shuffle compressor-specific code into private subclasses

This isn't implementing any new features, it is "just" moving code
around from FileFd methods which decided on each call how to handle the
request by including all logic for all possible compressor backends in
the method body to a model in which backend-specifics are implemented in
a FileFdPrivate subclass. This avoids a big chunk of #ifdef's and should
make it a tiny bit more obvious which backend uses which code.

The execution of the idea is slightly uglified by the need to preserve
ABI and API which causes liberal befriending.

Git-Dch: Ignore

2 files changed, 692 insertions, 635 deletions

diff --git a/apt-pkg/contrib/fileutl.cc b/apt-pkg/contrib/fileutl.cc
index c906a0c9e..06c4beeda 100644
--- a/apt-pkg/contrib/fileutl.cc
+++ b/apt-pkg/contrib/fileutl.cc
@@ -920,119 +920,636 @@ bool ChangeOwnerAndPermissionOfFile(char const * const requester, char const * c
 									/*}}}*/
 
 class FileFdPrivate {							/*{{{*/
-	public:
-#ifdef HAVE_ZLIB
-	gzFile gz;
-#endif
-#ifdef HAVE_BZ2
-	BZFILE* bz2;
-#endif
-#ifdef HAVE_LZMA
-	struct LZMAFILE {
-	   FILE* file;
-	   uint8_t buffer[4096];
-	   lzma_stream stream;
-	   lzma_ret err;
-	   bool eof;
-	   bool compressing;
-
-	   LZMAFILE() : file(NULL), eof(false), compressing(false) { buffer[0] = '\0'; }
-	   ~LZMAFILE() {
-	      if (compressing == true)
-	      {
-		for (;;) {
-			stream.avail_out = sizeof(buffer)/sizeof(buffer[0]);
-			stream.next_out = buffer;
-			err = lzma_code(&stream, LZMA_FINISH);
-			if (err != LZMA_OK && err != LZMA_STREAM_END)
-			{
-				_error->Error("~LZMAFILE: Compress finalisation failed");
-				break;
-			}
-			size_t const n =  sizeof(buffer)/sizeof(buffer[0]) - stream.avail_out;
-			if (n && fwrite(buffer, 1, n, file) != n)
-			{
-				_error->Errno("~LZMAFILE",_("Write error"));
-				break;
-			}
-			if (err == LZMA_STREAM_END)
-				break;
-		}
-	      }
-	      lzma_end(&stream);
-	      fclose(file);
-	   }
-	};
-	LZMAFILE* lzma;
-#endif
-	int compressed_fd;
-	pid_t compressor_pid;
-	bool pipe;
-	APT::Configuration::Compressor compressor;
-	unsigned int openmode;
-	unsigned long long seekpos;
-	FileFdPrivate() :
-#ifdef HAVE_ZLIB
-			  gz(NULL),
-#endif
-#ifdef HAVE_BZ2
-			  bz2(NULL),
-#endif
-#ifdef HAVE_LZMA
-			  lzma(NULL),
-#endif
-			  compressed_fd(-1), compressor_pid(-1), pipe(false),
-			  openmode(0), seekpos(0) {};
-	bool InternalClose(std::string const &FileName)
-	{
-	   if (false)
-	      /* dummy so that the rest can be 'else if's */;
+protected:
+   FileFd * const filefd;
+public:
+   int compressed_fd;
+   pid_t compressor_pid;
+   bool is_pipe;
+   APT::Configuration::Compressor compressor;
+   unsigned int openmode;
+   unsigned long long seekpos;
+   FileFdPrivate(FileFd * const pfilefd) : filefd(pfilefd),
+      compressed_fd(-1), compressor_pid(-1), is_pipe(false),
+      openmode(0), seekpos(0) {};
+
+   virtual bool InternalOpen(int const iFd, unsigned int const Mode) = 0;
+   virtual ssize_t InternalRead(void * const To, unsigned long long const Size) = 0;
+   virtual bool InternalReadError() { return filefd->FileFdErrno("read",_("Read error")); }
+   virtual char * InternalReadLine(char * const To, unsigned long long const Size)
+   {
+      unsigned long long read = 0;
+      while ((Size - 1) != read)
+      {
+	 unsigned long long done = 0;
+	 if (filefd->Read(To + read, 1, &done) == false)
+	    return NULL;
+	 if (done == 0)
+	    break;
+	 if (To[read++] == '\n')
+	    break;
+      }
+      if (read == 0)
+	 return NULL;
+      To[read] = '\0';
+      return To;
+   }
+   virtual ssize_t InternalWrite(void const * const From, unsigned long long const Size) = 0;
+   virtual bool InternalWriteError() { return filefd->FileFdErrno("write",_("Write error")); }
+   virtual bool InternalSeek(unsigned long long const To)
+   {
+      // Our poor man seeking is costly, so try to avoid it
+      unsigned long long const iseekpos = filefd->Tell();
+      if (iseekpos == To)
+	 return true;
+      else if (iseekpos < To)
+	 return filefd->Skip(To - iseekpos);
+
+      if ((openmode & FileFd::ReadOnly) != FileFd::ReadOnly)
+	 return filefd->FileFdError("Reopen is only implemented for read-only files!");
+      InternalClose(filefd->FileName);
+      if (filefd->iFd != -1)
+	 close(filefd->iFd);
+      filefd->iFd = -1;
+      if (filefd->TemporaryFileName.empty() == false)
+	 filefd->iFd = open(filefd->TemporaryFileName.c_str(), O_RDONLY);
+      else if (filefd->FileName.empty() == false)
+	 filefd->iFd = open(filefd->FileName.c_str(), O_RDONLY);
+      else
+      {
+	 if (compressed_fd > 0)
+	    if (lseek(compressed_fd, 0, SEEK_SET) != 0)
+	       filefd->iFd = compressed_fd;
+	 if (filefd->iFd < 0)
+	    return filefd->FileFdError("Reopen is not implemented for pipes opened with FileFd::OpenDescriptor()!");
+      }
+
+      if (filefd->OpenInternDescriptor(openmode, compressor) == false)
+	 return filefd->FileFdError("Seek on file %s because it couldn't be reopened", filefd->FileName.c_str());
+
+      if (To != 0)
+	 return filefd->Skip(To);
+
+      seekpos = To;
+      return true;
+   }
+   virtual bool InternalSkip(unsigned long long Over)
+   {
+      unsigned long long constexpr buffersize = 1024;
+      char buffer[buffersize];
+      while (Over != 0)
+      {
+	 unsigned long long toread = std::min(buffersize, Over);
+	 if (filefd->Read(buffer, toread) == false)
+	    return filefd->FileFdError("Unable to seek ahead %llu",Over);
+	 Over -= toread;
+      }
+      return true;
+   }
+   virtual bool InternalTruncate(unsigned long long const)
+   {
+      return filefd->FileFdError("Truncating compressed files is not implemented (%s)", filefd->FileName.c_str());
+   }
+   virtual unsigned long long InternalTell()
+   {
+      // In theory, we could just return seekpos here always instead of
+      // seeking around, but not all users of FileFd use always Seek() and co
+      // so d->seekpos isn't always true and we can just use it as a hint if
+      // we have nothing else, but not always as an authority…
+      return seekpos;
+   }
+   virtual unsigned long long InternalSize()
+   {
+      unsigned long long size = 0;
+      unsigned long long const oldSeek = filefd->Tell();
+      unsigned long long constexpr ignoresize = 1024;
+      char ignore[ignoresize];
+      unsigned long long read = 0;
+      do {
+	 if (filefd->Read(ignore, ignoresize, &read) == false)
+	 {
+	    filefd->Seek(oldSeek);
+	    return 0;
+	 }
+      } while(read != 0);
+      size = filefd->Tell();
+      filefd->Seek(oldSeek);
+      return size;
+   }
+   virtual bool InternalClose(std::string const &FileName) = 0;
+   virtual bool InternalStream() const { return false; }
+   virtual bool InternalAlwaysAutoClose() const { return true; }
+
+   virtual ~FileFdPrivate() {}
+};
+									/*}}}*/
+class GzipFileFdPrivate: public FileFdPrivate {				/*{{{*/
 #ifdef HAVE_ZLIB
-	   else if (gz != NULL) {
-	      int const e = gzclose(gz);
-	      gz = NULL;
-	      // gzdclose() on empty files always fails with "buffer error" here, ignore that
-	      if (e != 0 && e != Z_BUF_ERROR)
-		 return _error->Errno("close",_("Problem closing the gzip file %s"), FileName.c_str());
-	   }
-#endif
-#ifdef HAVE_BZ2
-	   else if (bz2 != NULL) {
-	      BZ2_bzclose(bz2);
-	      bz2 = NULL;
-	   }
-#endif
-#ifdef HAVE_LZMA
-	   else if (lzma != NULL) {
-	      delete lzma;
-	      lzma = NULL;
-	   }
+public:
+   gzFile gz;
+   virtual bool InternalOpen(int const iFd, unsigned int const Mode) override
