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diff --git a/llvm/lib/Support/TarWriter.cpp b/llvm/lib/Support/TarWriter.cpp
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+//===-- TarWriter.cpp - Tar archive file creator --------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// TarWriter class provides a feature to create a tar archive file.
+//
+// I put emphasis on simplicity over comprehensiveness when implementing this
+// class because we don't need a full-fledged archive file generator in LLVM
+// at the moment.
+//
+// The filename field in the Unix V7 tar header is 100 bytes. Longer filenames
+// are stored using the PAX extension. The PAX header is standardized in
+// POSIX.1-2001.
+//
+// The struct definition of UstarHeader is copied from
+// https://www.freebsd.org/cgi/man.cgi?query=tar&sektion=5
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/TarWriter.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/FileSystem.h"
+#include "llvm/Support/MathExtras.h"
+#include "llvm/Support/Path.h"
+
+using namespace llvm;
+
+// Each file in an archive must be aligned to this block size.
+static const int BlockSize = 512;
+
+struct UstarHeader {
+  char Name[100];
+  char Mode[8];
+  char Uid[8];
+  char Gid[8];
+  char Size[12];
+  char Mtime[12];
+  char Checksum[8];
+  char TypeFlag;
+  char Linkname[100];
+  char Magic[6];
+  char Version[2];
+  char Uname[32];
+  char Gname[32];
+  char DevMajor[8];
+  char DevMinor[8];
+  char Prefix[155];
+  char Pad[12];
+};
+static_assert(sizeof(UstarHeader) == BlockSize, "invalid Ustar header");
+
+static UstarHeader makeUstarHeader() {
+  UstarHeader Hdr = {};
+  memcpy(Hdr.Magic, "ustar", 5); // Ustar magic
+  memcpy(Hdr.Version, "00", 2);  // Ustar version
+  return Hdr;
+}
+
+// A PAX attribute is in the form of "<length> <key>=<value>\n"
+// where <length> is the length of the entire string including
+// the length field itself. An example string is this.
+//
+//   25 ctime=1084839148.1212\n
+//
+// This function create such string.
+static std::string formatPax(StringRef Key, StringRef Val) {
+  int Len = Key.size() + Val.size() + 3; // +3 for " ", "=" and "\n"
+
+  // We need to compute total size twice because appending
+  // a length field could change total size by one.
+  int Total = Len + Twine(Len).str().size();
+  Total = Len + Twine(Total).str().size();
+  return (Twine(Total) + " " + Key + "=" + Val + "\n").str();
+}
+
+// Headers in tar files must be aligned to 512 byte boundaries.
+// This function forwards the current file position to the next boundary.
+static void pad(raw_fd_ostream &OS) {
+  uint64_t Pos = OS.tell();
+  OS.seek(alignTo(Pos, BlockSize));
+}
+
+// Computes a checksum for a tar header.
+static void computeChecksum(UstarHeader &Hdr) {
+  // Before computing a checksum, checksum field must be
+  // filled with space characters.
+  memset(Hdr.Checksum, ' ', sizeof(Hdr.Checksum));
+
+  // Compute a checksum and set it to the checksum field.
+  unsigned Chksum = 0;
+  for (size_t I = 0; I < sizeof(Hdr); ++I)
+    Chksum += reinterpret_cast<uint8_t *>(&Hdr)[I];
+  snprintf(Hdr.Checksum, sizeof(Hdr.Checksum), "%06o", Chksum);
+}
+
+// Create a tar header and write it to a given output stream.
+static void writePaxHeader(raw_fd_ostream &OS, StringRef Path) {
+  // A PAX header consists of a 512-byte header followed
+  // by key-value strings. First, create key-value strings.
+  std::string PaxAttr = formatPax("path", Path);
+
+  // Create a 512-byte header.
