Fixes both bugs in issue #438.

[jpivarski]

But the second bug offers no clues as to how it can be solved.

Here's the issue: according to TObject's streamers (and a lot of other things), fBits is 4 bytes.

>>> import uproot
>>> t = uproot.open("uproot-issue-438b.root:Delphes")
>>> t.file.streamer_named("TObject").show()
TObject (v1)
    fUniqueID: unsigned int (TStreamerBasicType)
    fBits: unsigned int (TStreamerBasicType)

This is also true of almost all of the fBits branches:

>>> for x in t.keys(filter_name="*.fBits"):
...     print(f"{len(t[x].debug_array(0)) / 4:10} {x}")
... 
       1.0 Event/Event.fBits
       1.0 Weight/Weight.fBits
    7924.5 Particle/Particle.fBits
     273.0 Track/Track.fBits
     819.0 Tower/Tower.fBits
     273.0 EFlowTrack/EFlowTrack.fBits
     477.0 EFlowPhoton/EFlowPhoton.fBits
     267.0 EFlowNeutralHadron/EFlowNeutralHadron.fBits
       8.0 GenJet/GenJet.fBits
       1.0 GenMissingET/GenMissingET.fBits
       6.0 Jet/Jet.fBits
       0.0 Electron/Electron.fBits
       0.0 Photon/Photon.fBits
       0.0 Muon/Muon.fBits
       1.0 MissingET/MissingET.fBits
       1.0 ScalarHT/ScalarHT.fBits

The exception is Particle/Particle.fBits, whose first event has a number of bytes that does not divide evenly by 4. Upon closer inspection, it's clearly a 6-byte value.

>>> t["Particle/Particle.fBits"].debug_array(0).view([("x", ">i4"), ("y", ">i2")])
array([(50331664, 0), (50331664, 0), (50331664, 0), ..., (50331664, 0),
       (50331664, 0), (50331664, 0)], dtype=[('x', '>i4'), ('y', '>i2')])

However, there's nothing to distinguish this branch with 6 bytes:

>>> t["Particle/Particle.fBits"].all_members
{
    '@fUniqueID': 0,
    '@fBits': 55574528,
    'fName': 'Particle.fBits',
    'fTitle': 'fBits[Particle_]',
    'fFillColor': 0,
    'fFillStyle': 1001,
    'fCompress': 101,
    'fBasketSize': 64000,
    'fEntryOffsetLen': 10,
    'fWriteBasket': 33,
    'fEntryNumber': 100,
    'fIOFeatures': <ROOT::TIOFeatures at 0x7fd20af3d400>,
    'fOffset': 0,
    'fMaxBaskets': 34,
    'fSplitLevel': 0,
    'fEntries': 100,
    'fFirstEntry': 0,
    'fTotBytes': 1710622,
    'fZipBytes': 11966,
    'fBranches': <TObjArray of 0 items at 0x7fd20af3d460>,
    'fLeaves': <TObjArray of 1 items at 0x7fd20af3d5b0>,
    'fBasketBytes': array([381, 307, 382, 348, 276, 408, 359, 400, 376, 358, 339, 320, 324,
       362, 474, 310, 434, 366, 427, 282, 333, 452, 378, 378, 392, 382,
       370, 349, 392, 385, 365, 316, 241,   0], dtype=int32),
    'fBasketEntry': array([  0,   3,   5,   7,  11,  12,  15,  18,  21,  24,  27,  29,  31,
        33,  36,  41,  43,  46,  49,  53,  54,  56,  61,  64,  68,  73,
        77,  81,  83,  87,  92,  95,  97, 100]),
    'fBasketSeek': array([     375,    17300,   658211,   676369,  1374256,  1410371,
        2055073,  2668842,  2705428,  3223503,  3741825,  3748989,
        3777650,  4534932,  5093995,  5133565,  5736716,  6572626,
        7166521,  7186122,  7660358,  8287949,  8333770,  8858670,
        9413125,  9435318, 10023382, 10544552, 10584617, 11540595,
       12060644, 12084058, 12654014,        0]),
    'fClassName': <TString 'TObject' at 0x7fd20af9bb30>,
    'fParentName': <TString 'GenParticle' at 0x7fd20af9bba0>,
    'fClonesName': <TString '' at 0x7fd20af9bc10>,
    'fCheckSum': 2417737773,
    'fClassVersion': 1,
    'fID': 1,
    'fType': 31,
    'fStreamerType': 15,
    'fMaximum': 0,
    'fBranchCount': None,
    'fBranchCount2': None
}

from branches with 4 bytes:

