[io] Deprecate TMPIFile and the mpi build option

README/ReleaseNotes/v642/index.md

@@ -49,6 +49,7 @@ The following people have contributed to this new version:
 * Several enums that are redundant with `ROOT::ESTLType` are deprecated and will be removed in ROOT 6.44: `TClassEdit::ESTLType`, `TDictionary::ESTLType`, `TStreamerElement::ESTLType`. Please use `ROOT::ESTLType` instead.
 * The inclusion by external projects of Makefile templates contained within ROOT is deprecated in 6.42, a warning will be raised if you use them. These files will be removed in ROOT 7.
 * The conversion from Python set to **RooArgSet** is deprecated and won't work anymore in ROOT 6.44. The problem is that Python sets are unordered while RooArgSets are ordered, and this mismatch can lead to subtle problems later on. Prefer conversion from Python lists or tuples, which are ordered too.
+* The **TMPIFile** class and the `mpi` build option (not to be confused with `minuit2_mpi`, which is unaffected) are deprecated and will be removed in ROOT 6.44.
 * The ROOT IO capability for the `TMVA::Experimental::SOFIE::RModel` has been removed. Users should not be encouraged to serialize models in experimental classes. For the serialization of ONNX models one can already use ONNX directly, and even serialize the ONNX bytes to a ROOT file if required.
 * The **JsMVA** feature for interactive TMVA training in Jupyter notebooks is now removed. It was not functional for years and was therefore already excluded from ROOT 6.38. This also removes the `TMVA::IPythonInteractive` class and the related interactive-training interfaces from the TMVA method and fitter classes, such as `MethodBase::ExitFromTraining()` or `FitterBase::SetIPythonInteractive()`.
 

cmake/modules/RootBuildOptions.cmake

@@ -381,7 +381,7 @@ foreach(opt afdsmgrd afs alien bonjour builtin_afterimage builtin_davix castor c
 endforeach()
 
 #---Deprecated options------------------------------------------------------------------------
-foreach(opt r)
+foreach(opt mpi r)
   if(${opt})
     message(DEPRECATION ">>> Option '${opt}' is deprecated and will be removed in the next release of ROOT. Please contact root-dev@cern.ch should you still need it.")
   endif()

io/mpi/inc/TMPIFile.h

@@ -123,5 +123,5 @@ class TMPIFile : public TMemFile {
    void Close(Option_t *option = "") final;
 
    ClassDefOverride(TMPIFile, 0)
-};
+} R__DEPRECATED(6, 44, "TMPIFile is unmaintained. Write one file per process and merge with hadd/TFileMerger, or use ROOT::TBufferMerger instead.");
 #endif