+   {
+      if ((Mode & FileFd::ReadWrite) == FileFd::ReadWrite)
+	 gz = gzdopen(iFd, "r+");
+      else if ((Mode & FileFd::WriteOnly) == FileFd::WriteOnly)
+	 gz = gzdopen(iFd, "w");
+      else
+	 gz = gzdopen(iFd, "r");
+      filefd->Flags |= FileFd::Compressed;
+      return gz != nullptr;
+   }
+   virtual ssize_t InternalRead(void * const To, unsigned long long const Size) override
+   {
+      return gzread(gz, To, Size);
+   }
+   virtual bool InternalReadError() override
+   {
+      int err;
+      char const * const errmsg = gzerror(gz, &err);
+      if (err != Z_ERRNO)
+	 return filefd->FileFdError("gzread: %s (%d: %s)", _("Read error"), err, errmsg);
+      return FileFdPrivate::InternalReadError();
+   }
+   virtual char * InternalReadLine(char * const To, unsigned long long const Size) override
+   {
+      return gzgets(gz, To, Size);
+   }
+   virtual ssize_t InternalWrite(void const * const From, unsigned long long const Size) override
+   {
+      return gzwrite(gz,From,Size);
+   }
+   virtual bool InternalWriteError() override
+   {
+      int err;
+      char const * const errmsg = gzerror(gz, &err);
+      if (err != Z_ERRNO)
+	 return filefd->FileFdError("gzwrite: %s (%d: %s)", _("Write error"), err, errmsg);
+      return FileFdPrivate::InternalWriteError();
+   }
+   virtual bool InternalSeek(unsigned long long const To) override
+   {
+      off_t const res = gzseek(gz, To, SEEK_SET);
+      if (res != (off_t)To)
+	 return filefd->FileFdError("Unable to seek to %llu", To);
+
+      seekpos = To;
+      return true;
+   }
+   virtual bool InternalSkip(unsigned long long Over) override
+   {
+      off_t const res = gzseek(gz, Over, SEEK_CUR);
+      if (res < 0)
+	 return filefd->FileFdError("Unable to seek ahead %llu",Over);
+      seekpos = res;
+      return true;
+   }
+   virtual unsigned long long InternalTell() override
+   {
+      return gztell(gz);
+   }
+   virtual unsigned long long InternalSize() override
+   {
+      unsigned long long filesize = FileFdPrivate::InternalSize();
+      // only check gzsize if we are actually a gzip file, just checking for
+      // "gz" is not sufficient as uncompressed files could be opened with
+      // gzopen in "direct" mode as well
+      if (filesize == 0 || gzdirect(gz))
+	 return filesize;
+
+      off_t const oldPos = lseek(filefd->iFd, 0, SEEK_CUR);
+      /* unfortunately zlib.h doesn't provide a gzsize(), so we have to do
+       * this ourselves; the original (uncompressed) file size is the last 32
+       * bits of the file */
+      // FIXME: Size for gz-files is limited by 32bit… no largefile support
+      if (lseek(filefd->iFd, -4, SEEK_END) < 0)
+      {
+	 filefd->FileFdErrno("lseek","Unable to seek to end of gzipped file");
+	 return 0;
+      }
+      uint32_t size = 0;
+      if (read(filefd->iFd, &size, 4) != 4)
+      {
+	 filefd->FileFdErrno("read","Unable to read original size of gzipped file");
+	 return 0;
+      }
+      size = le32toh(size);
+
+      if (lseek(filefd->iFd, oldPos, SEEK_SET) < 0)
+      {
+	 filefd->FileFdErrno("lseek","Unable to seek in gzipped file");
+	 return 0;
+      }
+      return size;
+   }
+   virtual bool InternalClose(std::string const &FileName) override
+   {
+      if (gz == nullptr)
+	 return true;
+      int const e = gzclose(gz);
+      gz = nullptr;
+      // gzdclose() on empty files always fails with "buffer error" here, ignore that
+      if (e != 0 && e != Z_BUF_ERROR)
+	 return _error->Errno("close",_("Problem closing the gzip file %s"), FileName.c_str());
+      return true;
+   }
+
+   GzipFileFdPrivate(FileFd * const filefd) : FileFdPrivate(filefd), gz(nullptr) {}
+   virtual ~GzipFileFdPrivate() { InternalClose(""); }
 #endif
-	   return true;
-	}
-	bool CloseDown(std::string const &FileName)
-	{
-	   bool const Res = InternalClose(FileName);
-
-	   if (compressor_pid > 0)
-	      ExecWait(compressor_pid, "FileFdCompressor", true);
-	   compressor_pid = -1;
-
-	   return Res;
-	}
-	bool InternalStream() const {
-	   return false
+};
+									/*}}}*/
+class Bz2FileFdPrivate: public FileFdPrivate {				/*{{{*/
 #ifdef HAVE_BZ2
-	      || bz2 != NULL
+   BZFILE* bz2;
+public:
+   virtual bool InternalOpen(int const iFd, unsigned int const Mode) override
+   {
+      if ((Mode & FileFd::ReadWrite) == FileFd::ReadWrite)
+	 bz2 = BZ2_bzdopen(iFd, "r+");
+      else if ((Mode & FileFd::WriteOnly) == FileFd::WriteOnly)
+	 bz2 = BZ2_bzdopen(iFd, "w");
+      else
+	 bz2 = BZ2_bzdopen(iFd, "r");
+      filefd->Flags |= FileFd::Compressed;
+      return bz2 != nullptr;
+   }
+   virtual ssize_t InternalRead(void * const To, unsigned long long const Size) override
+   {
+      return BZ2_bzread(bz2, To, Size);
+   }
+   virtual bool InternalReadError() override
+   {
+      int err;
+      char const * const errmsg = BZ2_bzerror(bz2, &err);
+      if (err != BZ_IO_ERROR)
+	 return filefd->FileFdError("BZ2_bzread: %s %s (%d: %s)", filefd->FileName.c_str(), _("Read error"), err, errmsg);
+      return FileFdPrivate::InternalReadError();
+   }
+   virtual ssize_t InternalWrite(void const * const From, unsigned long long const Size) override
+   {
+      return BZ2_bzwrite(bz2, (void*)From, Size);
+   }
+   virtual bool InternalWriteError() override
+   {
+      int err;
+      char const * const errmsg = BZ2_bzerror(bz2, &err);
+      if (err != BZ_IO_ERROR)
+	 return filefd->FileFdError("BZ2_bzwrite: %s %s (%d: %s)", filefd->FileName.c_str(), _("Write error"), err, errmsg);
+      return FileFdPrivate::InternalWriteError();
+   }
+   virtual bool InternalStream() const override { return true; }
+   virtual bool InternalClose(std::string const &) override
+   {
+      if (bz2 == nullptr)
+	 return true;
+      BZ2_bzclose(bz2);
+      bz2 = nullptr;
+      return true;
+   }
+
+   Bz2FileFdPrivate(FileFd * const filefd) : FileFdPrivate(filefd), bz2(nullptr) {}
+   virtual ~Bz2FileFdPrivate() { InternalClose(""); }
 #endif
+};
+									/*}}}*/
+class LzmaFileFdPrivate: public FileFdPrivate {				/*{{{*/
 #ifdef HAVE_LZMA
-	      || lzma != NULL
+   struct LZMAFILE {
+      FILE* file;
+      uint8_t buffer[4096];
+      lzma_stream stream;
+      lzma_ret err;
+      bool eof;
+      bool compressing;
+
+      LZMAFILE() : file(nullptr), eof(false), compressing(false) { buffer[0] = '\0'; }
+      ~LZMAFILE()
+      {
+	 if (compressing == true)
+	 {
+	    size_t constexpr buffersize = sizeof(buffer)/sizeof(buffer[0]);
+	    while(true)
+	    {
+	       stream.avail_out = buffersize;
+	       stream.next_out = buffer;
+	       err = lzma_code(&stream, LZMA_FINISH);
+	       if (err != LZMA_OK && err != LZMA_STREAM_END)
+	       {
+		  _error->Error("~LZMAFILE: Compress finalisation failed");
+		  break;
+	       }
+	       size_t const n =  buffersize - stream.avail_out;
+	       if (n && fwrite(buffer, 1, n, file) != n)
+	       {
+		  _error->Errno("~LZMAFILE",_("Write error"));
+		  break;
+	       }
+	       if (err == LZMA_STREAM_END)
+		  break;
+	    }
+	 }
+	 lzma_end(&stream);
+	 fclose(file);
+      }
+   };
+   LZMAFILE* lzma;
+public:
+   virtual bool InternalOpen(int const iFd, unsigned int const Mode) override
+   {
+      if ((Mode & FileFd::ReadWrite) == FileFd::ReadWrite)
+	 return filefd->FileFdError("ReadWrite mode is not supported for lzma/xz files %s", filefd->FileName.c_str());
+
+      if (lzma == nullptr)
+	 lzma = new LzmaFileFdPrivate::LZMAFILE;
+      if ((Mode & FileFd::WriteOnly) == FileFd::WriteOnly)
+	 lzma->file = fdopen(iFd, "w");
+      else
