Avoid nilpointer on no File on Page

Fixes #5781

15 files changed, 213 insertions, 16 deletions

diff --git a/codegen/methods.go b/codegen/methods.go
index 007384f9b..ed8dba923 100644
--- a/codegen/methods.go
+++ b/codegen/methods.go
@@ -288,6 +288,12 @@ func (m Method) Declaration(receiver string) string {
 	return fmt.Sprintf("func (%s %s) %s%s %s", receiverShort(receiver), receiver, m.Name, m.inStr(), m.outStr())
 }
 
+// DeclarationNamed creates a method declaration (without any body) for the given receiver
+// with named return values.
+func (m Method) DeclarationNamed(receiver string) string {
+	return fmt.Sprintf("func (%s %s) %s%s %s", receiverShort(receiver), receiver, m.Name, m.inStr(), m.outStrNamed())
+}
+
 // Delegate creates a delegate call string.
 func (m Method) Delegate(receiver, delegate string) string {
 	ret := ""
@@ -336,6 +342,19 @@ func (m Method) outStr() string {
 	return "(" + strings.Join(m.Out, ", ") + ")"
 }
 
+func (m Method) outStrNamed() string {
+	if len(m.Out) == 0 {
+		return ""
+	}
+
+	outs := make([]string, len(m.Out))
+	for i := 0; i < len(outs); i++ {
+		outs[i] = fmt.Sprintf("o%d %s", i, m.Out[i])
+	}
+
+	return "(" + strings.Join(outs, ", ") + ")"
+}
+
 // Methods represents a list of methods for one or more interfaces.
 // The order matches the defined order in their source file(s).
 type Methods []Method
diff --git a/commands/convert.go b/commands/convert.go
index e7ba572bc..d0a46a641 100644
--- a/commands/convert.go
+++ b/commands/convert.go
@@ -143,7 +143,7 @@ func (cc *convertCmd) convertAndSavePage(p page.Page, site *hugolib.Site, target
 		}
 	}
 
-	if p.File() == nil {
+	if p.File().IsZero() {
 		// No content file.
 		return nil
 	}
diff --git a/deps/deps.go b/deps/deps.go
index 47159d017..fa62fe5ae 100644
--- a/deps/deps.go
+++ b/deps/deps.go
@@ -34,6 +34,9 @@ type Deps struct {
 	// Used to log errors that may repeat itself many times.
 	DistinctErrorLog *helpers.DistinctLogger
 
+	// Used to log warnings that may repeat itself many times.
+	DistinctWarningLog *helpers.DistinctLogger
+
 	// The templates to use. This will usually implement the full tpl.TemplateHandler.
 	Tmpl tpl.TemplateFinder `json:"-"`
 
@@ -233,11 +236,13 @@ func New(cfg DepsCfg) (*Deps, error) {
 	}
 
 	distinctErrorLogger := helpers.NewDistinctLogger(logger.ERROR)
+	distinctWarnLogger := helpers.NewDistinctLogger(logger.WARN)
 
 	d := &Deps{
 		Fs:                  fs,
 		Log:                 logger,
 		DistinctErrorLog:    distinctErrorLogger,
+		DistinctWarningLog:  distinctWarnLogger,
 		templateProvider:    cfg.TemplateProvider,
 		translationProvider: cfg.TranslationProvider,
 		WithTemplate:        cfg.WithTemplate,
diff --git a/hugolib/hugo_sites.go b/hugolib/hugo_sites.go
index 0c79f4bdb..9fe2a5bdb 100644
--- a/hugolib/hugo_sites.go
+++ b/hugolib/hugo_sites.go
@@ -577,7 +577,7 @@ func (cfg *BuildCfg) shouldRender(p *pageState) bool {
 		return true
 	}
 
