llvm/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/Support/Errc.h"
  15 #include "llvm/Support/FileSystem.h"
  16 #include "llvm/Support/Memory.h"
  17 #include "llvm/Support/TimeProfiler.h"
  18 #include <system_error>
  19
  20 #if !defined(_MSC_VER) && !defined(__MINGW32__)
  21 #include <unistd.h>
  22 #else
  23 #include <io.h>
  24 #endif
  25
  26 using namespace llvm;
  27 using namespace llvm::sys;
  28
  29 namespace {
  30 // A FileOutputBuffer which creates a temporary file in the same directory
  31 // as the final output file. The final output file is atomically replaced
  32 // with the temporary file on commit().
  33 class OnDiskBuffer : public FileOutputBuffer {
  34 public:
  35   OnDiskBuffer(StringRef Path, fs::TempFile Temp, fs::mapped_file_region Buf)
  36       : FileOutputBuffer(Path), Buffer(std::move(Buf)), Temp(std::move(Temp)) {}
  37
  38   uint8_t *getBufferStart() const override { return (uint8_t *)Buffer.data(); }
  39
  40   uint8_t *getBufferEnd() const override {
  41     return (uint8_t *)Buffer.data() + Buffer.size();
  42   }
  43
  44   size_t getBufferSize() const override { return Buffer.size(); }
  45
  46   Error commit() override {
  47     llvm::TimeTraceScope timeScope("Commit buffer to disk");
  48
  49     // Unmap buffer, letting OS flush dirty pages to file on disk.
  50     Buffer.unmap();
  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.unmap();
  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   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, std::size_t BufSize,
  79                  unsigned Mode)
  80       : FileOutputBuffer(Path), Buffer(Buf), BufferSize(BufSize),
  81         Mode(Mode) {}
  82
  83   uint8_t *getBufferStart() const override { return (uint8_t *)Buffer.base(); }
  84
  85   uint8_t *getBufferEnd() const override {
  86     return (uint8_t *)Buffer.base() + BufferSize;
  87   }
  88
  89   size_t getBufferSize() const override { return BufferSize; }
  90
  91   Error commit() override {
  92     if (FinalPath == "-") {
  93       llvm::outs() << StringRef((const char *)Buffer.base(), BufferSize);
  94       llvm::outs().flush();
  95       return Error::success();
  96     }
  97
  98     using namespace sys::fs;
  99     int FD;
 100     std::error_code EC;
 101     if (auto EC =
 102             openFileForWrite(FinalPath, FD, CD_CreateAlways, OF_None, Mode))
 103       return errorCodeToError(EC);
 104     raw_fd_ostream OS(FD, /*shouldClose=*/true, /*unbuffered=*/true);
 105     OS << StringRef((const char *)Buffer.base(), BufferSize);
 106     return Error::success();
 107   }
 108
 109 private:
 110   // Buffer may actually contain a larger memory block than BufferSize
 111   OwningMemoryBlock Buffer;
 112   size_t BufferSize;
 113   unsigned Mode;
 114 };
 115 } // namespace
 116
 117 static Expected<std::unique_ptr<InMemoryBuffer>>
 118 createInMemoryBuffer(StringRef Path, size_t Size, unsigned Mode) {
 119   std::error_code EC;
 120   MemoryBlock MB = Memory::allocateMappedMemory(
 121       Size, nullptr, sys::Memory::MF_READ | sys::Memory::MF_WRITE, EC);
 122   if (EC)
 123     return errorCodeToError(EC);
 124   return std::make_unique<InMemoryBuffer>(Path, MB, Size, Mode);
 125 }
 126
 127 static Expected<std::unique_ptr<FileOutputBuffer>>
 128 createOnDiskBuffer(StringRef Path, size_t Size, unsigned Mode) {
 129   Expected<fs::TempFile> FileOrErr =
 130       fs::TempFile::create(Path + ".tmp%%%%%%%", Mode);
 131   if (!FileOrErr)
 132     return FileOrErr.takeError();
 133   fs::TempFile File = std::move(*FileOrErr);
 134
 135   if (auto EC = fs::resize_file_before_mapping_readwrite(File.FD, Size)) {
 136     consumeError(File.discard());
 137     return errorCodeToError(EC);
 138   }
 139
 140   // Mmap it.
 141   std::error_code EC;
 142   fs::mapped_file_region MappedFile =
 143       fs::mapped_file_region(fs::convertFDToNativeFile(File.FD),
 144                              fs::mapped_file_region::readwrite, Size, 0, EC);
 145
 146   // mmap(2) can fail if the underlying filesystem does not support it.
 147   // If that happens, we fall back to in-memory buffer as the last resort.
 148   if (EC) {
 149     consumeError(File.discard());
 150     return createInMemoryBuffer(Path, Size, Mode);
 151   }
 152
 153   return std::make_unique<OnDiskBuffer>(Path, std::move(File),
 154                                          std::move(MappedFile));
 155 }
 156
 157 // Create an instance of FileOutputBuffer.
 158 Expected<std::unique_ptr<FileOutputBuffer>>
 159 FileOutputBuffer::create(StringRef Path, size_t Size, unsigned Flags) {
 160   // Handle "-" as stdout just like llvm::raw_ostream does.
 161   if (Path == "-")
 162     return createInMemoryBuffer("-", Size, /*Mode=*/0);
 163
 164   unsigned Mode = fs::all_read | fs::all_write;
 165   if (Flags & F_executable)
 166     Mode |= fs::all_exe;
 167
 168   // If Size is zero, don't use mmap which will fail with EINVAL.
 169   if (Size == 0)
 170     return createInMemoryBuffer(Path, Size, Mode);
 171
 172   fs::file_status Stat;
 173   fs::status(Path, Stat);
 174
 175   // Usually, we want to create OnDiskBuffer to create a temporary file in
 176   // the same directory as the destination file and atomically replaces it
 177   // by rename(2).
 178   //
 179   // However, if the destination file is a special file, we don't want to
 180   // use rename (e.g. we don't want to replace /dev/null with a regular
 181   // file.) If that's the case, we create an in-memory buffer, open the
 182   // destination file and write to it on commit().
 183   switch (Stat.type()) {
 184   case fs::file_type::directory_file:
 185     return errorCodeToError(errc::is_a_directory);
 186   case fs::file_type::regular_file:
 187   case fs::file_type::file_not_found:
 188   case fs::file_type::status_error:
 189     if (Flags & F_no_mmap)
 190       return createInMemoryBuffer(Path, Size, Mode);
 191     else
 192       return createOnDiskBuffer(Path, Size, Mode);
 193   default:
 194     return createInMemoryBuffer(Path, Size, Mode);
 195   }
 196 }