implement a buffer system for FileFd::ReadLine

The default implementation of ReadLine was very naive by just reading
each character one-by-one. That is kinda okay for libraries implementing
compression as they have internal buffers (but still not great), but
while working with files directly or via a pipe as there is no buffer
there so all those reads are in fact system calls.

This commit introduces an internal buffer in the FileFd implementation
which is only used by ReadLine. The more low-level Read and all other
actions remain unbuffered – they just changed to deal with potential
"left-overs" in the buffer correctly.

Closes: 808579

apt-pkg/contrib/fileutl.cc

@@ -922,6 +922,9 @@ bool ChangeOwnerAndPermissionOfFile(char const * const requester, char const * c
 class FileFdPrivate {							/*{{{*/
 protected:
    FileFd * const filefd;
+   static size_t constexpr buffersize_max = 1024;
+   char buffer[buffersize_max];
+   unsigned long long buffersize = 0;
 public:
    int compressed_fd;
    pid_t compressor_pid;
@@ -934,25 +937,92 @@ public:
       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)
+   ssize_t InternalRead(void * To, unsigned long long Size)
    {
-      unsigned long long read = 0;
-      while ((Size - 1) != read)
+      unsigned long long tmp = 0;
+      if (buffersize != 0)
       {
-	 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 (buffersize >= Size)
+	 {
+	    memcpy(To, buffer, Size);
+	    if (buffersize == Size)
+	    {
+	       buffersize = 0;
+	    }
+	    else
+	    {
+	       buffersize -= Size;
+	       memmove(buffer, buffer + Size, buffersize);
+	    }
+	    return Size;
+	 }
+	 else
+	 {
+	    memcpy(To, buffer, buffersize);
+	    Size -= buffersize;
+	    To = static_cast<char *>(To) + buffersize;
+	    tmp = buffersize;
+	    buffersize = 0;
+	 }
       }
-      if (read == 0)
-	 return NULL;
-      To[read] = '\0';
-      return To;
+      return InternalUnbufferedRead(To, Size) + tmp;
+   }
+   virtual ssize_t InternalUnbufferedRead(void * const To, unsigned long long const Size) = 0;
+   virtual bool InternalReadError() { return filefd->FileFdErrno("read",_("Read error")); }
+   virtual char * InternalReadLine(char * To, unsigned long long Size)
+   {
+      if (unlikely(Size == 0))
+	 return nullptr;
+      --Size;
+      To[0] = '\0';
+      if (unlikely(Size == 0))
+	 return To;
+      char * const InitialTo = To;
+
+      do {
+	 if (buffersize == 0)
+	 {
+	    unsigned long long actualread = 0;
+	    if (filefd->Read(buffer, buffersize_max, &actualread) == false)
+	       return nullptr;
+	    buffersize = actualread;
+	    if (buffersize == 0)
+	    {
+	       if (To == InitialTo)
+		  return nullptr;
+	       break;
+	    }
+	    filefd->Flags &= ~FileFd::HitEof;
+	 }
+
+	 unsigned long long const OutputSize = std::min(Size, buffersize);
+	 char const * const newline = static_cast<char const * const>(memchr(buffer, '\n', OutputSize));
+	 if (newline != nullptr)
+	 {
+	    size_t length = (newline - buffer);
+	    ++length;
+	    memcpy(To, buffer, length);
+	    To += length;
+	    if (length < buffersize)
+	    {
+	       buffersize -= length;
+	       memmove(buffer, buffer + length, buffersize);
+	    }
+	    else
+	       buffersize = 0;
+	    break;
+	 }
+	 else
+	 {
+	    memcpy(To, buffer, OutputSize);
+	    To += OutputSize;
+	    Size -= OutputSize;
+	    buffersize -= OutputSize;
+	    memmove(buffer, buffer + OutputSize, buffersize);
+	 }
+      } while (Size > 0);
+      *To = '\0';
+      return InitialTo;
    }
    virtual ssize_t InternalWrite(void const * const From, unsigned long long const Size) = 0;
    virtual bool InternalWriteError() { return filefd->FileFdErrno("write",_("Write error")); }
@@ -987,6 +1057,7 @@ public:
       if (filefd->OpenInternDescriptor(openmode, compressor) == false)
 	 return filefd->FileFdError("Seek on file %s because it couldn't be reopened", filefd->FileName.c_str());
 
+      buffersize = 0;
       if (To != 0)
 	 return filefd->Skip(To);
 