+	 lzma->file = fdopen(iFd, "r");
+      filefd->Flags |= FileFd::Compressed;
+      if (lzma->file == nullptr)
+	 return false;
+
+      uint32_t const xzlevel = 6;
+      uint64_t const memlimit = UINT64_MAX;
+      lzma_stream tmp_stream = LZMA_STREAM_INIT;
+      lzma->stream = tmp_stream;
+
+      if ((Mode & FileFd::WriteOnly) == FileFd::WriteOnly)
+      {
+	 if (compressor.Name == "xz")
+	 {
+	    if (lzma_easy_encoder(&lzma->stream, xzlevel, LZMA_CHECK_CRC32) != LZMA_OK)
+	       return false;
+	 }
+	 else
+	 {
+	    lzma_options_lzma options;
+	    lzma_lzma_preset(&options, xzlevel);
+	    if (lzma_alone_encoder(&lzma->stream, &options) != LZMA_OK)
+	       return false;
+	 }
+	 lzma->compressing = true;
+      }
+      else
+      {
+	 if (compressor.Name == "xz")
+	 {
+	    if (lzma_auto_decoder(&lzma->stream, memlimit, 0) != LZMA_OK)
+	       return false;
+	 }
+	 else
+	 {
+	    if (lzma_alone_decoder(&lzma->stream, memlimit) != LZMA_OK)
+	       return false;
+	 }
+	 lzma->compressing = false;
+      }
+      return true;
+   }
+   virtual ssize_t InternalRead(void * const To, unsigned long long const Size) override
+   {
+      ssize_t Res;
+      if (lzma->eof == true)
+	 return 0;
+
+      lzma->stream.next_out = (uint8_t *) To;
+      lzma->stream.avail_out = Size;
+      if (lzma->stream.avail_in == 0)
+      {
+	 lzma->stream.next_in = lzma->buffer;
+	 lzma->stream.avail_in = fread(lzma->buffer, 1, sizeof(lzma->buffer)/sizeof(lzma->buffer[0]), lzma->file);
+      }
+      lzma->err = lzma_code(&lzma->stream, LZMA_RUN);
+      if (lzma->err == LZMA_STREAM_END)
+      {
+	 lzma->eof = true;
+	 Res = Size - lzma->stream.avail_out;
+      }
+      else if (lzma->err != LZMA_OK)
+      {
+	 Res = -1;
+	 errno = 0;
+      }
+      else
+      {
+	 Res = Size - lzma->stream.avail_out;
+	 if (Res == 0)
+	 {
+	    // lzma run was okay, but produced no output…
+	    Res = -1;
+	    errno = EINTR;
+	 }
+      }
+      return Res;
+   }
+   virtual bool InternalReadError() override
+   {
+      return filefd->FileFdError("lzma_read: %s (%d)", _("Read error"), lzma->err);
+   }
+   virtual ssize_t InternalWrite(void const * const From, unsigned long long const Size) override
+   {
+      lzma->stream.next_in = (uint8_t *)From;
+      lzma->stream.avail_in = Size;
+      lzma->stream.next_out = lzma->buffer;
+      lzma->stream.avail_out = sizeof(lzma->buffer)/sizeof(lzma->buffer[0]);
+      lzma->err = lzma_code(&lzma->stream, LZMA_RUN);
+      if (lzma->err != LZMA_OK)
+	 return -1;
+      size_t const n = sizeof(lzma->buffer)/sizeof(lzma->buffer[0]) - lzma->stream.avail_out;
+      size_t const m = (n == 0) ? 0 : fwrite(lzma->buffer, 1, n, lzma->file);
+      if (m != n)
+	 return -1;
+      else
+	 return Size - lzma->stream.avail_in;
+   }
+   virtual bool InternalWriteError() override
+   {
+      return filefd->FileFdError("lzma_write: %s (%d)", _("Write error"), lzma->err);
+   }
+   virtual bool InternalStream() const override { return true; }
+   virtual bool InternalClose(std::string const &) override
+   {
+      delete lzma;
+      lzma = nullptr;
+      return true;
+   }
+
+   LzmaFileFdPrivate(FileFd * const filefd) : FileFdPrivate(filefd), lzma(nullptr) {}
+   virtual ~LzmaFileFdPrivate() { InternalClose(""); }
 #endif
-	      ;
-	}
+};
+									/*}}}*/
+class PipedFileFdPrivate: public FileFdPrivate				/*{{{*/
+/* if we don't have a specific class dealing with library calls, we (un)compress
+   by executing a specified binary and pipe in/out what we need */
+{
+public:
+   virtual bool InternalOpen(int const, unsigned int const Mode) override
+   {
+      // collect zombies here in case we reopen
+      if (compressor_pid > 0)
+	 ExecWait(compressor_pid, "FileFdCompressor", true);
+
+      if ((Mode & FileFd::ReadWrite) == FileFd::ReadWrite)
+	 return filefd->FileFdError("ReadWrite mode is not supported for file %s", filefd->FileName.c_str());
+
+      bool const Comp = (Mode & FileFd::WriteOnly) == FileFd::WriteOnly;
+      if (Comp == false)
+      {
+	 // Handle 'decompression' of empty files
+	 struct stat Buf;
+	 fstat(filefd->iFd, &Buf);
+	 if (Buf.st_size == 0 && S_ISFIFO(Buf.st_mode) == false)
+	    return true;
+
+	 // We don't need the file open - instead let the compressor open it
+	 // as he properly knows better how to efficiently read from 'his' file
+	 if (filefd->FileName.empty() == false)
+	 {
+	    close(filefd->iFd);
+	    filefd->iFd = -1;
+	 }
+      }
+
+      // Create a data pipe
+      int Pipe[2] = {-1,-1};
+      if (pipe(Pipe) != 0)
+	 return filefd->FileFdErrno("pipe",_("Failed to create subprocess IPC"));
+      for (int J = 0; J != 2; J++)
+	 SetCloseExec(Pipe[J],true);
+
+      compressed_fd = filefd->iFd;
+      is_pipe = true;
+
+      if (Comp == true)
+	 filefd->iFd = Pipe[1];
+      else
+	 filefd->iFd = Pipe[0];
+
+      // The child..
+      compressor_pid = ExecFork();
+      if (compressor_pid == 0)
+      {
+	 if (Comp == true)
+	 {
+	    dup2(compressed_fd,STDOUT_FILENO);
+	    dup2(Pipe[0],STDIN_FILENO);
+	 }
+	 else
+	 {
+	    if (compressed_fd != -1)
+	       dup2(compressed_fd,STDIN_FILENO);
+	    dup2(Pipe[1],STDOUT_FILENO);
+	 }
+	 int const nullfd = open("/dev/null", O_WRONLY);
+	 if (nullfd != -1)
+	 {
+	    dup2(nullfd,STDERR_FILENO);
+	    close(nullfd);
+	 }
+
+	 SetCloseExec(STDOUT_FILENO,false);
+	 SetCloseExec(STDIN_FILENO,false);
+
+	 std::vector<char const*> Args;
+	 Args.push_back(compressor.Binary.c_str());
+	 std::vector<std::string> const * const addArgs =
+	    (Comp == true) ? &(compressor.CompressArgs) : &(compressor.UncompressArgs);
+	 for (std::vector<std::string>::const_iterator a = addArgs->begin();
+	       a != addArgs->end(); ++a)
+	    Args.push_back(a->c_str());
+	 if (Comp == false && filefd->FileName.empty() == false)
+	 {
+	    // commands not needing arguments, do not need to be told about using standard output
+	    // in reality, only testcases with tools like cat, rev, rot13, … are able to trigger this
+	    if (compressor.CompressArgs.empty() == false && compressor.UncompressArgs.empty() == false)
+	       Args.push_back("--stdout");
+	    if (filefd->TemporaryFileName.empty() == false)
+	       Args.push_back(filefd->TemporaryFileName.c_str());
+	    else
+	       Args.push_back(filefd->FileName.c_str());
+	 }
+	 Args.push_back(NULL);
+
+	 execvp(Args[0],(char **)&Args[0]);
+	 cerr << _("Failed to exec compressor ") << Args[0] << endl;
+	 _exit(100);
+      }
+      if (Comp == true)
+	 close(Pipe[0]);
+      else
+	 close(Pipe[1]);
+
+      return true;
+   }
+   virtual ssize_t InternalRead(void * const To, unsigned long long const Size) override
+   {
+      return read(filefd->iFd, To, Size);
+   }
+   virtual ssize_t InternalWrite(void const * const From, unsigned long long const Size) override
+   {
+      return write(filefd->iFd, From, Size);
+   }
+   virtual bool InternalClose(std::string const &) override
+   {
+      bool Ret = true;
+      if (compressor_pid > 0)
+	 Ret &= ExecWait(compressor_pid, "FileFdCompressor", true);
+      compressor_pid = -1;
+      return Ret;
+   }
+   PipedFileFdPrivate(FileFd * const filefd) : FileFdPrivate(filefd) {}
+   virtual ~PipedFileFdPrivate() { InternalClose(""); }
+};
+									/*}}}*/
+class DirectFileFdPrivate: public FileFdPrivate				/*{{{*/
+{
+public:
+   virtual bool InternalOpen(int const, unsigned int const) override { return true; }
+   virtual ssize_t InternalRead(void * const To, unsigned long long const Size) override
+   {
+      return read(filefd->iFd, To, Size);
+   }
 
