[JainAyush-01]

This Pull request:

Fixes an in-place string mutation bug in TPluginManager::LoadHandlersFromPluginDirs that breaks plugin discovery when Root.PluginPath (or ROOT_PLUGIN_PATH) contains multiple colon-separated directories.

Changes or fixes:

Root Cause:

In commit e60640c (#18859), gSystem->ConcatFileName was replaced with gSystem->PrependPathName. Unlike ConcatFileName which returned a new heap-allocated string without modifying the input, PrependPathName(const char *dir, TString &name) modifies name in place.

In TPluginManager::LoadHandlersFromPluginDirs(const char *base):

TString sbase = base;
...
for (Int_t i = 0; i < dirs->GetEntriesFast(); i++) {
   ...
   if (!skip) {
      if (sbase != "") {
         const char *p = gSystem->PrependPathName(d, sbase);
         LoadHandlerMacros(p);
      }
   }
}

Because sbase was passed directly to PrependPathName without copying to a temporary variable:

• On iteration 0 (dir0 = "/custom/plugins"): sbase was mutated from "TArchiveFile" to "/custom/plugins/TArchiveFile".
• On iteration 1 (dir1 = "$ROOTSYS/etc/plugins"): PrependPathName prepended dir1 to the already mutated sbase, resulting in "$ROOTSYS/etc/plugins//custom/plugins/TArchiveFile".

As a result, whenever a user or experiment environment (e.g. ALICE O2 / JAliEn) prepended a custom directory to Root.PluginPath, all subsequent directories—including ROOT's own built-in plugins in $ROOTSYS/etc/plugins—failed to load.

Fix:

Copy sbase to a temporary TString temp = sbase; inside the loop before calling PrependPathName(d, temp), matching the pattern already used in the else branch (lines 545–546) and throughout TPluginManager.cxx.

Testing:

• Added GoogleTest regression test TEST(TPluginManager, MultiDirectoryPluginPath) in core/base/test/TPluginManagerTests.cxx.
• Verified that both custom plugins in a prepended directory and built-in plugins in subsequent directories are resolved properly.
• Ran test via ctest -R CorePluginManagerTests: 100% passed (0.26s).

Checklist:

• tested changes locally
• updated the docs (if necessary)

This PR fixes #23536

---

AI-assisted coding disclosure: I used an AI assistant to help navigate the codebase, structure the unit test, and format this PR description. I have personally reviewed, debugged, and tested the C++ changes locally and take full responsibility for them.

[dpiparo]

Did you have a look to https://github.com/root-project/root/blob/master/CONTRIBUTING.md#ai-assisted-coding-disclosure ?

[JainAyush-01]

Hi @dpiparo,

Thank you for pointing this out. I apologize, I missed the AI disclosure section in CONTRIBUTING.md.

I have updated the description here (and on #23791) with the disclosure. I used an LLM to help explore the codebase and structure the test, but I made sure to verify the bug and C++ logic locally, sbase was passed by non const reference TString and inside the vpaths loop, so sbase.Prepend(...) was repeatedly accumulating previous path segments instead of evaluating each search path independently.

Going forward, I will make sure all tool usage is properly disclosed according to the guidelines

[pcanal]

Thanks for you contribution. The test has been fixed and the PR has been extended as part of #23928