[core] Fix plugin lookup when Root.PluginPath has multiple directories

core/base/src/TPluginManager.cxx

@@ -532,7 +532,8 @@ void TPluginManager::LoadHandlersFromPluginDirs(const char *base)
       }
       if (!skip) {
          if (sbase != "") {
-            const char *p = gSystem->PrependPathName(d, sbase);
+            TString temp = sbase;
+            const char *p = gSystem->PrependPathName(d, temp);
             LoadHandlerMacros(p);
          } else {
             void *dirp = gSystem->OpenDirectory(d);

core/base/test/CMakeLists.txt

@@ -36,3 +36,5 @@ configure_file(Foo.C Foo.C COPYONLY)
 ROOT_ADD_GTEST(IncludePathTest IncludePathTest.cxx LIBRARIES Core)
 
 ROOT_ADD_GTEST(UUIDTest UUIDTest.cxx LIBRARIES Core)
+
+ROOT_ADD_GTEST(CorePluginManagerTests TPluginManagerTests.cxx LIBRARIES Core)

core/base/test/TPluginManagerTests.cxx

@@ -0,0 +1,73 @@
+#include "gtest/gtest.h"
+
+#include "TPluginManager.h"
+#include "TROOT.h"
+#include "TEnv.h"
+#include "TSystem.h"
+#include "TUUID.h"
+
+#include <fstream>
+
+// Regression test for https://github.com/root-project/root/issues/23536:
+// TPluginManager::FindHandler must correctly resolve plugins across all directories
+// when Root.PluginPath contains multiple colon-separated directory paths.
+TEST(TPluginManager, MultiDirectoryPluginPath)
+{
+   // Save original Root.PluginPath
+   const char *origPluginPath = gEnv->GetValue("Root.PluginPath", (char *)nullptr);
+   TString savedPath = origPluginPath ? origPluginPath : "";
+
+   // Construct default plugin path ($ROOTSYS/etc/plugins)
+   TString defaultPath = "plugins";
+   gSystem->PrependPathName(TROOT::GetEtcDir(), defaultPath);
+
+   // Create a temporary custom plugin directory with a custom macro
+   TUUID uuid;
+   TString tempDir = gSystem->TempDirectory();
+   TString customDir = TString::Format("test_plugins_%s", uuid.AsString());
+   gSystem->PrependPathName(tempDir.Data(), customDir);
+
+   TString baseSubdir = "TCustomPluginBase";
+   gSystem->PrependPathName(customDir.Data(), baseSubdir);
+   ASSERT_EQ(0, gSystem->mkdir(baseSubdir.Data(), /*recursive=*/true));
+
+   TString macroFile = "P010_TCustomPlugin.C";
+   gSystem->PrependPathName(baseSubdir.Data(), macroFile);
+
+   {
+      std::ofstream f(macroFile.Data());
+      ASSERT_TRUE(f.is_open());
+      f << "void P010_TCustomPlugin()\n"
+        << "{\n"
+        << "   gPluginMgr->AddHandler(\"TCustomPluginBase\", \".*\", \"TCustomClass\",\n"
+        << "      \"CustomLib\", \"TCustomClass()\");\n"
+        << "}\n";
+   }
+
+   // Set multi-directory Root.PluginPath: custom directory prepended before default directory
+   TString multiPath = customDir + ":" + defaultPath;
+   gEnv->SetValue("Root.PluginPath", multiPath.Data());
+
+   // 1. Verify that the custom plugin from the first directory is resolved
+   TPluginHandler *hCustom = gPluginMgr->FindHandler("TCustomPluginBase", "custom://test");
+   ASSERT_NE(hCustom, nullptr);
+   EXPECT_STREQ(hCustom->GetClass(), "TCustomClass");
+
+   // 2. Verify that built-in plugins from subsequent directories are resolved properly (not dropped or corrupted)
+   TPluginHandler *hArchive = gPluginMgr->FindHandler("TArchiveFile", "test.zip");
+   ASSERT_NE(hArchive, nullptr);
+   EXPECT_STREQ(hArchive->GetClass(), "TZIPFile");
+
+   TPluginHandler *hSQL = gPluginMgr->FindHandler("TSQLServer", "sqlite://test.db");
+   ASSERT_NE(hSQL, nullptr);
+   EXPECT_STREQ(hSQL->GetClass(), "TSQLiteServer");
+
+   // Clean up
+   gSystem->Unlink(macroFile.Data());
+   gSystem->Unlink(baseSubdir.Data());
+   gSystem->Unlink(customDir.Data());
+
+   if (!savedPath.IsNull()) {
+      gEnv->SetValue("Root.PluginPath", savedPath.Data());
+   }
+}