+   virtual ssize_t InternalWrite(void const * const From, unsigned long long const Size) override
+   {
+      return write(filefd->iFd, From, Size);
+   }
+   virtual bool InternalSeek(unsigned long long const To) override
+   {
+      off_t const res = lseek(filefd->iFd, To, SEEK_SET);
+      if (res != (off_t)To)
+	 return filefd->FileFdError("Unable to seek to %llu", To);
+      seekpos = To;
+      return true;
+   }
+   virtual bool InternalSkip(unsigned long long Over) override
+   {
+      off_t const res = lseek(filefd->iFd, Over, SEEK_CUR);
+      if (res < 0)
+	 return filefd->FileFdError("Unable to seek ahead %llu",Over);
+      seekpos = res;
+      return true;
+   }
+   virtual bool InternalTruncate(unsigned long long const To) override
+   {
+      if (ftruncate(filefd->iFd, To) != 0)
+	 return filefd->FileFdError("Unable to truncate to %llu",To);
+      return true;
+   }
+   virtual unsigned long long InternalTell() override
+   {
+      return lseek(filefd->iFd,0,SEEK_CUR);
+   }
+   virtual unsigned long long InternalSize() override
+   {
+      return filefd->FileSize();
+   }
+   virtual bool InternalClose(std::string const &) override { return true; }
+   virtual bool InternalAlwaysAutoClose() const override { return false; }
 
