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