hugolib: Enable override of theme base template only

This commit fixes the base template lookup order to match the behaviour of regular templates.

```
1. <current-path>/<template-name>-baseof.<suffix>, e.g. list-baseof.<suffix>.
2. <current-path>/baseof.<suffix>
3. _default/<template-name>-baseof.<suffix>, e.g. list-baseof.<suffix>.
4. _default/baseof.<suffix>

For each of the steps above, it will first look in the project, then, if theme is set,
in the theme's layouts folder.
```

Fixes #2783

hugolib/template_test.go

@@ -0,0 +1,145 @@
+// Copyright 2016 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.
+
+package hugolib
+
+import (
+	"path/filepath"
+	"testing"
+
+	"github.com/spf13/viper"
+)
+
+func TestBaseGoTemplate(t *testing.T) {
+	// Variants:
+	//   1. <current-path>/<template-name>-baseof.<suffix>, e.g. list-baseof.<suffix>.
+	//   2. <current-path>/baseof.<suffix>
+	//   3. _default/<template-name>-baseof.<suffix>, e.g. list-baseof.<suffix>.
+	//   4. _default/baseof.<suffix>
+	for i, this := range []struct {
+		setup  func(t *testing.T)
+		assert func(t *testing.T)
+	}{
+		{
+			// Variant 1
+			func(t *testing.T) {
+				writeSource(t, filepath.Join("layouts", "section", "sect-baseof.html"), `Base: {{block "main" .}}block{{end}}`)
+				writeSource(t, filepath.Join("layouts", "section", "sect.html"), `{{define "main"}}sect{{ end }}`)
+
+			},
+			func(t *testing.T) {
+				assertFileContent(t, filepath.Join("public", "sect", "index.html"), false, "Base: sect")
+			},
+		},
+		{
+			// Variant 2
+			func(t *testing.T) {
+				writeSource(t, filepath.Join("layouts", "baseof.html"), `Base: {{block "main" .}}block{{end}}`)
+				writeSource(t, filepath.Join("layouts", "index.html"), `{{define "main"}}index{{ end }}`)
+
+			},
+			func(t *testing.T) {
+				assertFileContent(t, filepath.Join("public", "index.html"), false, "Base: index")
+			},
+		},
+		{
+			// Variant 3
+			func(t *testing.T) {
+				writeSource(t, filepath.Join("layouts", "_default", "list-baseof.html"), `Base: {{block "main" .}}block{{end}}`)
+				writeSource(t, filepath.Join("layouts", "_default", "list.html"), `{{define "main"}}list{{ end }}`)
+
+			},
+			func(t *testing.T) {
+				assertFileContent(t, filepath.Join("public", "sect", "index.html"), false, "Base: list")
+			},
+		},
+		{
+			// Variant 4
+			func(t *testing.T) {
+				writeSource(t, filepath.Join("layouts", "_default", "baseof.html"), `Base: {{block "main" .}}block{{end}}`)
+				writeSource(t, filepath.Join("layouts", "_default", "list.html"), `{{define "main"}}list{{ end }}`)
+
+			},
+			func(t *testing.T) {
+				assertFileContent(t, filepath.Join("public", "sect", "index.html"), false, "Base: list")
+			},
+		},
+		{
+			// Variant 1, theme,  use project's base
+			func(t *testing.T) {
+				viper.Set("theme", "mytheme")
+				writeSource(t, filepath.Join("layouts", "section", "sect-baseof.html"), `Base: {{block "main" .}}block{{end}}`)
+				writeSource(t, filepath.Join("themes", "mytheme", "layouts", "section", "sect-baseof.html"), `Base Theme: {{block "main" .}}block{{end}}`)
+				writeSource(t, filepath.Join("layouts", "section", "sect.html"), `{{define "main"}}sect{{ end }}`)
+
+			},
+			func(t *testing.T) {
+				assertFileContent(t, filepath.Join("public", "sect", "index.html"), false, "Base: sect")
+			},
+		},
+		{
+			// Variant 1, theme,  use theme's base
+			func(t *testing.T) {
+				viper.Set("theme", "mytheme")
+				writeSource(t, filepath.Join("themes", "mytheme", "layouts", "section", "sect-baseof.html"), `Base Theme: {{block "main" .}}block{{end}}`)
+				writeSource(t, filepath.Join("layouts", "section", "sect.html"), `{{define "main"}}sect{{ end }}`)
+
+			},
+			func(t *testing.T) {
+				assertFileContent(t, filepath.Join("public", "sect", "index.html"), false, "Base Theme: sect")
+			},
+		},
+		{
+			// Variant 4, theme, use project's base
+			func(t *testing.T) {
+				viper.Set("theme", "mytheme")
+				writeSource(t, filepath.Join("layouts", "_default", "baseof.html"), `Base: {{block "main" .}}block{{end}}`)
+				writeSource(t, filepath.Join("themes", "mytheme", "layouts", "_default", "baseof.html"), `Base Theme: {{block "main" .}}block{{end}}`)
+				writeSource(t, filepath.Join("themes", "mytheme", "layouts", "_default", "list.html"), `{{define "main"}}list{{ end }}`)
+
+			},
+			func(t *testing.T) {
+				assertFileContent(t, filepath.Join("public", "sect", "index.html"), false, "Base: list")
+			},
+		},
+		{
+			// Variant 4, theme, use themes's base
+			func(t *testing.T) {
+				viper.Set("theme", "mytheme")
+				writeSource(t, filepath.Join("themes", "mytheme", "layouts", "_default", "baseof.html"), `Base Theme: {{block "main" .}}block{{end}}`)
+				writeSource(t, filepath.Join("themes", "mytheme", "layouts", "_default", "list.html"), `{{define "main"}}list{{ end }}`)
+
+			},
+			func(t *testing.T) {
+				assertFileContent(t, filepath.Join("public", "sect", "index.html"), false, "Base Theme: list")
+			},
+		},
+	} {
+
+		testCommonResetState()
+
+		writeSource(t, filepath.Join("content", "sect", "page.md"), `---
+title: Template test
+---
+Some content
+`)
+		this.setup(t)
+
+		if err := buildAndRenderSite(newSiteDefaultLang()); err != nil {
+			t.Fatalf("[%d] Failed to build site: %s", i, err)
+		}
+
+		this.assert(t)
+
+	}
+}