io: 64bit bytecount fix for Array and TString.

TString::WriteString was missing the push of the start position on top the stack (necessary
as the pop is unconditional in SetByteCount).

The startpos in TArray::ReadArray and TString::ReadString can become ULong64_t without any
issue.  If the length happens to be the same value as the 32bits marker ()
then it will be 'also' found on the stack

Same update for the (reading part of) TContainerConverters.cxx.

Author: pcanal

core/base/src/TString.cxx

@@ -1373,7 +1373,7 @@ TString *TString::ReadString(TBuffer &b, const TClass *clReq)
    b.InitMap();
 
    // Before reading object save start position
-   UInt_t startpos = UInt_t(b.Length());
+   ULong64_t startpos = ULong64_t(b.Length());
 
    ULong64_t tag;
    TClass *clRef = b.ReadClass(clReq, &tag);
@@ -1444,17 +1444,16 @@ void TString::WriteString(TBuffer &b, const TString *a)
 
    } else {
 
+      TClass *cl = a->IsA();
+
       // Reserve space for leading byte count
-      UInt_t cntpos = UInt_t(b.Length());
-      b.SetBufferOffset(Int_t(cntpos+sizeof(UInt_t)));
+      TBuffer::ByteCountWriter bcnt(b, cl);
 
-      TClass *cl = a->IsA();
       b.WriteClass(cl);
 
       ((TString *)a)->Streamer(b);
 
-      // Write byte count
-      b.SetByteCount(cntpos);
+      // The byte count is written automatically by the ByteCountWriter destructor
    }
 }
 

core/cont/src/TArray.cxx

@@ -45,7 +45,7 @@ TArray *TArray::ReadArray(TBuffer &b, const TClass *clReq)
    b.InitMap();
 
    // Before reading object save start position
-   UInt_t startpos = UInt_t(b.Length());
+   ULong64_t startpos = ULong64_t(b.Length());
 
    ULong64_t tag;
    TClass *clRef = b.ReadClass(clReq, &tag);

io/io/src/TContainerConverters.cxx

@@ -102,7 +102,7 @@ void TConvertClonesArrayToProxy::operator()(TBuffer &b, void *pmember, Int_t siz
          b.InitMap();
 
          // before reading object save start position
-         UInt_t startpos = b.Length();
+         ULong64_t startpos = b.Length();
 
          // attempt to load next object as TClass clCast
          ULong64_t tag;       // either tag or byte count