+  UstarHeader Hdr = makeUstarHeader();
+  snprintf(Hdr.Size, sizeof(Hdr.Size), "%011zo", PaxAttr.size());
+  Hdr.TypeFlag = 'x'; // PAX magic
+  computeChecksum(Hdr);
+
+  // Write them down.
+  OS << StringRef(reinterpret_cast<char *>(&Hdr), sizeof(Hdr));
+  OS << PaxAttr;
+  pad(OS);
+}
+
+// Path fits in a Ustar header if
+//
+// - Path is less than 100 characters long, or
+// - Path is in the form of "<prefix>/<name>" where <prefix> is less
+//   than or equal to 155 characters long and <name> is less than 100
+//   characters long. Both <prefix> and <name> can contain extra '/'.
+//
+// If Path fits in a Ustar header, updates Prefix and Name and returns true.
+// Otherwise, returns false.
+static bool splitUstar(StringRef Path, StringRef &Prefix, StringRef &Name) {
+  if (Path.size() < sizeof(UstarHeader::Name)) {
+    Prefix = "";
+    Name = Path;
+    return true;
+  }
+
+  size_t Sep = Path.rfind('/', sizeof(UstarHeader::Prefix) + 1);
+  if (Sep == StringRef::npos)
+    return false;
+  if (Path.size() - Sep - 1 >= sizeof(UstarHeader::Name))
+    return false;
+
+  Prefix = Path.substr(0, Sep);
+  Name = Path.substr(Sep + 1);
+  return true;
+}
+
+// The PAX header is an extended format, so a PAX header needs
+// to be followed by a "real" header.
+static void writeUstarHeader(raw_fd_ostream &OS, StringRef Prefix,
+                             StringRef Name, size_t Size) {
+  UstarHeader Hdr = makeUstarHeader();
+  memcpy(Hdr.Name, Name.data(), Name.size());
+  memcpy(Hdr.Mode, "0000664", 8);
+  snprintf(Hdr.Size, sizeof(Hdr.Size), "%011zo", Size);
+  memcpy(Hdr.Prefix, Prefix.data(), Prefix.size());
+  computeChecksum(Hdr);
+  OS << StringRef(reinterpret_cast<char *>(&Hdr), sizeof(Hdr));
+}
+
+// Creates a TarWriter instance and returns it.
+Expected<std::unique_ptr<TarWriter>> TarWriter::create(StringRef OutputPath,
+                                                       StringRef BaseDir) {
+  using namespace sys::fs;
+  int FD;
+  if (std::error_code EC =
+          openFileForWrite(OutputPath, FD, CD_CreateAlways, OF_None))
+    return make_error<StringError>("cannot open " + OutputPath, EC);
+  return std::unique_ptr<TarWriter>(new TarWriter(FD, BaseDir));
+}
+
+TarWriter::TarWriter(int FD, StringRef BaseDir)
+    : OS(FD, /*shouldClose=*/true, /*unbuffered=*/false), BaseDir(BaseDir) {}
+
+// Append a given file to an archive.
+void TarWriter::append(StringRef Path, StringRef Data) {
+  // Write Path and Data.
+  std::string Fullpath = BaseDir + "/" + sys::path::convert_to_slash(Path);
+
+  // We do not want to include the same file more than once.
+  if (!Files.insert(Fullpath).second)
+    return;
+
+  StringRef Prefix;
+  StringRef Name;
+  if (splitUstar(Fullpath, Prefix, Name)) {
+    writeUstarHeader(OS, Prefix, Name, Data.size());
+  } else {
+    writePaxHeader(OS, Fullpath);
+    writeUstarHeader(OS, "", "", Data.size());
+  }
+
+  OS << Data;
+  pad(OS);
+
+  // POSIX requires tar archives end with two null blocks.
+  // Here, we write the terminator and then seek back, so that
+  // the file being output is terminated correctly at any moment.
+  uint64_t Pos = OS.tell();
+  OS << std::string(BlockSize * 2, '\0');
+  OS.seek(Pos);
+  OS.flush();
+}