>>> t["Jet/Jet.fBits"].all_members
{
    '@fUniqueID': 0,
    '@fBits': 55574528,
    'fName': 'Jet.fBits',
    'fTitle': 'fBits[Jet_]',
    'fFillColor': 0,
    'fFillStyle': 1001,
    'fCompress': 101,
    'fBasketSize': 64000,
    'fEntryOffsetLen': 400,
    'fWriteBasket': 1,
    'fEntryNumber': 100,
    'fIOFeatures': <ROOT::TIOFeatures at 0x7fd209e72a30>,
    'fOffset': 0,
    'fMaxBaskets': 10,
    'fSplitLevel': 0,
    'fEntries': 100,
    'fFirstEntry': 0,
    'fTotBytes': 2131,
    'fZipBytes': 352,
    'fBranches': <TObjArray of 0 items at 0x7fd209e72a90>,
    'fLeaves': <TObjArray of 1 items at 0x7fd209e72be0>,
    'fBasketBytes': array([352,   0,   0,   0,   0,   0,   0,   0,   0,   0], dtype=int32),
    'fBasketEntry': array([  0, 100,   0,   0,   0,   0,   0,   0,   0,   0]),
    'fBasketSeek': array([14605752,        0,        0,        0,        0,        0,
              0,        0,        0,        0]),
    'fClassName': <TString 'TObject' at 0x7fd209e6e4a0>,
    'fParentName': <TString 'Jet' at 0x7fd209e6e510>,
    'fClonesName': <TString '' at 0x7fd209e6e580>,
    'fCheckSum': 2417737773,
    'fClassVersion': 1,
    'fID': 1,
    'fType': 31,
    'fStreamerType': 15,
    'fMaximum': 0,
    'fBranchCount': None,
    'fBranchCount2': None
}

So they can't be given different interpretations.

In the end, I think that the 6-bytes vs 4-bytes indicator might be in the fBits themselves. See the first 4 bytes of the first event:

>>> for x in t.keys(filter_name="*.fBits"):
...     print(f"{str(t[x].debug_array(0)[:4]):15} {x}")
... 
[3 0 0 0]       Event/Event.fBits
[3 0 0 0]       Weight/Weight.fBits
[ 3  0  0 16]   Particle/Particle.fBits
[ 3  0  0 16]   Track/Track.fBits
[ 3  0  0 16]   Tower/Tower.fBits
[ 3  0  0 16]   EFlowTrack/EFlowTrack.fBits
[ 3  0  0 16]   EFlowPhoton/EFlowPhoton.fBits
[ 3  0  0 16]   EFlowNeutralHadron/EFlowNeutralHadron.fBits
[3 0 0 0]       GenJet/GenJet.fBits
[3 0 0 0]       GenMissingET/GenMissingET.fBits
[3 0 0 0]       Jet/Jet.fBits
[]              Electron/Electron.fBits
[]              Photon/Photon.fBits
[]              Muon/Muon.fBits
[3 0 0 0]       MissingET/MissingET.fBits
[3 0 0 0]       ScalarHT/ScalarHT.fBits

The ones that have a 16 in them also have event sizes that are divisible by 6:

>>> for x in t.keys(filter_name="*.fBits"):
...     print(f"{len(t[x].debug_array(0)) / 6:10} {x}")
... 
0.6666666666666666 Event/Event.fBits
0.6666666666666666 Weight/Weight.fBits
    5283.0 Particle/Particle.fBits
     182.0 Track/Track.fBits
     546.0 Tower/Tower.fBits
     182.0 EFlowTrack/EFlowTrack.fBits
     318.0 EFlowPhoton/EFlowPhoton.fBits
     178.0 EFlowNeutralHadron/EFlowNeutralHadron.fBits
5.333333333333333 GenJet/GenJet.fBits
0.6666666666666666 GenMissingET/GenMissingET.fBits
       4.0 Jet/Jet.fBits
       0.0 Electron/Electron.fBits
       0.0 Photon/Photon.fBits
       0.0 Muon/Muon.fBits
0.6666666666666666 MissingET/MissingET.fBits
0.6666666666666666 ScalarHT/ScalarHT.fBits

That 16 could be kIsReferenced...