-	~FileFdPrivate() { CloseDown(""); }
+   DirectFileFdPrivate(FileFd * const filefd) : FileFdPrivate(filefd) {}
+   virtual ~DirectFileFdPrivate() { InternalClose(""); }
 };
 									/*}}}*/
 // FileFd Constructors							/*{{{*/
@@ -1212,8 +1729,6 @@ bool FileFd::Open(string FileName,unsigned int const Mode,APT::Configuration::Co
 }
 									/*}}}*/
 // FileFd::OpenDescriptor - Open a filedescriptor			/*{{{*/
-// ---------------------------------------------------------------------
-/* */
 bool FileFd::OpenDescriptor(int Fd, unsigned int const Mode, CompressMode Compress, bool AutoClose)
 {
    std::vector<APT::Configuration::Compressor> const compressors = APT::Configuration::getCompressors();
@@ -1271,41 +1786,36 @@ bool FileFd::OpenInternDescriptor(unsigned int const Mode, APT::Configuration::C
 {
    if (iFd == -1)
       return false;
-   if (compressor.Name == "." || compressor.Binary.empty() == true)
-      return true;
 
-#if defined HAVE_ZLIB || defined HAVE_BZ2 || defined HAVE_LZMA
-   // the API to open files is similar, so setup to avoid code duplicates later
-   // and while at it ensure that we close before opening (if its a reopen)
-   void* (*compress_open)(int, const char *) = NULL;
-   if (false)
-      /* dummy so that the rest can be 'else if's */;
-#define APT_COMPRESS_INIT(NAME,OPEN) \
-   else if (compressor.Name == NAME) \
-   { \
-      compress_open = (void*(*)(int, const char *)) OPEN; \
-      if (d != NULL) d->InternalClose(FileName); \
-   }
+   if (d != nullptr)
+      d->InternalClose(FileName);
+
+   if (d == nullptr)
+   {
+      if (false)
+	 /* dummy so that the rest can be 'else if's */;
+#define APT_COMPRESS_INIT(NAME, CONSTRUCTOR) \
+      else if (compressor.Name == NAME) \
+	 d = new CONSTRUCTOR(this)
 #ifdef HAVE_ZLIB
-   APT_COMPRESS_INIT("gzip", gzdopen)
+      APT_COMPRESS_INIT("gzip", GzipFileFdPrivate);
 #endif
 #ifdef HAVE_BZ2
-   APT_COMPRESS_INIT("bzip2", BZ2_bzdopen)
+      APT_COMPRESS_INIT("bzip2", Bz2FileFdPrivate);
 #endif
 #ifdef HAVE_LZMA
-   APT_COMPRESS_INIT("xz", fdopen)
-   APT_COMPRESS_INIT("lzma", fdopen)
+      APT_COMPRESS_INIT("xz", LzmaFileFdPrivate);
+      APT_COMPRESS_INIT("lzma", LzmaFileFdPrivate);
 #endif
 #undef APT_COMPRESS_INIT
-#endif
+      else if (compressor.Name == "." || compressor.Binary.empty() == true)
+	 d = new DirectFileFdPrivate(this);
+      else
+	 d = new PipedFileFdPrivate(this);
 