-	if cfg.whatChanged != nil && p.File() != nil {
+	if cfg.whatChanged != nil && !p.File().IsZero() {
 		return cfg.whatChanged.files[p.File().Filename()]
 	}
 
diff --git a/hugolib/page.go b/hugolib/page.go
index 576342cfa..99b771eee 100644
--- a/hugolib/page.go
+++ b/hugolib/page.go
@@ -236,7 +236,7 @@ func (p *pageState) TranslationKey() string {
 	p.translationKeyInit.Do(func() {
 		if p.m.translationKey != "" {
 			p.translationKey = p.Kind() + "/" + p.m.translationKey
-		} else if p.IsPage() && p.File() != nil {
+		} else if p.IsPage() && !p.File().IsZero() {
 			p.translationKey = path.Join(p.Kind(), filepath.ToSlash(p.File().Dir()), p.File().TranslationBaseName())
 		} else if p.IsNode() {
 			p.translationKey = path.Join(p.Kind(), p.SectionsPath())
@@ -462,7 +462,7 @@ func (p *pageState) Render(layout ...string) template.HTML {
 func (p *pageState) wrapError(err error) error {
 
 	var filename string
-	if p.File() != nil {
+	if !p.File().IsZero() {
 		filename = p.File().Filename()
 	}
 
@@ -665,7 +665,7 @@ func (p *pageState) parseError(err error, input []byte, offset int) error {
 }
 
 func (p *pageState) pathOrTitle() string {
-	if p.File() != nil {
+	if !p.File().IsZero() {
 		return p.File().Filename()
 	}
 
@@ -763,7 +763,7 @@ func (p *pageState) sortParentSections() {
 // For pages that do not (sections witout content page etc.), it returns the
 // virtual path, consistent with where you would add a source file.
 func (p *pageState) sourceRef() string {
-	if p.File() != nil {
+	if !p.File().IsZero() {
 		sourcePath := p.File().Path()
 		if sourcePath != "" {
 			return "/" + filepath.ToSlash(sourcePath)
diff --git a/hugolib/page__meta.go b/hugolib/page__meta.go
index 8532f5016..2c6b0a85d 100644
--- a/hugolib/page__meta.go
+++ b/hugolib/page__meta.go
@@ -222,7 +222,7 @@ func (p *pageMeta) Params() map[string]interface{} {
 }
 
 func (p *pageMeta) Path() string {
-	if p.File() != nil {
+	if !p.File().IsZero() {
 		return p.File().Path()
 	}
 	return p.SectionsPath()
@@ -256,7 +256,7 @@ func (p *pageMeta) Section() string {
 		return p.sections[0]
 	}
 
-	if p.File() != nil {
+	if !p.File().IsZero() {
 		return p.File().Section()
 	}
 
@@ -536,8 +536,8 @@ func (pm *pageMeta) setMetadata(p *pageState, frontmatter map[string]interface{}
 
 func (p *pageMeta) applyDefaultValues() error {
 	if p.markup == "" {
-		if p.File() != nil {
-			// Fall back to {file extension
+		if !p.File().IsZero() {
+			// Fall back to file extension
 			p.markup = helpers.GuessType(p.File().Ext())
 		}
 		if p.markup == "" {
diff --git a/hugolib/page__new.go b/hugolib/page__new.go
index 0f419b5da..0ac14a5e8 100644
--- a/hugolib/page__new.go
+++ b/hugolib/page__new.go
@@ -95,6 +95,10 @@ func newPageBase(metaProvider *pageMeta) (*pageState, error) {
 }
 
 func newPageFromMeta(metaProvider *pageMeta) (*pageState, error) {
+	if metaProvider.f == nil {
+		metaProvider.f = page.NewZeroFile(metaProvider.s.DistinctWarningLog)
+	}
+
 	ps, err := newPageBase(metaProvider)
 	if err != nil {
 		return nil, err
diff --git a/hugolib/page__paths.go b/hugolib/page__paths.go
index 0a5dad5ef..8bc7a7535 100644
--- a/hugolib/page__paths.go
+++ b/hugolib/page__paths.go
@@ -99,7 +99,7 @@ func createTargetPathDescriptor(s *Site, p page.Page, pm *pageMeta) (page.Target
 
