6.26 cannot read file written with 6.30.4 about root

[iarspider]

@wdconinc disclaimer: I am not part of ROOT team, so my interpretation could be wrong.

• the change in 6.30 was intentional,
• the change broke forward compatibility in older ROOT versions,

Yes

• in some cases we are waiting for new bugfix releases on older versions (i.e. 6.28),
• in other cases we are waiting for the bugfix to be merged in older versions as well (i.e. 6.26 and earlier).

I don't know if there is a plan to update older versions with this fix - in CMS we just cherry-picked the fix into our fork of ROOT.

[iarspider]

@hahnjo in CMS we saw this issue as well, fixed by rebuilding old ROOT versions with this commit - a434281 .

[wdconinc]

@iarspider Did your tests indicate that this is a regression introduced by that commit specifically, or that this issue was simply not there yet at the time of that commit and it could be any commit since then?

[iarspider]

@wdconinc sorry, pinged wrong person. The mentioned commit (aka PR #12845) contains the fix, but I don't know when that incompatibility was introduced.

[wdconinc]

Thanks, @iarspider. If I can summarize then:

• the change in 6.30 was intentional,
• the change broke forward compatibility in older ROOT versions,
• in some cases we are waiting for new bugfix releases on older versions (i.e. 6.28),
• in other cases we are waiting for the bugfix to be merged in older versions as well (i.e. 6.26 and earlier).

Is that an accurate summary? If so, do we have an estimate when the next 6.28 patch release will be, which will include this fix? Does this issue merit being summarized as a pinned issue at the top of the GitHub issues?

[wdconinc]

In CMS we just cherry-picked the fix into our fork of ROOT.

But that still forces everyone to use the CMS environment all the time. At our earlier stage of development, we have many users who use computing environments at clusters where we don't have control over the version installed, let alone patches installed.