-   if (d == NULL)
-   {
-      d = new FileFdPrivate();
       d->openmode = Mode;
       d->compressor = compressor;
-#if defined HAVE_ZLIB || defined HAVE_BZ2 || defined HAVE_LZMA
-      if ((Flags & AutoClose) != AutoClose && compress_open != NULL)
+      if ((Flags & AutoClose) != AutoClose && d->InternalAlwaysAutoClose())
       {
 	 // Need to duplicate fd here or gz/bz2 close for cleanup will close the fd as well
 	 int const internFd = dup(iFd);
@@ -1313,178 +1823,8 @@ bool FileFd::OpenInternDescriptor(unsigned int const Mode, APT::Configuration::C
 	    return FileFdErrno("OpenInternDescriptor", _("Could not open file descriptor %d"), iFd);
 	 iFd = internFd;
       }
-#endif
-   }
-
-#if defined HAVE_ZLIB || defined HAVE_BZ2 || defined HAVE_LZMA
-   if (compress_open != NULL)
-   {
-      void* compress_struct = NULL;
-      if ((Mode & ReadWrite) == ReadWrite)
-	 compress_struct = compress_open(iFd, "r+");
-      else if ((Mode & WriteOnly) == WriteOnly)
-	 compress_struct = compress_open(iFd, "w");
-      else
-	 compress_struct = compress_open(iFd, "r");
-      if (compress_struct == NULL)
-	 return false;
-
-      if (false)
-	 /* dummy so that the rest can be 'else if's */;
-#ifdef HAVE_ZLIB
-      else if (compressor.Name == "gzip")
-	 d->gz = (gzFile) compress_struct;
-#endif
-#ifdef HAVE_BZ2
-      else if (compressor.Name == "bzip2")
-	 d->bz2 = (BZFILE*) compress_struct;
-#endif
-#ifdef HAVE_LZMA
-      else if (compressor.Name == "xz" || compressor.Name == "lzma")
-      {
-	 uint32_t const xzlevel = 6;
-	 uint64_t const memlimit = UINT64_MAX;
-	 if (d->lzma == NULL)
-	    d->lzma = new FileFdPrivate::LZMAFILE;
-	 d->lzma->file = (FILE*) compress_struct;
-         lzma_stream tmp_stream = LZMA_STREAM_INIT;
-	 d->lzma->stream = tmp_stream;
-
-	 if ((Mode & ReadWrite) == ReadWrite)
-	    return FileFdError("ReadWrite mode is not supported for file %s", FileName.c_str());
-
-	 if ((Mode & WriteOnly) == WriteOnly)
-	 {
-	    if (compressor.Name == "xz")
-	    {
-	       if (lzma_easy_encoder(&d->lzma->stream, xzlevel, LZMA_CHECK_CRC32) != LZMA_OK)
-		  return false;
-	    }
-	    else
-	    {
-	       lzma_options_lzma options;
-	       lzma_lzma_preset(&options, xzlevel);
-	       if (lzma_alone_encoder(&d->lzma->stream, &options) != LZMA_OK)
-		  return false;
-	    }
-	    d->lzma->compressing = true;
-	 }
-	 else
-	 {
-	    if (compressor.Name == "xz")
-	    {
-	       if (lzma_auto_decoder(&d->lzma->stream, memlimit, 0) != LZMA_OK)
-		  return false;
-	    }
-	    else
-	    {
-	       if (lzma_alone_decoder(&d->lzma->stream, memlimit) != LZMA_OK)
-		  return false;
-	    }
-	    d->lzma->compressing = false;
-	 }
-      }
-#endif
-      Flags |= Compressed;
-      return true;
-   }
-#endif
-
-   // collect zombies here in case we reopen
-   if (d->compressor_pid > 0)
-      ExecWait(d->compressor_pid, "FileFdCompressor", true);
-
-   if ((Mode & ReadWrite) == ReadWrite)
-      return FileFdError("ReadWrite mode is not supported for file %s", FileName.c_str());
-
-   bool const Comp = (Mode & WriteOnly) == WriteOnly;
-   if (Comp == false)
-   {
-      // Handle 'decompression' of empty files
-      struct stat Buf;
-      fstat(iFd, &Buf);
-      if (Buf.st_size == 0 && S_ISFIFO(Buf.st_mode) == false)
-	 return true;
-
-      // We don't need the file open - instead let the compressor open it
-      // as he properly knows better how to efficiently read from 'his' file
-      if (FileName.empty() == false)
-      {
-	 close(iFd);
-	 iFd = -1;
-      }
-   }
-
-   // Create a data pipe
-   int Pipe[2] = {-1,-1};
-   if (pipe(Pipe) != 0)
-      return FileFdErrno("pipe",_("Failed to create subprocess IPC"));
-   for (int J = 0; J != 2; J++)
-      SetCloseExec(Pipe[J],true);
-
-   d->compressed_fd = iFd;
-   d->pipe = true;
-
-   if (Comp == true)
-      iFd = Pipe[1];
-   else
-      iFd = Pipe[0];
-
-   // The child..
-   d->compressor_pid = ExecFork();
-   if (d->compressor_pid == 0)
-   {
-      if (Comp == true)
-      {
-	 dup2(d->compressed_fd,STDOUT_FILENO);
-	 dup2(Pipe[0],STDIN_FILENO);
-      }
-      else
-      {
-	 if (d->compressed_fd != -1)
-	    dup2(d->compressed_fd,STDIN_FILENO);
-	 dup2(Pipe[1],STDOUT_FILENO);
-      }
-      int const nullfd = open("/dev/null", O_WRONLY);
-      if (nullfd != -1)
-      {
-	 dup2(nullfd,STDERR_FILENO);
-	 close(nullfd);
-      }
-
-      SetCloseExec(STDOUT_FILENO,false);
-      SetCloseExec(STDIN_FILENO,false);
-
-      std::vector<char const*> Args;
-      Args.push_back(compressor.Binary.c_str());
-      std::vector<std::string> const * const addArgs =
-		(Comp == true) ? &(compressor.CompressArgs) : &(compressor.UncompressArgs);
-      for (std::vector<std::string>::const_iterator a = addArgs->begin();
-	   a != addArgs->end(); ++a)
-	 Args.push_back(a->c_str());
-      if (Comp == false && FileName.empty() == false)
-      {
-	 // commands not needing arguments, do not need to be told about using standard output
-	 // in reality, only testcases with tools like cat, rev, rot13, … are able to trigger this
-	 if (compressor.CompressArgs.empty() == false && compressor.UncompressArgs.empty() == false)
-	    Args.push_back("--stdout");
-	 if (TemporaryFileName.empty() == false)
-	    Args.push_back(TemporaryFileName.c_str());
-	 else
-	    Args.push_back(FileName.c_str());
-      }
-      Args.push_back(NULL);
-
-      execvp(Args[0],(char **)&Args[0]);
-      cerr << _("Failed to exec compressor ") << Args[0] << endl;
-      _exit(100);
    }
-   if (Comp == true)
-      close(Pipe[0]);
-   else
-      close(Pipe[1]);
-
-   return true;
+   return d->InternalOpen(iFd, Mode);
 }
 									/*}}}*/
 // FileFd::~File - Closes the file					/*{{{*/
@@ -1495,7 +1835,7 @@ FileFd::~FileFd()
 {
    Close();
    if (d != NULL)
-      d->CloseDown(FileName);
+      d->InternalClose(FileName);
    delete d;
    d = NULL;
 }
@@ -1506,6 +1846,8 @@ FileFd::~FileFd()
    gracefully. */
 bool FileFd::Read(void *To,unsigned long long Size,unsigned long long *Actual)
 {
+   if (d == nullptr)
+      return false;
    ssize_t Res = 1;
    errno = 0;
    if (Actual != 0)
@@ -1513,54 +1855,7 @@ bool FileFd::Read(void *To,unsigned long long Size,unsigned long long *Actual)
    *((char *)To) = '\0';
    while (Res > 0 && Size > 0)
    {
-      if (false)
-	 /* dummy so that the rest can be 'else if's */;
-#ifdef HAVE_ZLIB
-      else if (d != NULL && d->gz != NULL)
-	 Res = gzread(d->gz,To,Size);
-#endif
-#ifdef HAVE_BZ2
-      else if (d != NULL && d->bz2 != NULL)
-	 Res = BZ2_bzread(d->bz2,To,Size);
-#endif
-#ifdef HAVE_LZMA
-      else if (d != NULL && d->lzma != NULL)
-      {
-	 if (d->lzma->eof == true)
-	    break;
-
-	 d->lzma->stream.next_out = (uint8_t *) To;
-	 d->lzma->stream.avail_out = Size;
-	 if (d->lzma->stream.avail_in == 0)
-	 {
-	    d->lzma->stream.next_in = d->lzma->buffer;
-	    d->lzma->stream.avail_in = fread(d->lzma->buffer, 1, sizeof(d->lzma->buffer)/sizeof(d->lzma->buffer[0]), d->lzma->file);
-	 }
-	 d->lzma->err = lzma_code(&d->lzma->stream, LZMA_RUN);
-	 if (d->lzma->err == LZMA_STREAM_END)
-	 {
-	    d->lzma->eof = true;
-	    Res = Size - d->lzma->stream.avail_out;
-	 }
-	 else if (d->lzma->err != LZMA_OK)
-	 {
-	    Res = -1;
-	    errno = 0;
-	 }
-	 else
-	 {
-	    Res = Size - d->lzma->stream.avail_out;
-	    if (Res == 0)
-	    {
-	       // lzma run was okay, but produced no output…
-	       Res = -1;
-	       errno = EINTR;
-	    }
-	 }
-      }
-#endif
-      else
-         Res = read(iFd,To,Size);
+      Res = d->InternalRead(To, Size);
 
       if (Res < 0)
       {
@@ -1571,31 +1866,7 @@ bool FileFd::Read(void *To,unsigned long long Size,unsigned long long *Actual)
 	    errno = 0;
 	    continue;
 	 }
-	 if (false)
-	    /* dummy so that the rest can be 'else if's */;
-#ifdef HAVE_ZLIB
-	 else if (d != NULL && d->gz != NULL)
-	 {
-	    int err;
-	    char const * const errmsg = gzerror(d->gz, &err);
-	    if (err != Z_ERRNO)
-	       return FileFdError("gzread: %s (%d: %s)", _("Read error"), err, errmsg);
-	 }
-#endif
-#ifdef HAVE_BZ2
-	 else if (d != NULL && d->bz2 != NULL)
-	 {
-	    int err;
-	    char const * const errmsg = BZ2_bzerror(d->bz2, &err);
-	    if (err != BZ_IO_ERROR)
-	       return FileFdError("BZ2_bzread: %s %s (%d: %s)", FileName.c_str(), _("Read error"), err, errmsg);
-	 }
-#endif
-#ifdef HAVE_LZMA
-	 else if (d != NULL && d->lzma != NULL)
-	    return FileFdError("lzma_read: %s (%d)", _("Read error"), d->lzma->err);
-#endif
-	 return FileFdErrno("read",_("Read error"));
+	 return d->InternalReadError();
       }
       