@@ -1016,7 +1087,7 @@ public:
       // 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;
+      return seekpos - buffersize;
    }
    virtual unsigned long long InternalSize()
    {
@@ -1058,7 +1129,7 @@ public:
       filefd->Flags |= FileFd::Compressed;
       return gz != nullptr;
    }
-   virtual ssize_t InternalRead(void * const To, unsigned long long const Size) override
+   virtual ssize_t InternalUnbufferedRead(void * const To, unsigned long long const Size) override
    {
       return gzread(gz, To, Size);
    }
@@ -1070,7 +1141,7 @@ public:
 	 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
+   virtual char * InternalReadLine(char * To, unsigned long long Size) override
    {
       return gzgets(gz, To, Size);
    }
@@ -1091,12 +1162,25 @@ public:
       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;
+      buffersize = 0;
       return true;
    }
    virtual bool InternalSkip(unsigned long long Over) override
    {
+      if (Over >= buffersize)
+      {
+	 Over -= buffersize;
+	 buffersize = 0;
+      }
+      else
+      {
+	 buffersize -= Over;
+	 memmove(buffer, buffer + Over, buffersize);
+	 return true;
+      }
+      if (Over == 0)
+	 return true;
       off_t const res = gzseek(gz, Over, SEEK_CUR);
       if (res < 0)
 	 return filefd->FileFdError("Unable to seek ahead %llu",Over);
@@ -1105,7 +1189,7 @@ public:
    }
    virtual unsigned long long InternalTell() override
    {
-      return gztell(gz);
+      return gztell(gz) - buffersize;
    }
    virtual unsigned long long InternalSize() override
    {
@@ -1173,7 +1257,7 @@ public:
       filefd->Flags |= FileFd::Compressed;
       return bz2 != nullptr;
    }
-   virtual ssize_t InternalRead(void * const To, unsigned long long const Size) override
+   virtual ssize_t InternalUnbufferedRead(void * const To, unsigned long long const Size) override
    {
       return BZ2_bzread(bz2, To, Size);
    }
@@ -1332,7 +1416,7 @@ public:
       }
       return true;
    }
-   virtual ssize_t InternalRead(void * const To, unsigned long long const Size) override
+   virtual ssize_t InternalUnbufferedRead(void * const To, unsigned long long const Size) override
    {
       ssize_t Res;
       if (lzma->eof == true)
@@ -1508,7 +1592,7 @@ public:
 
       return true;
    }
-   virtual ssize_t InternalRead(void * const To, unsigned long long const Size) override
+   virtual ssize_t InternalUnbufferedRead(void * const To, unsigned long long const Size) override
    {
       return read(filefd->iFd, To, Size);
    }
@@ -1532,13 +1616,18 @@ 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
+   virtual ssize_t InternalUnbufferedRead(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
    {
+      // files opened read+write are strange and only really "supported" for direct files
+      if (buffersize != 0)
+      {
+	 lseek(filefd->iFd, -buffersize, SEEK_CUR);
+	 buffersize = 0;
+      }
       return write(filefd->iFd, From, Size);
    }
    virtual bool InternalSeek(unsigned long long const To) override
@@ -1547,10 +1636,24 @@ public:
       if (res != (off_t)To)
 	 return filefd->FileFdError("Unable to seek to %llu", To);
       seekpos = To;
+      buffersize = 0;
       return true;
    }
    virtual bool InternalSkip(unsigned long long Over) override
    {
+      if (Over >= buffersize)
+      {
+	 Over -= buffersize;
+	 buffersize = 0;
+      }
+      else
+      {
+	 buffersize -= Over;
+	 memmove(buffer, buffer + Over, buffersize);
+	 return true;
+      }
+      if (Over == 0)
+	 return true;
       off_t const res = lseek(filefd->iFd, Over, SEEK_CUR);
       if (res < 0)
 	 return filefd->FileFdError("Unable to seek ahead %llu",Over);
@@ -1559,13 +1662,23 @@ public:
    }
    virtual bool InternalTruncate(unsigned long long const To) override
    {
+      if (buffersize != 0)
+      {
+	 unsigned long long const seekpos = lseek(filefd->iFd, 0, SEEK_CUR);
+	 if ((seekpos - buffersize) >= To)
+	    buffersize = 0;
+	 else if (seekpos >= To)
+	    buffersize = (To - seekpos);
+	 else
+	    buffersize = 0;
+      }
       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);
+      return lseek(filefd->iFd,0,SEEK_CUR) - buffersize;
    }
    virtual unsigned long long InternalSize() override
    {