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TarWriter: Emit PAX headers only when needed.

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We use PAX headers to store long filenames (>= 100 bytes).
It is not needed to emit PAX headers if filenames fit in the
Ustar header. This patch implements that optimization.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@291215 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Rui Ueyama 2017-01-06 05:33:45 +00:00
parent e3ce5381d8
commit cf9bb41b9b
1 changed files with 20 additions and 23 deletions
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43
lib/Support/TarWriter.cpp
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@ -9,21 +9,13 @@
//
// 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.
// 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, which is
// apparently too small. Various extensions were proposed and
// implemented to fix the issue. The writer implemented in this file
// uses PAX extension headers.
//
// Note that we emit PAX headers even if filenames fit in the V7
// header for the sake of simplicity. So, generated files are N
// kilobyte larger than the ideal where N is the number of files in
// archives. In practice, I think you don't need to worry about that.
//
// The PAX header is standardized in IEEE Std 1003.1-2001.
// 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
@ -68,8 +60,8 @@ static_assert(sizeof(UstarHeader) == BlockSize, "invalid Ustar header");
// 25 ctime=1084839148.1212\n
//
// This function create such string.
static std::string formatPax(StringRef Key, const Twine &Val) {
int Len = Key.size() + Val.str().size() + 3; // +3 for " ", "=" and "\n"
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.
@ -79,8 +71,7 @@ static std::string formatPax(StringRef Key, const Twine &Val) {
}
// Headers in tar files must be aligned to 512 byte boundaries.
// This function writes null bytes so that the file is a multiple
// of 512 bytes.
// 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));
@ -100,7 +91,7 @@ static void computeChecksum(UstarHeader &Hdr) {
}
// Create a tar header and write it to a given output stream.
static void writePaxHeader(raw_fd_ostream &OS, const Twine &Path) {
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);
@ -120,12 +111,12 @@ static void writePaxHeader(raw_fd_ostream &OS, const Twine &Path) {
// 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, size_t Size) {
static void writeUstarHeader(raw_fd_ostream &OS, StringRef Path, size_t Size) {
UstarHeader Hdr = {};
memcpy(Hdr.Name, Path.data(), Path.size());
strcpy(Hdr.Mode, "0000664");
sprintf(Hdr.Size, "%011lo", Size);
memcpy(Hdr.Magic, "ustar", 6);
computeChecksum(Hdr);
OS << StringRef(reinterpret_cast<char *>(&Hdr), sizeof(Hdr));
}
@ -154,8 +145,14 @@ TarWriter::TarWriter(int FD, StringRef BaseDir)
// Append a given file to an archive.
void TarWriter::append(StringRef Path, StringRef Data) {
// Write Path and Data.
writePaxHeader(OS, BaseDir + "/" + canonicalize(Path));
writeUstarHeader(OS, Data.size());
std::string S = BaseDir + "/" + canonicalize(Path) + "\0";
if (S.size() <= sizeof(UstarHeader::Name)) {
writeUstarHeader(OS, S, Data.size());
} else {
writePaxHeader(OS, S);
writeUstarHeader(OS, "", Data.size());
}
OS << Data;
pad(OS);