Allow producing forward compatible file for fBits value.

config/rootrc.in

@@ -476,6 +476,12 @@ RSA.KeyType:  1
 # Enable cross-protocol redirects
 TFile.CrossProtocolRedirects:  yes
 
+# Force the producing of files forward compatible with (unpatched) version
+# of ROOT older than v6.30 by recording the internal bits kIsOnHeap and
+# kNotDeleted; Older releases were not explicitly setting those bits to the
+# correct value but instead used verbatim the value stored in the file.
+# TFile.v630forwardCompatibility: no
+
 # List of S3 servers known to support multi-range HTTP GET requests.
 # This is the value sent back by the S3 server in the 'Server:' header
 # of the HTTP response.

core/base/src/TObject.cxx

@@ -913,14 +913,25 @@ void TObject::Streamer(TBuffer &R__b)
       }
    } else {
       R__b.WriteVersion(TObject::IsA());
+      // Can not read TFile.h here and avoid going through the interpreter by
+      // simply hard-coding this value.
+      // This **must** be equal to TFile::k630forwardCompatibility
+      constexpr int TFile__k630forwardCompatibility = BIT(2);
+      const auto parent = R__b.GetParent();
       if (!TestBit(kIsReferenced)) {
          R__b << fUniqueID;
-         R__b << (fBits & (~kIsOnHeap & ~kNotDeleted));
+         if (R__unlikely(parent && parent->TestBit(TFile__k630forwardCompatibility)))
+            R__b << fBits;
+         else
+            R__b << (fBits & (~kIsOnHeap & ~kNotDeleted));
       } else {
          //if the object is referenced, we must save its address/file_pid
          UInt_t uid = fUniqueID & 0xffffff;
          R__b << uid;
-         R__b << (fBits & (~kIsOnHeap & ~kNotDeleted));
+         if (R__unlikely(parent && parent->TestBit(TFile__k630forwardCompatibility)))
+            R__b << fBits;
+         else
+            R__b << (fBits & (~kIsOnHeap & ~kNotDeleted));
          TProcessID *pid = TProcessID::GetProcessWithUID(fUniqueID,this);
          //add uid to the TRefTable if there is one
          TRefTable *table = TRefTable::GetRefTable();

io/io/inc/TFile.h

@@ -181,6 +181,13 @@ class TFile : public TDirectoryFile {
 public:
    /// TFile status bits. BIT(13) is taken up by TObject
    enum EStatusBits {
+      // Produce files forward compatible with (unpatched) version older than
+      // v6.30 by recording the internal bits kIsOnHeap and kNotDeleted; Older
+      // releases were not explicitly setting those bits to the correct value
+      // but instead used verbatim the value stored in the file.
+      // Note that to avoid a circular dependency, this value is used
+      // hard coded in TObject.cxx.
+      k630forwardCompatibility = BIT(2),
       kRecovered     = BIT(10),
       kHasReferences = BIT(11),
       kDevNull       = BIT(12),

io/io/src/TFile.cxx

@@ -814,6 +814,11 @@ void TFile::Init(Bool_t create)
       //*-* -------------attempt recovering the file
       Bool_t tryrecover = (gEnv->GetValue("TFile.Recover", 1) == 1) ? kTRUE : kFALSE;
 
+      //*-* -------------Check if we need to enable forward compatible with version
+      //*-* -------------prior to v6.30
+      if (gEnv->GetValue("TFile.v630forwardCompatibility", 0) == 1)
+         SetBit(k630forwardCompatibility);
+
       //*-* -------------Read keys of the top directory
       if (fSeekKeys > fBEGIN && fEND <= size) {
          //normal case. Recover only if file has no keys

io/io/src/TStreamerInfoWriteBuffer.cxx

@@ -404,7 +404,12 @@ Int_t TStreamerInfo::WriteBufferAux(TBuffer &b, const T &arr,
 
          // special case for TObject::fBits in case of a referenced object
          case TStreamerInfo::kBits: { DOLOOP {
-            UInt_t *x=(UInt_t*)(arr[k]+ioffset); b << (*x & (~kIsOnHeap & ~kNotDeleted));
+            UInt_t *x=(UInt_t*)(arr[k]+ioffset);
+            const auto parent = b.GetParent();
+            if (R__unlikely(parent && parent->TestBit(TFile::k630forwardCompatibility)))
+               b << *x;
+            else
+               b << (*x & (~kIsOnHeap & ~kNotDeleted));
             if ((*x & kIsReferenced) != 0) {
                TObject *obj = (TObject*)(arr[k]+eoffset);
                TProcessID *pid = TProcessID::GetProcessWithUID(obj->GetUniqueID(),obj);