[io] always recreate free list in TFile::Recover()

This avoids tainting the recovered free list by stale entries that were
read before. It also allows for stronger assertions in TFree::AddFree().

Author: jblomer

io/io/src/TFile.cxx

@@ -2149,7 +2149,11 @@ Int_t TFile::Recover()
 
    fEND = Long64_t(size);
 
-   if (fWritable && !fFree) fFree  = new TList;
+   if (fWritable) {
+      if (!fFree)
+         fFree = new TList();
+      fFree->Delete();
+   }
 
    Int_t nrecov = 0;
    nwheader = 1024;
@@ -2222,12 +2226,7 @@ Int_t TFile::Recover()
    if (fWritable) {
       Long64_t max_file_size = Long64_t(kStartBigFile);
       if (max_file_size < fEND) max_file_size = fEND+1000000000;
-      TFree *last = (TFree*)fFree->Last();
-      if (last) {
-         last->AddFree(fFree,fEND,max_file_size);
-      } else {
-         new TFree(fFree,fEND,max_file_size);
-      }
+      new TFree(fFree, fEND, max_file_size);
       if (nrecov) Write();
    }
    return nrecov;

io/io/src/TFree.cxx

@@ -73,6 +73,8 @@ TFree *TFree::AddFree(TList *lfree, Long64_t first, Long64_t last)
    do {
       Long64_t curfirst = idcur->GetFirst();
       Long64_t curlast  = idcur->GetLast();
+      // We can't have overlapping free segments
+      assert((first < curfirst && last < curfirst) || first > curlast);
 
       if ((curlast == first - 1) && (last - curfirst < kMaxGapSize)) {
          idcur->SetLast(last);
@@ -104,6 +106,8 @@ TFree *TFree::AddFree(TList *lfree, Long64_t first, Long64_t last)
       idcur = (TFree*)lfree->After(idcur);
    } while (idcur);
 
+   // never here
+   assert(false);
    return nullptr;
 }