bug fix for skipping zero length record

Problem

One of my Fortran progame may write out a record of an empty array (for example, when no data point was found). When I try to read back the fortran file and skip this empty record using read(f) I got "attempting to read beyond record end" error. Reading the same file in Fortran has no problem.

Possible solution

In the src/withsubrecord.jl:

 49 function advance!( rec::RecordWithSubrecords )                                            
 50    #subreclen, sign = rdmarker(rec) # read trailing record marker                         
 51    #if subreclen != rec.subreclen                                                         
 52    #   fthrow("trailing subrecord marker doesn't match")                                  
 53    #end                                                                                   
 54    #**FIX HERE** advance to the next subrecord only when rec.more is true                                  
 55    if rec.more                                                                            
 56       subreclen, sign = rdmarker(rec) # read trailing record marker                       
 57       if subreclen != rec.subreclen                                                       
 58          fthrow("trailing subrecord marker doesn't match")                                
 59       end                                                                             
 60       subreclen, more = rdmarker(rec) # read next leading subrecord marker                
 61       rec.subleft = rec.subreclen = subreclen                                             
 62       rec.more = more                                                                     
 63    end                                                                                    
 64    nothing                                                                                
 65 end 

...

111 function Base.close( rec::RecordWithSubrecords )
112    if rec.writable
113       @assert rec.totleft == 0
114       @assert rec.subleft == 0
115       @assert !rec.more
116       wrmarker(rec, rec.subreclen, !rec.isfirst)
117    else
118       while rec.subleft > 0
119          skip(rec.io, rec.subleft)
120          rec.subleft = 0
121          advance!(rec)
122       end
123       @assert !rec.more
124       #**FIX HERE** read trailing record marker of the last subrecord
125       subreclen, sign = rdmarker(rec)
126       if subreclen != rec.subreclen
127          fthrow("trailing subrecord marker doesn't match")
128       end
129    end
130    nothing
131 end

Thank you!

Start in src/withsubrecord.jl with RecordWithSubrecords.advance! and Base.close, then reproduce reading a Fortran file containing an empty record with read(f). Trace how subrecord markers are consumed for zero-length records; done means the empty record can be skipped without reading beyond the record end and marker validation still succeeds.