Let the digit readers accept a timeframe without collisions

This fixes a crash in the ITS, MFT, MCH and MID digit readers when the digit
tree of a timeframe has no entry.

- A timeframe holds no collision at all whenever the interaction rate is low
  enough, and the digitiser then writes a valid tree with zero entries.
- The ITS/MFT reader guarded this with an assert, which is compiled out of
  every production build because ENABLE_CASSERT defaults to OFF, and then
  dereferenced branch addresses that GetEntry had not filled.
- The MCH and MID readers threw on the failed TTreeReader::Next().
- All four now send empty output and end the stream.
- The two asserts in the ITS/MFT connectTree become real errors for the same
  reason.

https://its.cern.ch/jira/browse/O2-7132

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: sawenzel

Detectors/ITSMFT/common/workflow/src/DigitReaderSpec.cxx

@@ -26,6 +26,7 @@
 #include "ITSMFTReconstruction/ChipMappingMFT.h"
 #include "SimulationDataFormat/MCCompLabel.h"
 #include "SimulationDataFormat/ConstMCTruthContainer.h"
+#include "SimulationDataFormat/MCTruthContainer.h"
 #include "DataFormatsITSMFT/PhysTrigger.h"
 #include "CommonUtils/NameConf.h"
 #include "CommonDataFormat/IRFrame.h"
@@ -101,7 +102,32 @@ void DigitReader<N>::run(ProcessingContext& pc)
   auto ent = mTree->GetReadEntry();
   if (!mUseIRFrames) {
     ent++;
-    assert(ent < mTree->GetEntries()); // this should not happen
+    if (ent >= mTree->GetEntries()) {
+      // A timeframe holds no collision at all whenever the interaction rate is low enough, and the
+      // digit tree then has no entry to read. Send empty output rather than dereferencing the
+      // branch addresses, which GetEntry has not filled. (This used to be an assert, which is
+      // compiled out of every production build since ENABLE_CASSERT defaults to OFF.)
+      LOG(info) << mDetName << "DigitReader has no entry to read, sending empty output";
+      for (uint32_t iLayer = 0; iLayer < mLayers; ++iLayer) {
+        pc.outputs().snapshot(Output{Origin, "DIGITSROF", iLayer}, std::vector<o2::itsmft::ROFRecord>{});
+        pc.outputs().snapshot(Output{Origin, "DIGITS", iLayer}, std::vector<o2::itsmft::Digit>{});
+        if (mUseMC) {
+          auto& sharedlabels = pc.outputs().make<o2::dataformats::ConstMCTruthContainer<o2::MCCompLabel>>(Output{Origin, "DIGITSMCTR", iLayer});
+          o2::dataformats::MCTruthContainer<o2::MCCompLabel> noLabels;
+          noLabels.flatten_to(sharedlabels);
+          pc.outputs().snapshot(Output{Origin, "DIGITSMC2ROF", iLayer}, std::vector<o2::itsmft::MC2ROFRecord>{});
+        }
+      }
+      if (mUseCalib) {
+        pc.outputs().snapshot(Output{Origin, "GBTCALIB", 0}, std::vector<o2::itsmft::GBTCalibData>{});
+      }
+      if (mTriggerOut) {
+        pc.outputs().snapshot(Output{Origin, "PHYSTRIG", 0}, std::vector<o2::itsmft::PhysTrigger>{});
+      }
+      pc.services().get<ControlService>().endOfStream();
+      pc.services().get<ControlService>().readyToQuit(QuitRequest::Me);
+      return;
+    }
     mTree->GetEntry(ent);
     for (uint32_t iLayer = 0; iLayer < mLayers; ++iLayer) {
       LOG(info) << mDetName << "DigitReader" << ((mDoStaggering) ? std::format(": {}", iLayer) : "") << " pushes " << mDigROFRec[iLayer]->size() << " ROFRecords, " << mDigits[iLayer]->size() << " digits at entry " << ent;
@@ -215,9 +241,13 @@ void DigitReader<N>::connectTree(const std::string& filename)
 {
   mTree.reset(nullptr); // in case it was already loaded
   mFile.reset(TFile::Open(filename.c_str()));
-  assert(mFile && !mFile->IsZombie());
+  if (!mFile || mFile->IsZombie()) {
+    throw std::runtime_error(std::format("Cannot open {}", filename));
+  }
   mTree.reset((TTree*)mFile->Get(mDigTreeName.c_str()));
-  assert(mTree);
+  if (!mTree) {
+    throw std::runtime_error(std::format("Tree {} not found in {}", mDigTreeName, filename));
+  }
   for (uint32_t iLayer = 0; iLayer < mLayers; ++iLayer) {
     setBranchAddress(mDigitROFBranchName, mDigROFRec[iLayer], iLayer);
     setBranchAddress(mDigitBranchName, mDigits[iLayer], iLayer);

