing file should be
        // semantically equivalent to the one we are trying to write, we give
        // AddBuffer a copy of the bytes we wrote in that case. We do this
        // instead of just using the existing file, because the pruner might
        // delete the file before we get a chance to use it.
        Error E = TempFile.keep(EntryPath);
        E = handleErrors(std::move(E), [&](const ECError &E) -> Error {
          std::error_code EC = E.convertToErrorCode();
          if (EC != errc::permission_denied)
            return errorCodeToError(EC);

          auto MBCopy = MemoryBuffer::getMemBufferCopy((*MBOrErr)->getBuffer(),
                                                       EntryPath);
          MBOrErr = std::move(MBCopy);

          // FIXME: should we consume the discard error?
          consumeError(TempFile.discard());

          return Error::success();
        });

        if (E)
          report_fatal_error(Twine("Failed to rename temporary file ") +
                             TempFile.TmpName + " to " + EntryPath + ": " +
                             toString(std::move(E)) + "\n");

        AddBuffer(Task, std::move(*MBOrErr));
      }
    };

    return [=](size_t Task) -> std::unique_ptr<NativeObjectStream> {
      // Write to a temporary to avoid race condition
      SmallString<64> TempFilenameModel;
      sys::path::append(TempFilenameModel, CacheDirectoryPath, "Thin-%%%%%%.tmp.o");
      Expected<sys::fs::TempFile> Temp = sys::fs::TempFile::create(
          TempFilenameModel, sys::fs::owner_read | sys::fs::owner_write);
      if (!Temp) {
        errs() << "Error: " << toString(Temp.takeError()) << "\n";
        report_fatal_error("ThinLTO: Can't get a temporary file");
      }

      // This CacheStream will move the temporary file into the cache when done.
      return llvm::make_unique<CacheStream>(
          llvm::make_unique<raw_fd_ostream>(Temp->FD, /* ShouldClose */ false),
          AddBuffer, std::move(*Temp), EntryPath.str(), Task);
    };
  };
}