Prevent infinite loops in 3D track matching - #310

Summary

• prevent the X/Y 3D matcher from decrementing a candidate-hit index below zero when the final hit in a view is outside the detector bounds
• apply the same fix to the U/V(/X) matcher, which used the same unsafe loop pattern
• recompute the loop condition before continuing after skipped hits

Failure mechanism

Both matchers walk candidate hits backward using signed indices. When an out-of-bounds hit was encountered, the code unconditionally decremented the corresponding index and immediately continued. If that hit was at index zero, the index became -1; the continue also skipped the loop-condition update. Subsequent iterations then read outside the vector and could loop indefinitely, crash, or appear nondeterministic depending on the memory contents.

This was observed in a mini-validation run of PR #301. ConvertToTMSTree.exe stayed at 100% CPU for more than 20 minutes after the final spill was sliced, versus about 200 seconds for the previous complete run. Repeated GDB samples all landed in the detector-bound checks in TrackMatching3D_XY() even though there were only two X candidates, two Y candidates, and eight hits in each current candidate. The matcher itself is unchanged by PR #301; that PR's changed hit population exposed this pre-existing bug.

The fix stops matching when an out-of-bounds hit exhausts a required view, otherwise advances safely and updates the loop condition before continuing.

Similar-code audit

The U/V(/X) matcher contained the same unguarded decrement followed by continue, so it is fixed here too. Other reverse-index loops in TMS_Reco.cpp guard decrements with index > 0 or break immediately after reaching the boundary.

Validation

• git diff --check
• targeted loop-control checks for an out-of-bounds hit at index zero and at a positive index
• source audit of decrement/continue and reverse-index patterns in src/ and app/

A full build/run is not available in the local VM because the experiment ROOT/CLHEP environment is not installed. The triggering full sample can be rerun in the FNAL environment.