From 54ef9733b2797e5250781cf2e4eb165e14206afd Mon Sep 17 00:00:00 2001
From: Sebastien Binet <binet@cern.ch>
Date: Thu, 14 Nov 2024 17:24:43 +0100
Subject: [PATCH] groot/riofs: drop mmap-ing ROOT files by default, add
 file+mmap:// convenience protocol

Fixes #1014.

Signed-off-by: Sebastien Binet <binet@cern.ch>
---
 groot/example_test.go     | 51 +++++++++++++++++++++++++++++++++++++++
 groot/riofs/fileplugin.go |  7 ++++++
 2 files changed, 58 insertions(+)

diff --git a/groot/example_test.go b/groot/example_test.go
index d80992e81..9a3265860 100644
--- a/groot/example_test.go
+++ b/groot/example_test.go
@@ -226,3 +226,54 @@ func ExampleOpen_graph() {
 	// (x,y)[2] = (+3.000000e+00, +6.000000e+00)
 	// (x,y)[3] = (+4.000000e+00, +8.000000e+00)
 }
+
+// ExampleOpen_file shows how users can open a local ROOT file with the 'file://' protocol.
+func ExampleOpen_file() {
+	f, err := groot.Open("file://./testdata/simple.root")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer f.Close()
+
+	for _, key := range f.Keys() {
+		fmt.Printf("key:  %q cycle=%d title=%q\n", key.Name(), key.Cycle(), key.Title())
+	}
+
+	obj, err := f.Get("tree")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	tree := obj.(rtree.Tree)
+	fmt.Printf("tree: %q, entries=%d\n", tree.Name(), tree.Entries())
+
+	// Output:
+	// key:  "tree" cycle=1 title="fake data"
+	// tree: "tree", entries=4
+}
+
+// ExampleOpen_mmap shows how users can open and mmap a local ROOT file.
+// mmap-ing a file may be useful for performances reasons.
+func ExampleOpen_mmap() {
+	f, err := groot.Open("file+mmap://./testdata/simple.root")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer f.Close()
+
+	for _, key := range f.Keys() {
+		fmt.Printf("key:  %q cycle=%d title=%q\n", key.Name(), key.Cycle(), key.Title())
+	}
+
+	obj, err := f.Get("tree")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	tree := obj.(rtree.Tree)
+	fmt.Printf("tree: %q, entries=%d\n", tree.Name(), tree.Entries())
+
+	// Output:
+	// key:  "tree" cycle=1 title="fake data"
+	// tree: "tree", entries=4
+}
diff --git a/groot/riofs/fileplugin.go b/groot/riofs/fileplugin.go
index e2bd0f053..cb8362ff0 100644
--- a/groot/riofs/fileplugin.go
+++ b/groot/riofs/fileplugin.go
@@ -7,6 +7,7 @@ package riofs
 import (
 	"fmt"
 	"net/url"
+	"os"
 	"sort"
 	"strings"
 	"sync"
@@ -70,9 +71,15 @@ func openFile(path string) (Reader, error) {
 
 func openLocalFile(path string) (Reader, error) {
 	path = strings.TrimPrefix(path, "file://")
+	return os.Open(path)
+}
+
+func mmapLocalFile(path string) (Reader, error) {
+	path = strings.TrimPrefix(path, "file+mmap://")
 	return mmap.Open(path)
 }
 
 func init() {
 	Register("file", openLocalFile)
+	Register("file+mmap", mmapLocalFile)
 }