TTreeCache::FillBuffer spurrious error.

[pcanal]

As seen in cms-sw/cmssw#33361, in some normal usage circumstances, TTreeCache::FillBuffer will issue an Error message (turned into an exception by CMSSW):

fCurrentClusterStart=11348 fEntryCurrent=11392 fNextClusterStart=11394
but fEntryCurrent should not be in between the two 

[pcanal]

The error message is inaccurate, it did not take into account jagged filling of the TTreeCache. In this case, the cache was filled with a little more than one cluster and when it needs to do the next refill it restarted from the cluster start boundary of that partially downloaded cluster which is “indeed” within the range of the last TreeCache fill (i.e. the error).

[pcanal]

We did not see the problem with a local file because the TTreeCache usage is different. CMSSW take note of whether prefetching (asynchronous reads) is available for a while or not. In the setup CMSSW has, the prefetching (asynchronous reads) is available for the local file but not for the network/remote file. In addition when prefetching (asynchronous reads) is not available, CMSSW uses multiple TTreeCache for a given TTree while it uses only one when prefetching (asynchronous reads) is available. This results in the pattern of filling to be different between the 2 cases.