>>> np.array([uproot.const.kIsReferenced], ">i4").view("u1")
array([ 0,  0,  0, 16], dtype=uint8)

If this is the issue, then the fBits would have to become an object type (uproot.AsObjects) that includes the extra 2 bytes if it sees a 16. That would be slow to deserialize (before AwkwardForth) and annoying because nobody cares about fBits.

I had another idea: why not just read them in as 1-byte values? Then at least it doesn't break.

>>> import uproot
>>> t = uproot.open("uproot-issue-438b.root:Delphes")
>>> for x in t.keys(filter_name="*.fBits"):
...     print(t[x].array())
... 
[[3, 0, 0, 0], [3, 0, 0, 0], [3, 0, 0, 0, ... 0, 0, 0], [3, 0, 0, 0], [3, 0, 0, 0]]
[[3, 0, 0, 0], [3, 0, 0, 0], [3, 0, 0, 0, ... 0, 0, 0], [3, 0, 0, 0], [3, 0, 0, 0]]
[[3, 0, 0, 16, 0, 0, 3, 0, 0, 16, 0, 0, 3, ... 3, 0, 0, 16, 0, 0, 3, 0, 0, 16, 0, 0]]
[[3, 0, 0, 16, 0, 0, 3, 0, 0, 16, 0, 0, 3, ... 3, 0, 0, 16, 0, 0, 3, 0, 0, 16, 0, 0]]
[[3, 0, 0, 16, 0, 0, 3, 0, 0, 16, 0, 0, 3, ... 3, 0, 0, 16, 0, 0, 3, 0, 0, 16, 0, 0]]
[[3, 0, 0, 16, 0, 0, 3, 0, 0, 16, 0, 0, 3, ... 3, 0, 0, 16, 0, 0, 3, 0, 0, 16, 0, 0]]
[[3, 0, 0, 16, 0, 0, 3, 0, 0, 16, 0, 0, 3, ... 3, 0, 0, 16, 0, 0, 3, 0, 0, 16, 0, 0]]
[[3, 0, 0, 16, 0, 0, 3, 0, 0, 16, 0, 0, 3, ... 3, 0, 0, 16, 0, 0, 3, 0, 0, 16, 0, 0]]
[[3, 0, 0, 0, 3, 0, 0, 0, 3, 0, 0, 0, 3, 0, ... [3, 0, 0, 0, 3, 0, 0, 0, 3, 0, 0, 0]]
[[3, 0, 0, 0], [3, 0, 0, 0], [3, 0, 0, 0, ... 0, 0, 0], [3, 0, 0, 0], [3, 0, 0, 0]]
[[3, 0, 0, 0, 3, 0, 0, 0, 3, 0, 0, 0, 3, 0, ... [3, 0, 0, 0, 3, 0, 0, 0, 3, 0, 0, 0]]
[[], [], [], [], [], [], [], [], [], [], ... [], [], [], [], [], [], [], [], [], []]
[[], [], [], [], [], [], [], [], [], [], ... [], [], [], [], [], [], [], [], [], []]
[[], [], [], [], [], [], [], [], [], [], ... [], [], [], [], [], [], [], [], [], []]
[[3, 0, 0, 0], [3, 0, 0, 0], [3, 0, 0, 0, ... 0, 0, 0], [3, 0, 0, 0], [3, 0, 0, 0]]
[[3, 0, 0, 0], [3, 0, 0, 0], [3, 0, 0, 0, ... 0, 0, 0], [3, 0, 0, 0], [3, 0, 0, 0]]

That's the second commit.