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[io] Wrong member offset in streamer info with read rules
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Description
For a class with read rules, data members may report wrong offsets (see reproducer).
Reproducer
In the attached reproducer, the output of offset is
$ ./offset
0
8
0
StreamerInfo for class: Event, version=3, checksum=0x153ea7a
Event@@3 @@alloc offset= 0 type=1001
float pt offset= 8 type= 5 (cached)
float pt offset= 0 type= 5 (write)
Event@@3 @@dealloc offset= 0 type=1002
i= 0, @@alloc type=1001, offset= 0, len=1, method=0
i= 1, pt type= 5, offset= 8, len=1, method=0 [cached]
i= 2, pt type= 5, offset= 0, len=1, method=0 [write]
i= 3, @@dealloc type=1002, offset= 0, len=1, method=0
Demonstrating that pt is reported with an offset of 8 by the streamer info element.
The wrong offset in the streamer info seems to stem from TStreamerInfo::BuildOld, which adds a second, faulty cached member to the list of streamer info elements.
ROOT version
master
Installation method
From sources
Operating system
Linux
Additional context
No response
The output is as intended but the problem is that TStreamerInfo::GetDataMemberOffset is returning the offset of the cached member rather than the write (or repeat) member (i.e. the real live member).
