Generate separate data and common files.

The goal is to resolve #23, to make it possible to use vfsgen multiple
times in same package.

Previously, that would cause an error because the same types would be
defined in each generated file.

The approach to solve that is to generate two files:

-	VFS data file
-	common type definitions file

This avoids the multiple declaration issues in an efficient way.

A downside is having 2 files as generated output, instead of one.

generator.go

@@ -22,8 +22,22 @@ import (
 func Generate(input http.FileSystem, opt Options) error {
 	opt.fillMissing()
 
-	// Create output file.
-	f, err := os.Create(opt.Filename)
+	// Create common output file.
+	f, err := os.Create(opt.CommonFilename)
+	if err != nil {
+		return err
+	}
+
+	err = t.ExecuteTemplate(f, "Common", opt)
+	if err != nil {
+		f.Close()
+		return err
+	}
+
+	f.Close()
+
+	// Create data output file.
+	f, err = os.Create(opt.DataFilename)
 	if err != nil {
 		return err
 	}
@@ -45,11 +59,6 @@ func Generate(input http.FileSystem, opt Options) error {
 		return err
 	}
 
-	err = t.ExecuteTemplate(f, "Trailer", toc)
-	if err != nil {
-		return err
-	}
-
 	// Trim any potential excess.
 	cur, err := f.Seek(0, io.SeekCurrent)
 	if err != nil {
@@ -241,17 +250,14 @@ var t = template.Must(template.New("").Funcs(template.FuncMap{
 {{end}}package {{.PackageName}}
 
 import (
-	"bytes"
-	"compress/gzip"
-	"fmt"
-	"io"
-	"io/ioutil"
 	"net/http"
 	"os"
-	pathpkg "path"
 	"time"
 )
 
+// We imported "os", but may end up not using it. Avoid unused import error.
+var _ os.FileInfo
+
 {{comment .VariableComment}}
 var {{.VariableName}} = func() http.FileSystem {
 	mustUnmarshalTextTime := func(text string) time.Time {
@@ -307,7 +313,24 @@ var {{.VariableName}} = func() http.FileSystem {
 
 
 
-{{define "Trailer"}}
+{{define "Common"}}// Code generated by vfsgen; DO NOT EDIT
+
+{{with .BuildTags}}// +build {{.}}
+
+{{end}}package {{.PackageName}}
+
+import (
+	"bytes"
+	"compress/gzip"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"net/http"
+	"os"
+	pathpkg "path"
+	"time"
+)
+
 type vfsgen۰FS map[string]interface{}
 
 func (fs vfsgen۰FS) Open(path string) (http.File, error) {
@@ -317,7 +340,7 @@ func (fs vfsgen۰FS) Open(path string) (http.File, error) {
 		return nil, &os.PathError{Op: "open", Path: path, Err: os.ErrNotExist}
 	}
 
-	switch f := f.(type) {{"{"}}{{if .HasCompressedFile}}
+	switch f := f.(type) {
 	case *vfsgen۰CompressedFileInfo:
 		gr, err := gzip.NewReader(bytes.NewReader(f.compressedContent))
 		if err != nil {
@@ -327,12 +350,12 @@ func (fs vfsgen۰FS) Open(path string) (http.File, error) {
 		return &vfsgen۰CompressedFile{
 			vfsgen۰CompressedFileInfo: f,
 			gr: gr,
-		}, nil{{end}}{{if .HasFile}}
+		}, nil
 	case *vfsgen۰FileInfo:
 		return &vfsgen۰File{
 			vfsgen۰FileInfo: f,
 			Reader:          bytes.NewReader(f.content),
-		}, nil{{end}}
+		}, nil
 	case *vfsgen۰DirInfo:
 		return &vfsgen۰Dir{
 			vfsgen۰DirInfo: f,
@@ -342,7 +365,7 @@ func (fs vfsgen۰FS) Open(path string) (http.File, error) {
 		panic(fmt.Sprintf("unexpected type %T", f))
 	}
 }
-{{if .HasCompressedFile}}
+
 // vfsgen۰CompressedFileInfo is a static definition of a gzip compressed file.
 type vfsgen۰CompressedFileInfo struct {
 	name              string
@@ -413,11 +436,7 @@ func (f *vfsgen۰CompressedFile) Seek(offset int64, whence int) (int64, error) {
 func (f *vfsgen۰CompressedFile) Close() error {
 	return f.gr.Close()
 }
-{{else}}
-// We already imported "compress/gzip" and "io/ioutil", but ended up not using them. Avoid unused import error.
-var _ = gzip.Reader{}
-var _ = ioutil.Discard
-{{end}}{{if .HasFile}}
+
 // vfsgen۰FileInfo is a static definition of an uncompressed file (because it's not worth gzip compressing).
 type vfsgen۰FileInfo struct {
 	name    string
@@ -448,10 +467,7 @@ type vfsgen۰File struct {
 func (f *vfsgen۰File) Close() error {
 	return nil
 }
-{{else}}
-// We already imported "bytes", but ended up not using it. Avoid unused import error.
-var _ = bytes.Reader{}
-{{end}}
+
 // vfsgen۰DirInfo is a static definition of a directory.
 type vfsgen۰DirInfo struct {
 	name    string