 	d := s.Deps
 
-	if p.File() != nil {
+	if !p.File().IsZero() {
 		dir = p.File().Dir()
 		baseName = p.File().TranslationBaseName()
 	}
diff --git a/hugolib/page_test.go b/hugolib/page_test.go
index 5e9ac696c..1e362e0dc 100644
--- a/hugolib/page_test.go
+++ b/hugolib/page_test.go
@@ -18,6 +18,8 @@ import (
 	"html/template"
 	"os"
 
+	"github.com/gohugoio/hugo/common/loggers"
+
 	"path/filepath"
 	"strings"
 	"testing"
@@ -1166,6 +1168,12 @@ Content:{{ .Content }}
 
 }
 
+// https://github.com/gohugoio/hugo/issues/5781
+func TestPageWithZeroFile(t *testing.T) {
+	newTestSitesBuilder(t).WithLogger(loggers.NewWarningLogger()).WithSimpleConfigFile().
+		WithTemplatesAdded("index.html", "{{ .File.Filename }}{{ with .File }}{{ .Dir }}{{ end }}").Build(BuildCfg{})
+}
+
 func TestShouldBuild(t *testing.T) {
 	t.Parallel()
 	var past = time.Date(2009, 11, 17, 20, 34, 58, 651387237, time.UTC)
diff --git a/hugolib/site.go b/hugolib/site.go
index 145ae2d49..1bf2d639c 100644
--- a/hugolib/site.go
+++ b/hugolib/site.go
@@ -789,11 +789,11 @@ func (s *siteRefLinker) refLink(ref string, source interface{}, relative bool, o
 	if refURL.Fragment != "" {
 		_ = target
 		link = link + "#" + refURL.Fragment
-		if pctx, ok := target.(pageContext); ok && target.File() != nil && !pctx.getRenderingConfig().PlainIDAnchors {
+		if pctx, ok := target.(pageContext); ok && !target.File().IsZero() && !pctx.getRenderingConfig().PlainIDAnchors {
 			if refURL.Path != "" {
 				link = link + ":" + target.File().UniqueID()
 			}
-		} else if pctx, ok := p.(pageContext); ok && p.File() != nil && !pctx.getRenderingConfig().PlainIDAnchors {
+		} else if pctx, ok := p.(pageContext); ok && !p.File().IsZero() && !pctx.getRenderingConfig().PlainIDAnchors {
 			link = link + ":" + p.File().UniqueID()
 		}
 
diff --git a/magefile.go b/magefile.go
index 04f0499a2..3b74a7e94 100644
--- a/magefile.go
+++ b/magefile.go
@@ -89,6 +89,7 @@ func Generate() error {
 		// TODO(bep) check: stat ./resources/page/*autogen*: no such file or directory
 		"./resources/page/page_marshaljson.autogen.go",
 		"./resources/page/page_wrappers.autogen.go",
+		"./resources/page/zero_file.autogen.go",
 	}
 
 	for _, pattern := range goFmtPatterns {
diff --git a/resources/page/page_generate/generate_page_wrappers.go b/resources/page/page_generate/generate_page_wrappers.go
index 54e1f272e..739d7a00e 100644
--- a/resources/page/page_generate/generate_page_wrappers.go
+++ b/resources/page/page_generate/generate_page_wrappers.go
@@ -63,6 +63,10 @@ func Generate(c *codegen.Inspector) error {
 		return errors.Wrap(err, "failed to generate deprecate wrappers")
 	}
 
+	if err := generateFileIsZeroWrappers(c); err != nil {
+		return errors.Wrap(err, "failed to generate file wrappers")
+	}
+
 	return nil
 }
 
