[Root7] A path to reducing global object registration and management.

[hageboeck]

This is the start of a longer-term effort to transform ROOT into working with less globals, making ownership of objects clearer, and going more towards standard C++ behaviour instead of implicit ownership. One could call this the "ROOT 7 mode". In order to allow for early testing, this mode is introduced as an opt-in feature in ROOT 6.

What this PR brings

TROOT.h gets three new functions (names up for discussion):

   namespace ROOT::Experimental {
   void EnableObjectAutoRegistration();
   void DisableObjectAutoRegistration();
   bool ObjectAutoRegistrationEnabled();
   }

https://github.com/hageboeck/root/blob/040980428a842a3314c75bde1ba9e6ae208fe8b1/core/base/inc/TROOT.h#L100-L104

The naming and API is inspired by EnableImplicitMT() and its colleagues. These determine whether TObject-derived classes auto-register themselves to gDirectory when created. A well-known example is TH1 and all its derived classes, but what's covered by these functions will be extended further on the path to ROOT 7. Currently, this mode also covers RooFit objects, TGraph, and TEfficiency.
An extensive list of objects with auto registration and their state (maybe not fully complete) can be found below.

A lot of preparatory work was done to prepare ROOT to work both with auto registration on and off. A detailed list of this work can be found at the end. Currently, there is one commit that switches auto registration off across the entire CI to demonstrate that ROOT already today works in this mode. Once the PR is ready to merge, this commit will be removed to not break the workflows of ROOT's users who did not opt in. A single (or a few; TBD) builds with auto registration off will be added to the CI to keep test coverage for this feature.
TODO:

• Remove the commit that globally switches auto registration off.

Difference to TH1::AddDirectoryStatus()

The ROOT 7 mode with auto registration off might look similar to TH1::AddDirectoryStatus(), but there are important differences:

• DirectoryStatus is a static (global) bool while ObjectAutoRegistrationEnabled() is thread_local
• DirectoryStatus only affects histogram-like objects (see table at the end). AutoRegistrationEnabled should eventually extend to more cases, e.g. RooFit objects, TEventList (planned), functions (planned).
• DirectoryStatus affects the behaviour of TFile::Get. file->Get("name"); file->Get("name") returns the same object when directory status is true, and it reads a new object every time when it's false. I consider it best to not interfere here, because one can use TFile if such behaviour is desired, and RFile if not. AutoRegistrationEnabled() should only decide what happens to new objects, not objects read from a file.
• DirectoryStatus cannot be set from the environment, and its default cannot be changed. It is a non-thread-safe runtime feature. AutoRegistrationEnabled() can be initialised from the environment (thread safe), and its state can be changed per thread, which might be vital for using it in MT frameworks.

Setting defaults

Some users or frameworks might be interested in operating in ROOT 7 mode already today, but they don't want to add calls to the above functions in each thread of their code. Especially for testing, they should be able to set a default without recompiling. Defaults can be set in a .rootrc:

Root.ObjectAutoRegistration: 0

and also using an environment variable:

ROOT_OBJECT_AUTO_REGISTRATION=0 ./executable --args

When any object queries ObjectAutoRegistrationEnabled(), a thread-safe initialisation is performed, taking defaults from the environment above. If no defaults are set, registration defaults to "on" in ROOT 6 and "off" in ROOT 7.

This, however, might lead to an "it worked on my machine" problem: Some users might have altered their environment, while others might have chosen the opposite. To make it obvious when a ROOT 6 runs in ROOT 7 preview mode, info messages such as the following are printed if the environment modifies the default:

$ ./executable --args
Info in <TROOT>: Object auto registration disabled using the environment variable ROOT_OBJECT_AUTO_REGISTRATION

(A similar message exists for rootrc)

List of classes covered (or intended to be covered) by "DisableObjectAutoRegistration"

	Auto registration disabled with DisableObjectAutoRegistration()	How could it be disabled before?	Comments
TH1 and derived	Yes	TH1::AddDirectoryStatus()
TGraph2D	Yes	TH1::AddDirectoryStatus()
RooPlot	Yes	RooPlot::addDirectoryStatus()
TEfficiency	Yes (New)	Was inconsistent
TProfile2D	Yes	TH1::AddDirectoryStatus()
RooDataHist	Already off	Already off
RooDataSet	Already off	Already off
RooDirItem	Already off	Already off
RooFitResult	Already off	Already off
TEntryList	Yes (in the future)	Not possible	To be done later
TEventList	Yes (in the future)	Not possible	To be done later
TTree	Probably no (to be decided)		Unclear if it should be changed. It would be broken without being attached to a file
TBranch	No		It needs to know what its tree is
TChain, TNTuple	No		Same as for TTree
TFunction	Probably yes, in the future. To be decided		This is a registration not to gDirectory but to a global list of functions. Way forward to be decided.

TODOs:

• Add tests for implicit / explicit registration of TH1s
• Deprecate TH1::AddDirectory() as suggested in Replace TH1::AddDirectory by TDirectory::TContext #21523 ?
  • In Doxygen, it is already marked as deprecated, but a C++ deprecation should probably happen a bit later on the way to ROOT 7.
  • Merge the PRs below to make ROOT ready for running both with and without auto registration.

Preparatory work

The following PRs were part of this branch, and have been split off over time to separate the introduction of this feature from preparing ROOT to work both with auto registration off and on:

• [roottest] Fix roottest trying to invoke /usr/bin/root.exe in tests. #20818
• [cmake] Simplify logic disabling runPyClassTest.C on Windows #20830
• A few minor fixes in roottest #20832
• Introduce a RUNPATH for compiled macros. #20833
• Updates to stressgraphics #20886
• [core] Make TDirectory::Append tolerant to identical objects. #20888
• [RF] Prepare RooFit and HistFactory for ROOT without implicit ownership #20889
• [roottest] Move hist ownership test from diffs to programmatic test. #20919
• [hist] Make two tests independent of gDirectory #20920
• [PyROOT] Remove a test that worked just by chance. #20921
• [HF] Prepare HistFactory for ROOT without implicit ownership #20922
• [RF] Prepare RooFit for ROOT without implicit ownership. #20978
• [PyROOT] Prepare tests for ROOT 7 #21224
• Prepare roottest for ROOT 7's ownership model #21225
• Prepare tutorials for the ROOT 7 object ownership model #21226
• Prepare the "stress" tests for ROOT 7 ownership model. #21228
• Prepare mlp for ROOT 7 ownership model. #21229
• [RF] Remove all RooDirItem::appendToDir calls. #21231
• [core] Deprecate TDirectory::AddDirectoryStatus() #21248
• Minimise direct usage of gDirectory, use TContext instead. #21287
• [hist] Prepare for explicit object ownership model #21296
• [net] Correctly restore gFile. #21319
• [roottest] Fix the multicore/fork test. #21341
• Replace TH1::AddDirectory by TDirectory::TContext #21523
• [roottest] Refactor custom_diff.py #21587
• [tmva] Retire TH1::AddDirectory use in TMVA. #21646
• Make TTree::Draw() independent of histogram auto registration, suppress a warning #21648
• [ruff] Try to run all range checks before exiting. #21679
• [roottest] Fix test dependencies using fixtures #21708

[github-actions]

Test Results

    22 files      22 suites   3d 7h 37m 40s ⏱️
 3 833 tests  3 831 ✅  1 💤 1 ❌
75 672 runs  75 653 ✅ 18 💤 1 ❌

For more details on these failures, see this check.

Results for commit cdbb2be.

♻️ This comment has been updated with latest results.

Please tell us why you approve this request.