Handle case for empty ROOT file

An empty ROOT files contains no ProcessHistory which causes problems when filling the ProductRegistry at construction time. Now skip empty files until we find a file with data.

Author: Dr15Jones

IOPool/Input/src/RootFile.cc

@@ -563,9 +563,13 @@ namespace edm {
       processingOrderMerge(*processHistoryRegistry_, processingOrder);
       newReg->setProcessOrder(processingOrder);
 
-      // freeze the product registry
-      newReg->setFrozen(inputType != InputType::Primary);
-      productRegistry_.reset(newReg.release());
+      if (not processingOrder.empty()) {
+        // freeze the product registry
+        newReg->setFrozen(inputType != InputType::Primary);
+        productRegistry_.reset(newReg.release());
+      } else {
+        productRegistry_ = std::make_shared<ProductRegistry>();
+      }
     }
 
     // Set up information from the product registry.
@@ -626,6 +630,7 @@ namespace edm {
 
   RootFile::~RootFile() {}
 
+  bool RootFile::empty() const { return runTree_.entries() == 0; }
   void RootFile::readEntryDescriptionTree(EntryDescriptionMap& entryDescriptionMap, InputType inputType) {
     // Called only for old format files.
     // We use a smart pointer so the tree will be deleted after use, and not kept for the life of the file.

IOPool/Input/src/RootFile.h

@@ -155,6 +155,8 @@ namespace edm {
     std::array<bool, NumBranchTypes> const& hasNewlyDroppedBranch() const { return hasNewlyDroppedBranch_; }
     bool branchListIndexesUnchanged() const { return branchListIndexesUnchanged_; }
     bool modifiedIDs() const { return daqProvenanceHelper_.get() != nullptr; }
+    //Are there no data stored in the file?
+    bool empty() const;
     std::shared_ptr<FileBlock> createFileBlock();
     void updateFileBlock(FileBlock&);
 

IOPool/Input/src/RootPrimaryFileSequence.cc

@@ -66,7 +66,7 @@ namespace edm {
     // Open the first file.
     for (setAtFirstFile(); !noMoreFiles(); setAtNextFile()) {
       initFile(input_.skipBadFiles());
-      if (rootFile())
+      if (rootFile() and not rootFile()->empty())
         break;
     }
     if (rootFile()) {