@@ -199,6 +203,68 @@ type pageDeprecated struct {
 	return nil
 }
 
+func generateFileIsZeroWrappers(c *codegen.Inspector) error {
+	filename := filepath.Join(c.ProjectRootDir, packageDir, "zero_file.autogen.go")
+	f, err := os.Create(filename)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	// Generate warnings for zero file access
+
+	warning := func(name string, tp reflect.Type) string {
+		msg := fmt.Sprintf(".File.%s on zero object. Wrap it in if or with: {{ with .File }}{{ .%s }}{{ end }}", name, name)
+
+		return fmt.Sprintf("z.log.Println(%q)", msg)
+	}
+
+	var buff bytes.Buffer
+
+	methods := c.MethodsFromTypes([]reflect.Type{reflect.TypeOf((*source.File)(nil)).Elem()}, nil)
+
+	for _, m := range methods {
+		if m.Name == "IsZero" {
+			continue
+		}
+		fmt.Fprint(&buff, m.DeclarationNamed("zeroFile"))
+		fmt.Fprintln(&buff, " {")
+		fmt.Fprintf(&buff, "\t%s\n", warning(m.Name, m.Owner))
+		if len(m.Out) > 0 {
+			fmt.Fprintln(&buff, "\treturn")
+		}
+		fmt.Fprintln(&buff, "}")
+
+	}
+
+	pkgImports := append(methods.Imports(), "github.com/gohugoio/hugo/helpers", "github.com/gohugoio/hugo/source")
+
+	fmt.Fprintf(f, `%s
+
+package page
+
+%s
+
+// ZeroFile represents a zero value of source.File with warnings if invoked.
+type zeroFile struct {	
+	log *helpers.DistinctLogger 
+}
+
+func NewZeroFile(log *helpers.DistinctLogger) source.File {
+	return zeroFile{log: log}
+}
+
+func (zeroFile) IsZero() bool {
+	return true
+}
+
+%s
+
+`, header, importsString(pkgImports), buff.String())
+
+	return nil
+}
+
 func importsString(imps []string) string {
 	if len(imps) == 0 {
 		return ""
diff --git a/resources/page/pages_sort.go b/resources/page/pages_sort.go
index eb3a28247..7b2a34a6a 100644
--- a/resources/page/pages_sort.go
+++ b/resources/page/pages_sort.go
@@ -51,8 +51,8 @@ var DefaultPageSort = func(p1, p2 Page) bool {
 	if p1.Weight() == p2.Weight() {
 		if p1.Date().Unix() == p2.Date().Unix() {
 			if p1.LinkTitle() == p2.LinkTitle() {
-				if p1.File() == nil || p2.File() == nil {
-					return p1.File() == nil
+				if p1.File().IsZero() || p2.File().IsZero() {
+					return p1.File().IsZero()
 				}
 				return p1.File().Filename() < p2.File().Filename()
 			}
@@ -77,7 +77,7 @@ var languagePageSort = func(p1, p2 Page) bool {
 	if p1.Language().Weight == p2.Language().Weight {
 		if p1.Date().Unix() == p2.Date().Unix() {
 			if p1.LinkTitle() == p2.LinkTitle() {
-				if p1.File() != nil && p2.File() != nil {
+				if !p1.File().IsZero() && !p2.File().IsZero() {
 					return p1.File().Filename() < p2.File().Filename()
 				}
 			}
diff --git a/resources/page/zero_file.autogen.go b/resources/page/zero_file.autogen.go
new file mode 100644
index 000000000..eec1dd66d
--- /dev/null
+++ b/resources/page/zero_file.autogen.go
@@ -0,0 +1,88 @@
+// Copyright 2019 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// This file is autogenerated.
+
+package page
+
+import (
+	"github.com/gohugoio/hugo/helpers"
+	"github.com/gohugoio/hugo/source"
+	"os"
+)
+
+// ZeroFile represents a zero value of source.File with warnings if invoked.
+type zeroFile struct {