       To = (char *)To + Res;
@@ -1621,110 +1892,37 @@ bool FileFd::Read(void *To,unsigned long long Size,unsigned long long *Actual)
 									/*}}}*/
 // FileFd::ReadLine - Read a complete line from the file		/*{{{*/
 // ---------------------------------------------------------------------
-/* Beware: This method can be quiet slow for big buffers on UNcompressed
+/* Beware: This method can be quite slow for big buffers on UNcompressed
    files because of the naive implementation! */
 char* FileFd::ReadLine(char *To, unsigned long long const Size)
 {
    *To = '\0';
-#ifdef HAVE_ZLIB
-   if (d != NULL && d->gz != NULL)
-      return gzgets(d->gz, To, Size);
-#endif
-
-   unsigned long long read = 0;
-   while ((Size - 1) != read)
-   {
-      unsigned long long done = 0;
-      if (Read(To + read, 1, &done) == false)
-	 return NULL;
-      if (done == 0)
-	 break;
-      if (To[read++] == '\n')
-	 break;
-   }
-   if (read == 0)
-      return NULL;
-   To[read] = '\0';
-   return To;
+   if (d == nullptr)
+      return nullptr;
+   return d->InternalReadLine(To, Size);
 }
 									/*}}}*/
 // FileFd::Write - Write to the file					/*{{{*/
-// ---------------------------------------------------------------------
-/* */
 bool FileFd::Write(const void *From,unsigned long long Size)
 {
+   if (d == nullptr)
+      return false;
    ssize_t Res = 1;
    errno = 0;
    while (Res > 0 && Size > 0)
    {
-      if (false)
-	 /* dummy so that the rest can be 'else if's */;
-#ifdef HAVE_ZLIB
-      else if (d != NULL && d->gz != NULL)
-	 Res = gzwrite(d->gz,From,Size);
-#endif
-#ifdef HAVE_BZ2
-      else if (d != NULL && d->bz2 != NULL)
-	 Res = BZ2_bzwrite(d->bz2,(void*)From,Size);
-#endif
-#ifdef HAVE_LZMA
-      else if (d != NULL && d->lzma != NULL)
-      {
-	 d->lzma->stream.next_in = (uint8_t *)From;
-	 d->lzma->stream.avail_in = Size;
-	 d->lzma->stream.next_out = d->lzma->buffer;
-	 d->lzma->stream.avail_out = sizeof(d->lzma->buffer)/sizeof(d->lzma->buffer[0]);
-	 d->lzma->err = lzma_code(&d->lzma->stream, LZMA_RUN);
-	 if (d->lzma->err != LZMA_OK)
-	    return false;
-	 size_t const n = sizeof(d->lzma->buffer)/sizeof(d->lzma->buffer[0]) - d->lzma->stream.avail_out;
-	 size_t const m = (n == 0) ? 0 : fwrite(d->lzma->buffer, 1, n, d->lzma->file);
-	 if (m != n)
-	    Res = -1;
-	 else
-	    Res = Size - d->lzma->stream.avail_in;
-      }
-#endif
-      else
-	 Res = write(iFd,From,Size);
-
+      Res = d->InternalWrite(From, Size);
       if (Res < 0 && errno == EINTR)
 	 continue;
       if (Res < 0)
-      {
-	 if (false)
-	    /* dummy so that the rest can be 'else if's */;
-#ifdef HAVE_ZLIB
-	 else if (d != NULL && d->gz != NULL)
-	 {
-	    int err;
-	    char const * const errmsg = gzerror(d->gz, &err);
-	    if (err != Z_ERRNO)
-	       return FileFdError("gzwrite: %s (%d: %s)", _("Write error"), err, errmsg);
-	 }
-#endif
-#ifdef HAVE_BZ2
-	 else if (d != NULL && d->bz2 != NULL)
-	 {
-	    int err;
-	    char const * const errmsg = BZ2_bzerror(d->bz2, &err);
-	    if (err != BZ_IO_ERROR)
-	       return FileFdError("BZ2_bzwrite: %s (%d: %s)", _("Write error"), err, errmsg);
-	 }
-#endif
-#ifdef HAVE_LZMA
-	 else if (d != NULL && d->lzma != NULL)
-	    return FileFdErrno("lzma_fwrite", _("Write error"));
-#endif
-	 return FileFdErrno("write",_("Write error"));
-      }
-      
+	 return d->InternalWriteError();
+
       From = (char const *)From + Res;
       Size -= Res;
       if (d != NULL)
 	 d->seekpos += Res;
    }
-   
+
    if (Size == 0)
       return true;
 
