Fix FairRootManager::GetObject

[YanzhaoW]

Describe your proposal.

Fix the UB found in the issue #1556 related to FairRootManager::GetObject.

Cause of the issue

The branch address of the TTree must be fixed during the data reading. In other words, if an object of type T needs to be read from a tree, the value T** must be fixed and cannot be changed during the whole reading process. However, in the GetObject function, the variable newentry is a reference to the T* pointer stored in the std::vector<T*>. If the size of the vector grows beyond its capacity, the whole vector will be moved to another memory location, which means the T** value will be changed.

This makes sense as in my real practice from R3BRoot, I didn't get all zeros but rather direct segmentation fault.

Let newentry refer to the T* in the object of std::map<TString, TObject*> fixes the issue. std::map doesn't reallocate its entries during the runtime.

TODO

1. AddMemoryBranch now returns TObject*&, which is ugly and asking for troubles in the future. Unfortunately, there isn't any better way to deal with this issue if we still use TTree raw API for the data reading. It must take a TObject** for calling SetBranchAddress.
2. As I have suggested from #1556, TTreeReader is a much better choice as we don't need to deal with T** situation anymore.

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Checklist:

• [x] Followed the Contributing Guidelines

[coderabbitai[bot]]

Walkthrough

## Walkthrough The `FairRootManager` class has undergone updates to enhance branch handling efficiency. The `ActivateBranch` method's logic has improved to better handle existing branches and the `AddMemoryBranch` now returns a reference to the added branch object, instead of void, to provide more control and information back to the calling context. ## Changes | Files | Change Summary | |---------------------------------------------------|----------------| | `fairroot/base/steer/FairRootManager.cxx`, `fairroot/base/steer/FairRootManager.h` | Updated `ActivateBranch` logic for better efficiency in handling branches and modified `AddMemoryBranch` to return a reference to `TObject*`. |

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Recent review details

**Configuration used: CodeRabbit UI** **Review profile: CHILL**

Commits

Files that changed from the base of the PR and between 94c52ab9b2a2adba4b7c5ba77ff615a821bb5b85 and 3f1b6ab8523edd557719d40f923fd5f0a9cf0c2a.

Files selected for processing (1)

* fairroot/base/steer/FairRootManager.h (2 hunks)

Files skipped from review as they are similar to previous changes (1)

* fairroot/base/steer/FairRootManager.h

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[ChristianTackeGSI]

Thanks for analyzing this!

(We came to an alike result).

Our quick fix would be to replace the std::vector by a std::list. This is probably the fix that we would like to deploy for 19.0.1.

[YanzhaoW]

Hi, @ChristianTackeGSI

I don't quite understand the purpose of fObj2. When you call GetObject, if the branch has not been added and does't exist in the tree, you push nullptr to the vector. If the branch exists, you push the same pointer twice into the vector. On the other hand, you also have fMap for the book-keeping of the branches.

[ChristianTackeGSI]

Right, fObj2 is a bit strange!

We would like to have a minimal fix for 19.0.1 first. And the minimal one is to use std::list.

For 19.1 we can then slowly work on further refactoring and improving GetObject.

And: Thanks for the reference to TTreeReader!

[YanzhaoW]

Ok, Fair enough.

Should I close this PR and you create another one using std::list? Or I should just change it to std::list here?

[ChristianTackeGSI]

Ok, Fair enough.

Should I close this PR and you create another one using std::list? Or I should just change it to std::list here?

How ever you like. :-)

@dennisklein wanted to create a proper test first anyway.

[YanzhaoW]

@ChristianTackeGSI

Is this minimum change OK for you?

[YanzhaoW]

There is another issue about branch IO of FairRootManger:

There are two different, independent branch registries: fAnyBranchMap and fMap. These are used by different APIs of FairRootManager:

1. fAnyBranchMap is used by RegisterAny and InitObjectAs.
2. fMap is used by Register and GetObject.

Any branch registered in one way cannot be read by another way.

What's even more "interesting" is that the event header (online run) is always registered in fMap but not in fAnyBranchMap. So you may find that you can get a valid event header from GetObject but mysteriously nullptr from InitObjectAs.

@ChristianTackeGSI @dennisklein

[ChristianTackeGSI]

It looks like it will take a bit longer to create proper tests for this. So merging now, so that the fix is in place at least.

[ChristianTackeGSI]

BTW: I think, we need a ChangeLog entry for this?