imports: Process should accept nil src

The comment of Process() implies src can be nil, but it didn't handle
nil src correctly before because parse() doesn't expect nil src.
Passing []byte(nil) to parser.ParseFile() results in error and parse()
tries to parse again by modifying src if src is statement list.

This problem isn't related with cmd/goimports because goimports doesn't
pass []byte(nil) to Process() as src.

Fixes golang/go#19676

Change-Id: Idbaa6646c3907690763eabc798860c56bb9963d4
Reviewed-on: https://go-review.googlesource.com/38613
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

imports/fix_test.go

@@ -936,6 +936,72 @@ func TestFixImports(t *testing.T) {
 	}
 }
 
+func TestProcess_nil_src(t *testing.T) {
+	dir, err := ioutil.TempDir("", "goimports-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+	tests := []struct {
+		name    string
+		in, out string
+	}{
+		{
+			name: "nil-src",
+			in: `package foo
+func bar() {
+fmt.Println("hi")
+}
+`,
+			out: `package foo
+
+import "fmt"
+
+func bar() {
+	fmt.Println("hi")
+}
+`,
+		},
+		{
+			name: "missing package",
+			in: `
+func bar() {
+fmt.Println("hi")
+}
+`,
+			out: `
+import "fmt"
+
+func bar() {
+	fmt.Println("hi")
+}
+`,
+		},
+	}
+
+	options := &Options{
+		TabWidth:  8,
+		TabIndent: true,
+		Comments:  true,
+		Fragment:  true,
+	}
+
+	for _, tt := range tests {
+		filename := filepath.Join(dir, tt.name+".go")
+		if err := ioutil.WriteFile(filename, []byte(tt.in), 0666); err != nil {
+			t.Fatal(err)
+		}
+		buf, err := Process(filename, nil, options)
+		if err != nil {
+			t.Errorf("error on %q: %v", tt.name, err)
+			continue
+		}
+		if got := string(buf); got != tt.out {
+			t.Errorf("results diff on %q\nGOT:\n%s\nWANT:\n%s\n", tt.name, got, tt.out)
+		}
+	}
+}
+
 // Test support for packages in GOPATH that are actually symlinks.
 // Also test that a symlink loop does not block the process.
 func TestImportSymlinks(t *testing.T) {

imports/imports.go

@@ -18,6 +18,7 @@ import (
 	"go/printer"
 	"go/token"
 	"io"
+	"io/ioutil"
 	"regexp"
 	"strconv"
 	"strings"
@@ -47,6 +48,13 @@ func Process(filename string, src []byte, opt *Options) ([]byte, error) {
 	if opt == nil {
 		opt = &Options{Comments: true, TabIndent: true, TabWidth: 8}
 	}
+	if src == nil {
+		b, err := ioutil.ReadFile(filename)
+		if err != nil {
+			return nil, err
+		}
+		src = b
+	}
 
 	fileSet := token.NewFileSet()
 	file, adjust, err := parse(fileSet, filename, src, opt)