@@ -1753,117 +1951,31 @@ bool FileFd::Write(int Fd, const void *From, unsigned long long Size)
 }
 									/*}}}*/
 // FileFd::Seek - Seek in the file					/*{{{*/
-// ---------------------------------------------------------------------
-/* */
 bool FileFd::Seek(unsigned long long To)
 {
+   if (d == nullptr)
+      return false;
    Flags &= ~HitEof;
-
-   if (d != NULL && (d->pipe == true || d->InternalStream() == true))
-   {
-      // Our poor man seeking in pipes is costly, so try to avoid it
-      unsigned long long seekpos = Tell();
-      if (seekpos == To)
-	 return true;
-      else if (seekpos < To)
-	 return Skip(To - seekpos);
-
-      if ((d->openmode & ReadOnly) != ReadOnly)
-	 return FileFdError("Reopen is only implemented for read-only files!");
-      d->InternalClose(FileName);
-      if (iFd != -1)
-	 close(iFd);
-      iFd = -1;
-      if (TemporaryFileName.empty() == false)
-	 iFd = open(TemporaryFileName.c_str(), O_RDONLY);
-      else if (FileName.empty() == false)
-	 iFd = open(FileName.c_str(), O_RDONLY);
-      else
-      {
-	 if (d->compressed_fd > 0)
-	    if (lseek(d->compressed_fd, 0, SEEK_SET) != 0)
-	       iFd = d->compressed_fd;
-	 if (iFd < 0)
-	    return FileFdError("Reopen is not implemented for pipes opened with FileFd::OpenDescriptor()!");
-      }
-
-      if (OpenInternDescriptor(d->openmode, d->compressor) == false)
-	 return FileFdError("Seek on file %s because it couldn't be reopened", FileName.c_str());
-
-      if (To != 0)
-	 return Skip(To);
-
-      d->seekpos = To;
-      return true;
-   }
-   off_t res;
-#ifdef HAVE_ZLIB
-   if (d != NULL && d->gz)
-      res = gzseek(d->gz,To,SEEK_SET);
-   else
-#endif
-      res = lseek(iFd,To,SEEK_SET);
-   if (res != (off_t)To)
-      return FileFdError("Unable to seek to %llu", To);
-
-   if (d != NULL)
-      d->seekpos = To;
-   return true;
+   return d->InternalSeek(To);
 }
 									/*}}}*/
-// FileFd::Skip - Seek in the file					/*{{{*/
-// ---------------------------------------------------------------------
-/* */
+// FileFd::Skip - Skip over data in the file				/*{{{*/
 bool FileFd::Skip(unsigned long long Over)
 {
-   if (d != NULL && (d->pipe == true || d->InternalStream() == true))
-   {
-      char buffer[1024];
-      while (Over != 0)
-      {
-	 unsigned long long toread = std::min((unsigned long long) sizeof(buffer), Over);
-	 if (Read(buffer, toread) == false)
-	    return FileFdError("Unable to seek ahead %llu",Over);
-	 Over -= toread;
-      }
-      return true;
-   }
-
-   off_t res;
-#ifdef HAVE_ZLIB
-   if (d != NULL && d->gz != NULL)
-      res = gzseek(d->gz,Over,SEEK_CUR);
-   else
-#endif
-      res = lseek(iFd,Over,SEEK_CUR);
-   if (res < 0)
-      return FileFdError("Unable to seek ahead %llu",Over);
-   if (d != NULL)
-      d->seekpos = res;
-
-   return true;
+   if (d == nullptr)
+      return false;
+   return d->InternalSkip(Over);
 }
 									/*}}}*/
-// FileFd::Truncate - Truncate the file 				/*{{{*/
-// ---------------------------------------------------------------------
-/* */
+// FileFd::Truncate - Truncate the file					/*{{{*/
 bool FileFd::Truncate(unsigned long long To)
 {
+   if (d == nullptr)
+      return false;
    // truncating /dev/null is always successful - as we get an error otherwise
    if (To == 0 && FileName == "/dev/null")
       return true;
-#if defined HAVE_ZLIB || defined HAVE_BZ2 || defined HAVE_LZMA
-   if (d != NULL && (d->InternalStream() == true
-#ifdef HAVE_ZLIB
-	    || d->gz != NULL
-#endif
-	    ))
-      return FileFdError("Truncating compressed files is not implemented (%s)", FileName.c_str());
-#endif
-   if (ftruncate(iFd,To) != 0)
-      return FileFdError("Unable to truncate to %llu",To);
-
-   return true;
+   return d->InternalTruncate(To);
 }
 									/*}}}*/
 // FileFd::Tell - Current seek position					/*{{{*/
@@ -1871,30 +1983,18 @@ bool FileFd::Truncate(unsigned long long To)
 /* */
 unsigned long long FileFd::Tell()
 {
-   // In theory, we could just return seekpos here always instead of
-   // seeking around, but not all users of FileFd use always Seek() and co
-   // so d->seekpos isn't always true and we can just use it as a hint if
-   // we have nothing else, but not always as an authority…
-   if (d != NULL && (d->pipe == true || d->InternalStream() == true))
-      return d->seekpos;
-
-   off_t Res;
-#ifdef HAVE_ZLIB
-   if (d != NULL && d->gz != NULL)
-     Res = gztell(d->gz);
-   else
-#endif
-     Res = lseek(iFd,0,SEEK_CUR);
+   if (d == nullptr)
+      return false;
+   off_t const Res = d->InternalTell();
    if (Res == (off_t)-1)
       FileFdErrno("lseek","Failed to determine the current file position");
-   if (d != NULL)
-      d->seekpos = Res;
+   d->seekpos = Res;
    return Res;
 }
 									/*}}}*/
 static bool StatFileFd(char const * const msg, int const iFd, std::string const &FileName, struct stat &Buf, FileFdPrivate * const d) /*{{{*/
 {
-   bool ispipe = (d != NULL && d->pipe == true);
+   bool ispipe = (d != NULL && d->is_pipe == true);
    if (ispipe == false)
    {
       if (fstat(iFd,&Buf) != 0)
@@ -1911,7 +2011,7 @@ static bool StatFileFd(char const * const msg, int const iFd, std::string const
       // we set it here, too, as we get the info here for free
       // in theory the Open-methods should take care of it already
       if (d != NULL)
-	 d->pipe = true;
+	 d->is_pipe = true;
       if (stat(FileName.c_str(), &Buf) != 0)
 	 return _error->Errno("fstat", "Unable to determine %s for file %s", msg, FileName.c_str());
    }
@@ -1943,64 +2043,11 @@ time_t FileFd::ModificationTime()
 }
 									/*}}}*/
 // FileFd::Size - Return the size of the content in the file		/*{{{*/
-// ---------------------------------------------------------------------
-/* */
 unsigned long long FileFd::Size()
 {
-   unsigned long long size = FileSize();
-
-   // for compressor pipes st_size is undefined and at 'best' zero,
-   // so we 'read' the content and 'seek' back - see there
-   if (d != NULL && (d->pipe == true || (d->InternalStream() == true && size > 0)))
-   {
-      unsigned long long const oldSeek = Tell();
-      char ignore[1000];
-      unsigned long long read = 0;
-      do {
-	 if (Read(ignore, sizeof(ignore), &read) == false)
-	 {
-	    Seek(oldSeek);
-	    return 0;
-	 }
-      } while(read != 0);
-      size = Tell();
-      Seek(oldSeek);
-   }
-#ifdef HAVE_ZLIB
-   // only check gzsize if we are actually a gzip file, just checking for
-   // "gz" is not sufficient as uncompressed files could be opened with
-   // gzopen in "direct" mode as well
-   else if (d != NULL && d->gz && !gzdirect(d->gz) && size > 0)
-   {
-       off_t const oldPos = lseek(iFd,0,SEEK_CUR);
-       /* unfortunately zlib.h doesn't provide a gzsize(), so we have to do
-	* this ourselves; the original (uncompressed) file size is the last 32
-	* bits of the file */
-       // FIXME: Size for gz-files is limited by 32bit… no largefile support
-       if (lseek(iFd, -4, SEEK_END) < 0)
-       {
-	  FileFdErrno("lseek","Unable to seek to end of gzipped file");
-	  return 0;
-       }
-       uint32_t size = 0;
-       if (read(iFd, &size, 4) != 4)
-       {
-	  FileFdErrno("read","Unable to read original size of gzipped file");
-	  return 0;
-       }
-       size = le32toh(size);
-
-       if (lseek(iFd, oldPos, SEEK_SET) < 0)
-       {
-	  FileFdErrno("lseek","Unable to seek in gzipped file");
-	  return 0;
-       }
-
-       return size;
-   }
-#endif
-
-   return size;
+   if (d == nullptr)
+      return false;
+   return d->InternalSize();
 }
 									/*}}}*/
 // FileFd::Close - Close the file if the close flag is set		/*{{{*/
@@ -2020,7 +2067,7 @@ bool FileFd::Close()
 
    if (d != NULL)
    {
-      Res &= d->CloseDown(FileName);
+      Res &= d->InternalClose(FileName);
       delete d;
       d = NULL;
    }
@@ -2086,14 +2133,18 @@ bool FileFd::FileFdError(const char *Description,...) {
    return false;
 }
 									/*}}}*/
-
-gzFile FileFd::gzFd() {
+gzFile FileFd::gzFd() {							/*{{{*/
 #ifdef HAVE_ZLIB
-   return d->gz;
+   GzipFileFdPrivate * const gzipd = dynamic_cast<GzipFileFdPrivate*>(d);
+   if (gzipd == nullptr)
+      return nullptr;
+   else
+      return gzipd->gz;
 #else
-   return NULL;
+   return nullptr;
 #endif
 }
+									/*}}}*/
 
 // Glob - wrapper around "glob()"					/*{{{*/
 std::vector<std::string> Glob(std::string const &pattern, int flags)
diff --git a/apt-pkg/contrib/fileutl.h b/apt-pkg/contrib/fileutl.h
index 5809bf1de..57d3ab841 100644
--- a/apt-pkg/contrib/fileutl.h
+++ b/apt-pkg/contrib/fileutl.h
@@ -41,6 +41,12 @@ using std::string;
 class FileFdPrivate;
 class FileFd
 {
+   friend class FileFdPrivate;
+   friend class GzipFileFdPrivate;
+   friend class Bz2FileFdPrivate;
+   friend class LzmaFileFdPrivate;
+   friend class DirectFileFdPrivate;
+   friend class PipedFileFdPrivate;
    protected:
    int iFd;