Add an ftruncate call paired with fallocate to play safe with some FSes (namely, BtrFS fallocate sets file size to allocated size, i.e. multiple of block size)

src/libutil/archive.cc

@@ -181,6 +181,8 @@ static void parseContents(ParseSink & sink, Source & source, const Path & path)
         left -= n;
     }
 
+    sink.finalizeContents(size);
+
     readPadding(size, source);
 }
 
@@ -310,6 +312,12 @@ struct RestoreSink : ParseSink
         writeFull(fd, data, len);
     }
 
+    void finalizeContents(unsigned long long size)
+    {
+        errno = ftruncate(fd, size);
+        if (errno) throw SysError(format("truncating file to its allocated length of %1% bytes") % size);
+    }
+
     void createSymlink(const Path & path, const string & target)
     {
         Path p = dstPath + path;