imports: fix reading from stdin on Windows

Don't panic when reading from stdin on Windows. This is a regression
from https://golang.org/cl/43454

Also fix some weird behavior with stdin processing I noticed during
reviewing the code: don't allow the -w (write) flag, and adust the
filename shown with the -d (diff) flag.

Fixes golang/go#20941

Change-Id: I73d0a1dc74c919238a3bb72823585bbf1b7daba1
Reviewed-on: https://go-review.googlesource.com/47810
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Jones <rbjones@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/goimports/goimports.go

@@ -144,12 +144,19 @@ func processFile(filename string, in io.Reader, out io.Writer, argType argumentT
 			fmt.Fprintln(out, filename)
 		}
 		if *write {
+			if argType == fromStdin {
+				// filename is "<standard input>"
+				return errors.New("can't use -w on stdin")
+			}
 			err = ioutil.WriteFile(filename, res, 0)
 			if err != nil {
 				return err
 			}
 		}
 		if *doDiff {
+			if argType == fromStdin {
+				filename = "stdin.go" // because <standard input>.orig looks silly
+			}
 			data, err := diff(src, res, filename)
 			if err != nil {
 				return fmt.Errorf("computing diff: %s", err)

imports/fix.go

@@ -254,11 +254,13 @@ func fixImports(fset *token.FileSet, f *ast.File, filename string) (added []stri
 	results := make(chan result)
 	for pkgName, symbols := range refs {
 		go func(pkgName string, symbols map[string]bool) {
+			if packageInfo != nil {
 				sibling := packageInfo.Imports[pkgName]
 				if sibling.Path != "" {
 					results <- result{ipath: sibling.Path, name: sibling.Alias}
 					return
 				}
+			}
 			ipath, rename, err := findImport(pkgName, symbols, filename)
 			r := result{ipath: ipath, err: err}
 			if rename {

imports/fix_test.go

@@ -1803,3 +1803,14 @@ func TestShouldTraverse(t *testing.T) {
 		}
 	}
 }
+
+// Issue 20941: this used to panic on Windows.
+func TestProcessStdin(t *testing.T) {
+	got, err := Process("<standard input>", []byte("package main\nfunc main() {\n\tfmt.Println(123)\n}\n"), nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(string(got), `"fmt"`) {
+		t.Errorf("expected fmt import; got: %s", got)
+	}
+}