generator_test.go

@@ -79,20 +79,22 @@ func TestGenerate_buildAndGofmt(t *testing.T) {
 	}
 
 	for _, test := range tests {
-		filename := filepath.Join(tempDir, test.filename)
+		dataFilename := filepath.Join(tempDir, test.filename)
+		commonFilename := filepath.Join(tempDir, "vfsgencommon.go")
 
 		err = vfsgen.Generate(test.fs, vfsgen.Options{
-			Filename:    filename,
-			PackageName: "test",
+			DataFilename:   dataFilename,
+			CommonFilename: commonFilename,
+			PackageName:    "test",
 		})
 		if err != nil {
 			t.Fatalf("vfsgen.Generate() failed: %v", err)
 		}
 
-		if out, err := exec.Command("go", "build", filename).CombinedOutput(); err != nil {
+		if out, err := exec.Command("go", "build", dataFilename, commonFilename).CombinedOutput(); err != nil {
 			t.Errorf("err: %v\nout: %s", err, out)
 		}
-		if out, err := exec.Command("gofmt", "-d", "-s", filename).Output(); err != nil || len(out) != 0 {
+		if out, err := exec.Command("gofmt", "-d", "-s", dataFilename, commonFilename).Output(); err != nil || len(out) != 0 {
 			t.Errorf("gofmt issue\nerr: %v\nout: %s", err, out)
 		}
 	}

options.go

@@ -7,9 +7,13 @@ import (
 
 // Options for vfsgen code generation.
 type Options struct {
-	// Filename of the generated Go code output (including extension).
+	// DataFilename is the filename of the generated VFS data Go code (including extension).
 	// If left empty, it defaults to "{{toLower .VariableName}}_vfsdata.go".
-	Filename string
+	DataFilename string
+
+	// CommonFilename is the filename of the generated common vfsgen Go code (including extension).
+	// If left empty, it defaults to "vfsgencommon.go".
+	CommonFilename string
 
 	// PackageName is the name of the package in the generated code.
 	// If left empty, it defaults to "main".
@@ -33,11 +37,14 @@ func (opt *Options) fillMissing() {
 	if opt.PackageName == "" {
 		opt.PackageName = "main"
 	}
+	if opt.CommonFilename == "" {
+		opt.CommonFilename = "vfsgencommon.go"
+	}
 	if opt.VariableName == "" {
 		opt.VariableName = "assets"
 	}
-	if opt.Filename == "" {
-		opt.Filename = fmt.Sprintf("%s_vfsdata.go", strings.ToLower(opt.VariableName))
+	if opt.DataFilename == "" {
+		opt.DataFilename = fmt.Sprintf("%s_vfsdata.go", strings.ToLower(opt.VariableName))
 	}
 	if opt.VariableComment == "" {
 		opt.VariableComment = fmt.Sprintf("%s statically implements the virtual filesystem provided to vfsgen.", opt.VariableName)

test/test_gen.go

@@ -23,8 +23,9 @@ func main() {
 	}))
 
 	err := vfsgen.Generate(fs, vfsgen.Options{
-		Filename:    "test_vfsdata_test.go",
-		PackageName: "test_test",
+		DataFilename:   "test_vfsdata_test.go",
+		CommonFilename: "vfsgencommon_test.go",
+		PackageName:    "test_test",
 	})
 	if err != nil {
 		log.Fatalln(err)