From 5bc348d9f781dd05ef26e45edd7e948b02331b81 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Alberto=20Francisco=20Solaz=20Garc=C3=ADa?=
 <albfsg@proton.me>
Date: Fri, 7 Aug 2026 14:07:57 +0200
Subject: [PATCH 1/2] [ntuple] Auto-enable counters if metrics are also enabled

---
 tree/ntuple/inc/ROOT/RNTupleMetrics.hxx | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/tree/ntuple/inc/ROOT/RNTupleMetrics.hxx b/tree/ntuple/inc/ROOT/RNTupleMetrics.hxx
index 5137a4bde1e0c..8cbc78a8842c5 100644
--- a/tree/ntuple/inc/ROOT/RNTupleMetrics.hxx
+++ b/tree/ntuple/inc/ROOT/RNTupleMetrics.hxx
@@ -312,6 +312,8 @@ public:
       auto counter = std::make_unique<std::remove_pointer_t<CounterPtrT>>(name, std::forward<Args>(args)...);
       auto ptrCounter = counter.get();
       fCounters.emplace_back(std::move(counter));
+      if (fIsEnabled)
+         ptrCounter->Enable();
       return ptrCounter;
    }
 

From 6b687ea5c4974e69a89cac1c18dcfae4bbdf2f4e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Alberto=20Francisco=20Solaz=20Garc=C3=ADa?=
 <albfsg@proton.me>
Date: Fri, 7 Aug 2026 14:08:57 +0200
Subject: [PATCH 2/2] [ntuple] Enable metrics if ROOT_EXPORT_RNTUPLE_METRICS is
 set

In this commit, `ROOT_EXPORT_RNTUPLE_METRICS` is an optional environment
variable that if set enables the metrics, but does not trigger metric
storage after the program ends.

In a future commit, we will expect the optional environment variable
`ROOT_EXPORT_RNTUPLE_METRICS` to be set to a path. If done, metrics will be
enabled and RNTupleMetrics will store the metrics there as a `.root`
file.
---
 roottest/root/ntuple/metrics/CMakeLists.txt   |  8 +++++
 .../ntuple/metrics/metrics_env_disabled.ref   |  3 ++
 .../ntuple/metrics/metrics_env_enabled.ref    |  3 ++
 .../ntuple/metrics/test_rntuple_metrics_env.C | 32 +++++++++++++++++++
 tree/ntuple/inc/ROOT/RNTupleMetrics.hxx       | 11 ++++++-
 tree/ntuple/src/RNTupleMetrics.cxx            | 12 +++++++
 6 files changed, 68 insertions(+), 1 deletion(-)
 create mode 100644 roottest/root/ntuple/metrics/CMakeLists.txt
 create mode 100644 roottest/root/ntuple/metrics/metrics_env_disabled.ref
 create mode 100644 roottest/root/ntuple/metrics/metrics_env_enabled.ref
 create mode 100644 roottest/root/ntuple/metrics/test_rntuple_metrics_env.C

