lib/Support/FileOutputBuffer.cpp

   1 //===- FileOutputBuffer.cpp - File Output Buffer ----------------*- C++ -*-===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // Utility for creating a in-memory buffer that will be written to a file.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #include "llvm/Support/FileOutputBuffer.h"
  14 #include "llvm/ADT/STLExtras.h"
  15 #include "llvm/ADT/SmallString.h"
  16 #include "llvm/Support/Errc.h"
  17 #include "llvm/Support/Memory.h"
  18 #include "llvm/Support/Path.h"
  19 #include <system_error>
  20
  21 #if !defined(_MSC_VER) && !defined(__MINGW32__)
  22 #include <unistd.h>
  23 #else
  24 #include <io.h>
  25 #endif
  26
  27 using namespace llvm;
  28 using namespace llvm::sys;
  29
  30 namespace {
  31 // A FileOutputBuffer which creates a temporary file in the same directory
  32 // as the final output file. The final output file is atomically replaced
  33 // with the temporary file on commit().
  34 class OnDiskBuffer : public FileOutputBuffer {
  35 public:
  36   OnDiskBuffer(StringRef Path, fs::TempFile Temp,
  37                std::unique_ptr<fs::mapped_file_region> Buf)
  38       : FileOutputBuffer(Path), Buffer(std::move(Buf)), Temp(std::move(Temp)) {}
  39
  40   uint8_t *getBufferStart() const override { return (uint8_t *)Buffer->data(); }
  41
  42   uint8_t *getBufferEnd() const override {
  43     return (uint8_t *)Buffer->data() + Buffer->size();
  44   }
  45
  46   size_t getBufferSize() const override { return Buffer->size(); }
  47
  48   Error commit() override {
  49     // Unmap buffer, letting OS flush dirty pages to file on disk.
  50     Buffer.reset();
  51
  52     // Atomically replace the existing file with the new one.
  53     return Temp.keep(FinalPath);
  54   }
  55
  56   ~OnDiskBuffer() override {
  57     // Close the mapping before deleting the temp file, so that the removal
  58     // succeeds.
  59     Buffer.reset();
  60     consumeError(Temp.discard());
  61   }
  62
  63   void discard() override {
  64     // Delete the temp file if it still was open, but keeping the mapping
  65     // active.
  66     consumeError(Temp.discard());
  67   }
  68
  69 private:
  70   std::unique_ptr<fs::mapped_file_region> Buffer;
  71   fs::TempFile Temp;
  72 };
  73
  74 // A FileOutputBuffer which keeps data in memory and writes to the final
  75 // output file on commit(). This is used only when we cannot use OnDiskBuffer.
  76 class InMemoryBuffer : public FileOutputBuffer {
  77 public:
  78   InMemoryBuffer(StringRef Path, MemoryBlock Buf, unsigned Mode)
  79       : FileOutputBuffer(Path), Buffer(Buf), Mode(Mode) {}
  80
  81   uint8_t *getBufferStart() const override { return (uint8_t *)Buffer.base(); }
  82
  83   uint8_t *getBufferEnd() const override {
  84     return (uint8_t *)Buffer.base() + Buffer.size();
  85   }
  86
  87   size_t getBufferSize() const override { return Buffer.size(); }
  88
  89   Error commit() override {
  90     if (FinalPath == "-") {
  91       llvm::outs() << StringRef((const char *)Buffer.base(), Buffer.size());
  92       llvm::outs().flush();
  93       return Error::success();
  94     }
  95
  96     using namespace sys::fs;
  97     int FD;
  98     std::error_code EC;
  99     if (auto EC =
 100             openFileForWrite(FinalPath, FD, CD_CreateAlways, OF_None, Mode))
 101       return errorCodeToError(EC);
 102     raw_fd_ostream OS(FD, /*shouldClose=*/true, /*unbuffered=*/true);
