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//===-- TarWriter.cpp - Tar archive file creator --------------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// 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");
// 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 = {};
snprintf(Hdr.Size, sizeof(Hdr.Size), "%011zo", PaxAttr.size());
Hdr.TypeFlag = 'x'; // PAX magic
memcpy(Hdr.Magic, "ustar", 6); // Ustar magic
computeChecksum(Hdr);
// Write them down.
OS << StringRef(reinterpret_cast<char *>(&Hdr), sizeof(Hdr));
OS << PaxAttr;
pad(OS);
}
// In the Ustar header, a path can be split at any '/' to store
// a path into UstarHeader::Name and UstarHeader::Prefix. This
// function splits a given path for that purpose.
static std::pair<StringRef, StringRef> splitPath(StringRef Path) {
if (Path.size() <= sizeof(UstarHeader::Name))
return {"", Path};
size_t Sep = Path.rfind('/', sizeof(UstarHeader::Name) + 1);
if (Sep == StringRef::npos)
return {"", Path};
return {Path.substr(0, Sep), Path.substr(Sep + 1)};
}
// Returns true if a given path can be stored to a Ustar header
// without the PAX extension.
static bool fitsInUstar(StringRef Path) {
StringRef Prefix;
StringRef Name;
std::tie(Prefix, Name) = splitPath(Path);
return Name.size() <= sizeof(UstarHeader::Name);
}
// 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 Path, size_t Size) {
StringRef Prefix;
StringRef Name;
std::tie(Prefix, Name) = splitPath(Path);
UstarHeader Hdr = {};
memcpy(Hdr.Name, Name.data(), Name.size());
memcpy(Hdr.Mode, "0000664", 8);
snprintf(Hdr.Size, sizeof(Hdr.Size), "%011zo", Size);
memcpy(Hdr.Magic, "ustar", 6);
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) {
int FD;
if (std::error_code EC = openFileForWrite(OutputPath, FD, sys::fs::F_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 S = BaseDir + "/" + sys::path::convert_to_slash(Path) + "\0";
if (fitsInUstar(S)) {
writeUstarHeader(OS, S, Data.size());
} else {
writePaxHeader(OS, S);
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();
}
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