diff --git a/roottest/root/ntuple/metrics/CMakeLists.txt b/roottest/root/ntuple/metrics/CMakeLists.txt
new file mode 100644
index 0000000000000..24fb47b881f7b
--- /dev/null
+++ b/roottest/root/ntuple/metrics/CMakeLists.txt
@@ -0,0 +1,8 @@
+ROOTTEST_ADD_TEST(metrics_env_enabled
+                  MACRO test_rntuple_metrics_env.C
+                  ENVIRONMENT ROOT_EXPORT_RNTUPLE_METRICS=metrics_env_enabled.root
+                  OUTREF metrics_env_enabled.ref)
+
+ROOTTEST_ADD_TEST(metrics_env_disabled
+                  MACRO test_rntuple_metrics_env.C
+                  OUTREF metrics_env_disabled.ref)
diff --git a/roottest/root/ntuple/metrics/metrics_env_disabled.ref b/roottest/root/ntuple/metrics/metrics_env_disabled.ref
new file mode 100644
index 0000000000000..71b9e67cef4cb
--- /dev/null
+++ b/roottest/root/ntuple/metrics/metrics_env_disabled.ref
@@ -0,0 +1,3 @@
+Processing test_rntuple_metrics_env.C...
+Metrics enabled: false
+nPageCommitted: 0
diff --git a/roottest/root/ntuple/metrics/metrics_env_enabled.ref b/roottest/root/ntuple/metrics/metrics_env_enabled.ref
new file mode 100644
index 0000000000000..289372a209b92
--- /dev/null
+++ b/roottest/root/ntuple/metrics/metrics_env_enabled.ref
@@ -0,0 +1,3 @@
+Processing test_rntuple_metrics_env.C...
+Metrics enabled: true
+nPageCommitted: 1
diff --git a/roottest/root/ntuple/metrics/test_rntuple_metrics_env.C b/roottest/root/ntuple/metrics/test_rntuple_metrics_env.C
new file mode 100644
index 0000000000000..f3baefae64ffc
--- /dev/null
+++ b/roottest/root/ntuple/metrics/test_rntuple_metrics_env.C
@@ -0,0 +1,32 @@
+#include <ROOT/RNTupleModel.hxx>
+#include <ROOT/RNTupleWriter.hxx>
+
+#include <TSystem.h>
+
+#include <cstddef>
+#include <iostream>
+#include <string>
+#include <utility>
+
+void test_rntuple_metrics_env()
+{
+   constexpr std::size_t kNEntries = 1000;
+   const std::string kFileName{"test_metrics_env.root"};
+
+   auto model = ROOT::RNTupleModel::Create();
+   auto pt = model->MakeField<float>("f");
+
+   auto writer = ROOT::RNTupleWriter::Recreate(std::move(model), "ntpl", kFileName);
+   for (std::size_t i = 0; i < kNEntries; ++i) {
+      *pt = static_cast<float>(i);
+      writer->Fill();
+   }
+   writer->CommitDataset();
+
+   const auto &metrics = writer->GetMetrics();
+   const auto *nPageCommitted = metrics.GetCounter("RNTupleWriter.RPageSinkBuf.RPageSinkFile.nPageCommitted");
+
+   std::cout << "Metrics enabled: " << std::boolalpha << metrics.IsEnabled() << std::endl;
+   std::cout << "nPageCommitted: " << (nPageCommitted ? std::to_string(nPageCommitted->GetValueAsInt()) : "?")
+             << std::endl;
+}
diff --git a/tree/ntuple/inc/ROOT/RNTupleMetrics.hxx b/tree/ntuple/inc/ROOT/RNTupleMetrics.hxx
index 8cbc78a8842c5..965234e254851 100644
--- a/tree/ntuple/inc/ROOT/RNTupleMetrics.hxx
+++ b/tree/ntuple/inc/ROOT/RNTupleMetrics.hxx
@@ -282,6 +282,9 @@ using RNTupleAtomicTimer = RNTupleTimer<RNTupleAtomicCounter, RNTupleTickCounter
 \brief A collection of Counter objects with a name, a unit, and a description.
 
 The class owns the counters.
+
+If the environment variable `ROOT_EXPORT_RNTUPLE_METRICS` is set, metrics are automatically enabled on
+construction, and any counter added afterwards through MakeCounter() is enabled as well.
 */
 // clang-format on
 class RNTupleMetrics {
@@ -297,7 +300,12 @@ private:
    bool Contains(const std::string &name) const;
 
 public:
-   explicit RNTupleMetrics(const std::string &name) : fName(name) {}
+   explicit RNTupleMetrics(const std::string &name) : fName(name)
+   {
+      // TODO: Use the value of `GetMetricsExportPath` to save the contents of the metrics in a `.root` file
+      if (!GetMetricsExportPath().empty())
+         Enable();
+   }
    RNTupleMetrics(const RNTupleMetrics &other) = delete;
    RNTupleMetrics & operator=(const RNTupleMetrics &other) = delete;
    RNTupleMetrics(RNTupleMetrics &&other) = default;
@@ -324,6 +332,7 @@ public:
    const RNTuplePerfCounter *GetCounter(std::string_view name) const;
 
    void ObserveMetrics(RNTupleMetrics &observee);
+   static const std::string &GetMetricsExportPath();
 
    void Print(std::ostream &output, const std::string &prefix = "") const;
    void Enable();
diff --git a/tree/ntuple/src/RNTupleMetrics.cxx b/tree/ntuple/src/RNTupleMetrics.cxx
index c02fbd3d88675..5bd4538bfae15 100644
--- a/tree/ntuple/src/RNTupleMetrics.cxx
+++ b/tree/ntuple/src/RNTupleMetrics.cxx
@@ -14,6 +14,8 @@
 
 #include <ROOT/RNTupleMetrics.hxx>
 
+#include <TSystem.h>
+
 #include <ostream>
 
 #include <iostream>
@@ -90,3 +92,13 @@ void ROOT::Experimental::Detail::RNTupleMetrics::ObserveMetrics(RNTupleMetrics &
 {
    fObservedMetrics.push_back(&observee);
 }
+
+const std::string &ROOT::Experimental::Detail::RNTupleMetrics::GetMetricsExportPath()
+{
+   static const std::string path = []() -> std::string {
+      if (const char *env = gSystem->Getenv("ROOT_EXPORT_RNTUPLE_METRICS"); env && *env)
+         return env;
+      return "";
+   }();
+   return path;
+}