 103     OS << StringRef((const char *)Buffer.base(), Buffer.size());
 104     return Error::success();
 105   }
 106
 107 private:
 108   OwningMemoryBlock Buffer;
 109   unsigned Mode;
 110 };
 111 } // namespace
 112
 113 static Expected<std::unique_ptr<InMemoryBuffer>>
 114 createInMemoryBuffer(StringRef Path, size_t Size, unsigned Mode) {
 115   std::error_code EC;
 116   MemoryBlock MB = Memory::allocateMappedMemory(
 117       Size, nullptr, sys::Memory::MF_READ | sys::Memory::MF_WRITE, EC);
 118   if (EC)
 119     return errorCodeToError(EC);
 120   return llvm::make_unique<InMemoryBuffer>(Path, MB, Mode);
 121 }
 122
 123 static Expected<std::unique_ptr<FileOutputBuffer>>
 124 createOnDiskBuffer(StringRef Path, size_t Size, unsigned Mode) {
 125   Expected<fs::TempFile> FileOrErr =
 126       fs::TempFile::create(Path + ".tmp%%%%%%%", Mode);
 127   if (!FileOrErr)
 128     return FileOrErr.takeError();
 129   fs::TempFile File = std::move(*FileOrErr);
 130
 131 #ifndef _WIN32
 132   // On Windows, CreateFileMapping (the mmap function on Windows)
 133   // automatically extends the underlying file. We don't need to
 134   // extend the file beforehand. _chsize (ftruncate on Windows) is
 135   // pretty slow just like it writes specified amount of bytes,
 136   // so we should avoid calling that function.
 137   if (auto EC = fs::resize_file(File.FD, Size)) {
 138     consumeError(File.discard());
 139     return errorCodeToError(EC);
 140   }
 141 #endif
 142
 143   // Mmap it.
 144   std::error_code EC;
 145   auto MappedFile = llvm::make_unique<fs::mapped_file_region>(
 146       File.FD, fs::mapped_file_region::readwrite, Size, 0, EC);
 147
 148   // mmap(2) can fail if the underlying filesystem does not support it.
 149   // If that happens, we fall back to in-memory buffer as the last resort.
 150   if (EC) {
 151     consumeError(File.discard());
 152     return createInMemoryBuffer(Path, Size, Mode);
 153   }
 154
 155   return llvm::make_unique<OnDiskBuffer>(Path, std::move(File),
 156                                          std::move(MappedFile));
 157 }
 158
 159 // Create an instance of FileOutputBuffer.
 160 Expected<std::unique_ptr<FileOutputBuffer>>
 161 FileOutputBuffer::create(StringRef Path, size_t Size, unsigned Flags) {
 162   // Handle "-" as stdout just like llvm::raw_ostream does.
 163   if (Path == "-")
 164     return createInMemoryBuffer("-", Size, /*Mode=*/0);
 165
 166   unsigned Mode = fs::all_read | fs::all_write;
 167   if (Flags & F_executable)
 168     Mode |= fs::all_exe;
 169
 170   fs::file_status Stat;
 171   fs::status(Path, Stat);
 172
 173   // Usually, we want to create OnDiskBuffer to create a temporary file in
 174   // the same directory as the destination file and atomically replaces it
 175   // by rename(2).
 176   //
 177   // However, if the destination file is a special file, we don't want to
 178   // use rename (e.g. we don't want to replace /dev/null with a regular
 179   // file.) If that's the case, we create an in-memory buffer, open the
 180   // destination file and write to it on commit().
 181   switch (Stat.type()) {
 182   case fs::file_type::directory_file:
 183     return errorCodeToError(errc::is_a_directory);
 184   case fs::file_type::regular_file:
 185   case fs::file_type::file_not_found:
 186   case fs::file_type::status_error:
 187     return createOnDiskBuffer(Path, Size, Mode);
 188   default:
 189     return createInMemoryBuffer(Path, Size, Mode);
 190   }
 191 }