getNumberOfEvents changes the internal state of the file

jstrube commented 4 years ago

OS version: SLC6, Ubuntu 18.04
Compiler version: GCC 6.2, gcc 7.4
Package version: v02-12-01
Reproduced by: call readNextEvent(), followed by getNumberOfEvents(), followed by readNextEvent(). You have just read the same event twice!
Goal: It is not obvious that getNumberOfEvents leads to this behavior, nor should it. The name suggest that this is a getter method. However, it looks like the implementation changes the internal state of the file. Tested on /pnfs/desy.de/ilc/prod/ilc/mc-opt-3/ild/dst-merged/500-TDR_ws/2f_Z_hadronic/ILD_l5_o1_v02/v02-00-01/rv02-00-01.sv02-00-01.mILD_l5_o1_v02.E500-TDR_ws.I250114.P2f_z_h.eL.pR.n033.d_dstm_10410_96.slcio

rete commented 4 years ago

Thanks for reporting this. Indeed, calling getNumberOfEvents() is doing the following:

ask the random access manager to read the index at the end of the file
populate the event map
reset the stream pointer to begin This is why you read twice the same event I guess

gaede commented 4 years ago

I agree that this seems not obvious - should change the documentation.

iLCSoft / LCIO