Detectors/MUON/MCH/IO/src/DigitReaderSpec.cxx

@@ -37,6 +37,7 @@
 #include "DataFormatsMCH/ROFRecord.h"
 #include "Framework/ConfigParamRegistry.h"
 #include "Framework/ControlService.h"
+#include "Framework/Logger.h"
 #include "Framework/DataSpecUtils.h"
 #include "Framework/Task.h"
 #include "Framework/WorkflowSpec.h"
@@ -109,6 +110,20 @@ class DigitsReaderDeviceDPL
 
   void sendNextTF(ProcessingContext& pc)
   {
+    // A timeframe holds no collision at all whenever the interaction rate is low enough, and the
+    // digit tree then has no entry. Send empty containers and finish, rather than throwing.
+    if (mTreeReader.GetEntries() == 0) {
+      LOG(info) << "digit tree has no entry, sending empty output";
+      pc.outputs().snapshot(OutputRef{"rofs"}, std::vector<ROFRecord>{});
+      pc.outputs().snapshot(OutputRef{"digits"}, std::vector<Digit>{});
+      if (mUseMC) {
+        pc.outputs().snapshot(OutputRef{"labels"}, dataformats::MCTruthContainer<MCCompLabel>{});
+      }
+      pc.services().get<ControlService>().endOfStream();
+      pc.services().get<ControlService>().readyToQuit(QuitRequest::Me);
+      return;
+    }
+
     // load the next TF and check its validity (missing branch, ...)
     if (!mTreeReader.Next()) {
       throw std::invalid_argument(mTreeReader.fgEntryStatusText[mTreeReader.GetEntryStatus()]);

Detectors/MUON/MID/Workflow/src/DigitReaderSpec.cxx

@@ -28,6 +28,7 @@
 
 #include "Framework/ConfigParamRegistry.h"
 #include "Framework/ControlService.h"
+#include "Framework/Logger.h"
 #include "Framework/DataSpecUtils.h"
 #include "Framework/Task.h"
 #include "Framework/WorkflowSpec.h"
@@ -103,6 +104,20 @@ class DigitsReaderDeviceDPL
 
   void sendNextTF(ProcessingContext& pc)
   {
+    // A timeframe holds no collision at all whenever the interaction rate is low enough, and the
+    // digit tree then has no entry. Send empty containers and finish, rather than throwing.
+    if (mTreeReader.GetEntries() == 0) {
+      LOG(info) << "digit tree has no entry, sending empty output";
+      pc.outputs().snapshot(OutputRef{"rofs"}, std::vector<ROFRecord>{});
+      pc.outputs().snapshot(OutputRef{"digits"}, std::vector<ColumnData>{});
+      if (mUseMC) {
+        pc.outputs().snapshot(OutputRef{"labels"}, dataformats::MCTruthContainer<MCLabel>{});
+      }
+      pc.services().get<ControlService>().endOfStream();
+      pc.services().get<ControlService>().readyToQuit(QuitRequest::Me);
+      return;
+    }
+
     // load the next TF and check its validity (missing branch, ...)
     if (!mTreeReader.Next()) {
       throw std::invalid_argument(mTreeReader.fgEntryStatusText[mTreeReader.GetEntryStatus()]);