hugolib: Add .Site.Params.mainSections

Fixes #3206

hugolib/site.go

@@ -1698,6 +1698,36 @@ func (s *Site) assembleSections() {
 				wp.Page.PrevInSection = s.Sections[k][i+1].Page
 			}
 		}
+
+	}
+
+	var (
+		sectionsParamId      = "mainSections"
+		sectionsParamIdLower = strings.ToLower(sectionsParamId)
+		mainSections         interface{}
+		found                bool
+	)
+
+	if mainSections, found = s.Info.Params[sectionsParamIdLower]; !found {
+		// Pick the section with most regular pages
+		var (
+			chosenSection string
+			pageCount     int
+		)
+
+		for sect, pages := range s.Sections {
+			if pages.Count() >= pageCount {
+				chosenSection = sect
+				pageCount = pages.Count()
+			}
+		}
+		mainSections = []string{chosenSection}
+
+		// Try to make this as backwards compatible as possible.
+		s.Info.Params[sectionsParamId] = mainSections
+		s.Info.Params[sectionsParamIdLower] = mainSections
+	} else {
+		s.Info.Params[sectionsParamId] = mainSections
 	}
 }
 

hugolib/site_test.go

@@ -400,6 +400,8 @@ func doTestSectionNaming(t *testing.T, canonify, uglify, pluralize bool) {
 
 	sources := []source.ByteSource{
 		{Name: filepath.FromSlash("sect/doc1.html"), Content: []byte("doc1")},
+		// Add one more page to sect to make sure sect is picked in mainSections
+		{Name: filepath.FromSlash("sect/sect.html"), Content: []byte("sect")},
 		{Name: filepath.FromSlash("Fish and Chips/doc2.html"), Content: []byte("doc2")},
 		{Name: filepath.FromSlash("ラーメン/doc3.html"), Content: []byte("doc3")},
 	}
@@ -419,6 +421,11 @@ func doTestSectionNaming(t *testing.T, canonify, uglify, pluralize bool) {
 	writeSource(t, fs, filepath.Join("layouts", "_default/list.html"), "{{.Title}}")
 
 	s := buildSingleSite(t, deps.DepsCfg{Fs: fs, Cfg: cfg}, BuildCfg{})
+
+	mainSections, err := s.Info.Param("mainSections")
+	require.NoError(t, err)
+	require.Equal(t, mainSections, []string{"sect"})
+
 	th := testHelper{s.Cfg, s.Fs, t}
 	tests := []struct {
 		doc         string