From 8d42a7942ad2cc3ecff2ad90502c7e9e87da5d52 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B8rn=20Erik=20Pedersen?=
 <bjorn.erik.pedersen@gmail.com>
Date: Thu, 1 Feb 2024 16:35:41 +0100
Subject: [PATCH] Improve nilpointer error message

---
 common/herrors/errors.go  | 21 +++++++++++++++++++++
 hugolib/paginator_test.go | 12 ++++++++++++
 hugolib/site_render.go    |  3 ++-
 3 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/common/herrors/errors.go b/common/herrors/errors.go
index 59739a86a..7c389c1ae 100644
--- a/common/herrors/errors.go
+++ b/common/herrors/errors.go
@@ -19,8 +19,10 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"regexp"
 	"runtime"
 	"runtime/debug"
+	"strings"
 	"time"
 )
 
@@ -116,3 +118,22 @@ func IsNotExist(err error) bool {
 
 	return false
 }
+
+var nilPointerErrRe = regexp.MustCompile(`at <(.*)>: error calling (.*?): runtime error: invalid memory address or nil pointer dereference`)
+
+func ImproveIfNilPointer(inErr error) (outErr error) {
+	outErr = inErr
+
+	m := nilPointerErrRe.FindStringSubmatch(inErr.Error())
+	if len(m) == 0 {
+		return
+	}
+	call := m[1]
+	field := m[2]
+	parts := strings.Split(call, ".")
+	receiverName := parts[len(parts)-2]
+	receiver := strings.Join(parts[:len(parts)-1], ".")
+	s := fmt.Sprintf("– %s is nil; wrap it in if or with: {{ with %s }}{{ .%s }}{{ end }}", receiverName, receiver, field)
+	outErr = errors.New(nilPointerErrRe.ReplaceAllString(inErr.Error(), s))
+	return
+}
diff --git a/hugolib/paginator_test.go b/hugolib/paginator_test.go
index 2ce07805e..fb592bed1 100644
--- a/hugolib/paginator_test.go
+++ b/hugolib/paginator_test.go
@@ -170,3 +170,15 @@ Paginator: {{ .Paginator }}
 	b.Assert(err, qt.IsNotNil)
 	b.Assert(err.Error(), qt.Contains, `error calling Paginator: pagination not supported for this page: kind: "page"`)
 }
+
+func TestNilPointerErrorMessage(t *testing.T) {
+	files := `
+-- hugo.toml --
+-- content/p1.md --
+-- layouts/_default/single.html --
+Home Filename: {{ site.Home.File.Filename }}
+`
+	b, err := TestE(t, files)
+	b.Assert(err, qt.IsNotNil)
+	b.Assert(err.Error(), qt.Contains, `_default/single.html:1:22: executing "_default/single.html" – File is nil; wrap it in if or with: {{ with site.Home.File }}{{ .Filename }}{{ end }}`)
+}
diff --git a/hugolib/site_render.go b/hugolib/site_render.go
index 138530680..ef3e3aeef 100644
--- a/hugolib/site_render.go
+++ b/hugolib/site_render.go
@@ -20,6 +20,7 @@ import (
 	"strings"
 	"sync"
 
+	"github.com/gohugoio/hugo/common/herrors"
 	"github.com/gohugoio/hugo/hugolib/doctree"
 
 	"github.com/gohugoio/hugo/config"
@@ -110,7 +111,7 @@ func (s *Site) renderPages(ctx *siteRenderContext) error {
 
 	err := <-errs
 	if err != nil {
-		return fmt.Errorf("failed to render pages: %w", err)
+		return fmt.Errorf("failed to render pages: %w", herrors.ImproveIfNilPointer(err))
 	}
 	return nil
 }