+	log *helpers.DistinctLogger
+}
+
+func NewZeroFile(log *helpers.DistinctLogger) source.File {
+	return zeroFile{log: log}
+}
+
+func (zeroFile) IsZero() bool {
+	return true
+}
+
+func (z zeroFile) Path() (o0 string) {
+	z.log.Println(".File.Path on zero object. Wrap it in if or with: {{ with .File }}{{ .Path }}{{ end }}")
+	return
+}
+func (z zeroFile) Section() (o0 string) {
+	z.log.Println(".File.Section on zero object. Wrap it in if or with: {{ with .File }}{{ .Section }}{{ end }}")
+	return
+}
+func (z zeroFile) Lang() (o0 string) {
+	z.log.Println(".File.Lang on zero object. Wrap it in if or with: {{ with .File }}{{ .Lang }}{{ end }}")
+	return
+}
+func (z zeroFile) Filename() (o0 string) {
+	z.log.Println(".File.Filename on zero object. Wrap it in if or with: {{ with .File }}{{ .Filename }}{{ end }}")
+	return
+}
+func (z zeroFile) Dir() (o0 string) {
+	z.log.Println(".File.Dir on zero object. Wrap it in if or with: {{ with .File }}{{ .Dir }}{{ end }}")
+	return
+}
+func (z zeroFile) Extension() (o0 string) {
+	z.log.Println(".File.Extension on zero object. Wrap it in if or with: {{ with .File }}{{ .Extension }}{{ end }}")
+	return
+}
+func (z zeroFile) Ext() (o0 string) {
+	z.log.Println(".File.Ext on zero object. Wrap it in if or with: {{ with .File }}{{ .Ext }}{{ end }}")
+	return
+}
+func (z zeroFile) LogicalName() (o0 string) {
+	z.log.Println(".File.LogicalName on zero object. Wrap it in if or with: {{ with .File }}{{ .LogicalName }}{{ end }}")
+	return
+}
+func (z zeroFile) BaseFileName() (o0 string) {
+	z.log.Println(".File.BaseFileName on zero object. Wrap it in if or with: {{ with .File }}{{ .BaseFileName }}{{ end }}")
+	return
+}
+func (z zeroFile) TranslationBaseName() (o0 string) {
+	z.log.Println(".File.TranslationBaseName on zero object. Wrap it in if or with: {{ with .File }}{{ .TranslationBaseName }}{{ end }}")
+	return
+}
+func (z zeroFile) ContentBaseName() (o0 string) {
+	z.log.Println(".File.ContentBaseName on zero object. Wrap it in if or with: {{ with .File }}{{ .ContentBaseName }}{{ end }}")
+	return
+}
+func (z zeroFile) UniqueID() (o0 string) {
+	z.log.Println(".File.UniqueID on zero object. Wrap it in if or with: {{ with .File }}{{ .UniqueID }}{{ end }}")
+	return
+}
+func (z zeroFile) FileInfo() (o0 os.FileInfo) {
+	z.log.Println(".File.FileInfo on zero object. Wrap it in if or with: {{ with .File }}{{ .FileInfo }}{{ end }}")
+	return
+}
diff --git a/source/fileInfo.go b/source/fileInfo.go
index 3f262fb5e..072b55b6c 100644
--- a/source/fileInfo.go
+++ b/source/fileInfo.go
@@ -53,6 +53,8 @@ type fileOverlap interface {
 	// Lang is the language code for this page. It will be the
 	// same as the site's language code.
 	Lang() string
+
+	IsZero() bool
 }
 
 type FileWithoutOverlap interface {
@@ -187,6 +189,10 @@ func (fi *FileInfo) Open() (hugio.ReadSeekCloser, error) {
 	return f, err
 }
 
+func (fi *FileInfo) IsZero() bool {
+	return fi == nil
+}
+
 // We create a lot of these FileInfo objects, but there are parts of it used only
 // in some cases that is slightly expensive to construct.
 func (fi *FileInfo) init() {