use a dynamic buffer for ReadLine

We don't need the buffer that often - only for ReadLine - as it is only
occasionally used, so it is actually more efficient to allocate it if
needed instead of statically by default. It also allows the caller to
influence the buffer size instead of hardcoding it.

Git-Dch: Ignore

apt-pkg/contrib/fileutl.cc

@@ -922,8 +922,8 @@ 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];
+   size_t buffersize_max = 0;
+   std::unique_ptr<char> buffer;
    unsigned long long buffersize = 0;
 public:
    int compressed_fd;
@@ -932,7 +932,7 @@ public:
    APT::Configuration::Compressor compressor;
    unsigned int openmode;
    unsigned long long seekpos;
-   FileFdPrivate(FileFd * const pfilefd) : filefd(pfilefd),
+   FileFdPrivate(FileFd * const pfilefd) : filefd(pfilefd), buffer(nullptr),
       compressed_fd(-1), compressor_pid(-1), is_pipe(false),
       openmode(0), seekpos(0) {};
 
@@ -944,7 +944,7 @@ public:
       {
 	 if (buffersize >= Size)
 	 {
-	    memcpy(To, buffer, Size);
+	    memcpy(To, buffer.get(), Size);
 	    if (buffersize == Size)
 	    {
 	       buffersize = 0;
@@ -952,13 +952,13 @@ public:
 	    else
 	    {
 	       buffersize -= Size;
-	       memmove(buffer, buffer + Size, buffersize);
+	       memmove(buffer.get(), buffer.get() + Size, buffersize);
 	    }
 	    return Size;
 	 }
 	 else
 	 {
-	    memcpy(To, buffer, buffersize);
+	    memcpy(To, buffer.get(), buffersize);
 	    Size -= buffersize;
 	    To = static_cast<char *>(To) + buffersize;
 	    tmp = buffersize;
@@ -982,12 +982,19 @@ public:
       do {
 	 if (buffersize == 0)
 	 {
+	    if (buffer.get() == nullptr)
+	    {
+	       buffer.reset(new char[Size]);
+	       buffersize_max = Size;
+	    }
 	    unsigned long long actualread = 0;
-	    if (filefd->Read(buffer, buffersize_max, &actualread) == false)
+	    if (filefd->Read(buffer.get(), buffersize_max, &actualread) == false)
 	       return nullptr;
 	    buffersize = actualread;
 	    if (buffersize == 0)
 	    {
+	       buffer.reset(nullptr);
+	       buffersize_max = 0;
 	       if (To == InitialTo)
 		  return nullptr;
 	       break;
@@ -996,17 +1003,17 @@ public:
 	 }
 
 	 unsigned long long const OutputSize = std::min(Size, buffersize);
-	 char const * const newline = static_cast<char const * const>(memchr(buffer, '\n', OutputSize));
+	 char const * const newline = static_cast<char const * const>(memchr(buffer.get(), '\n', OutputSize));
 	 if (newline != nullptr)
 	 {
-	    size_t length = (newline - buffer);
+	    size_t length = (newline - buffer.get());
 	    ++length;
-	    memcpy(To, buffer, length);
+	    memcpy(To, buffer.get(), length);
 	    To += length;
 	    if (length < buffersize)
 	    {
 	       buffersize -= length;
-	       memmove(buffer, buffer + length, buffersize);
+	       memmove(buffer.get(), buffer.get() + length, buffersize);
 	    }
 	    else
 	       buffersize = 0;
@@ -1014,11 +1021,11 @@ public:
 	 }
 	 else
 	 {
-	    memcpy(To, buffer, OutputSize);
+	    memcpy(To, buffer.get(), OutputSize);
 	    To += OutputSize;
 	    Size -= OutputSize;
 	    buffersize -= OutputSize;
-	    memmove(buffer, buffer + OutputSize, buffersize);
+	    memmove(buffer.get(), buffer.get() + OutputSize, buffersize);
 	 }
       } while (Size > 0);
       *To = '\0';
@@ -1176,7 +1183,7 @@ public:
       else
       {
 	 buffersize -= Over;
-	 memmove(buffer, buffer + Over, buffersize);
+	 memmove(buffer.get(), buffer.get() + Over, buffersize);
 	 return true;
       }
       if (Over == 0)
@@ -1649,7 +1656,7 @@ public:
       else
       {
 	 buffersize -= Over;
-	 memmove(buffer, buffer + Over, buffersize);
+	 memmove(buffer.get(), buffer.get() + Over, buffersize);
 	 return true;
       }
       if (Over == 0)