go/format, cmd/gofmt: fix issues with partial Go code with indent

Fixes golang#5551.
Fixes golang#4449.

Adds tests for both issues.

Note that the two issues occur only when formatting partial Go code
with indent.

The best way to understand the change is as follows: I took the code
of cmd/gofmt and go/format, combined it into one unified code that
does not suffer from either 4449 nor 5551, and then applied that code
to both cmd/gofmt and go/format.

As a result, there is now much more identical code between the two
packages, making future code deduplication easier (it was not possible
to do that now without adding public APIs, which I was advised not to
do at this time).

More specifically, I took the parse() of cmd/gofmt which correctly
preserves comments (issue 5551) and modified it to fix issue where
it would sometimes modify literal values (issue 4449).

I ended up removing the matchSpace() function because it no longer
needed to do some of its work (insert indent), and a part of its work
had to be done in advance (determining the indentation of first code
line), because that calculation is required for cfg.Fprint() to run.

adjustIndent is used to adjust the indent of cfg.Fprint() to compensate
for the body of wrapper func being indented by one level. This allows
to get rid of the bytes.Replace text manipulation of inner content,
which was problematic and sometimes altered raw string literals (issue
4449). This means that sometimes the value of cfg.Indent is negative,
but that works as expected.

So now the algorithm for formatting partial Go code is:

1. Determine and prepend leading space of original source.
2. Determine and prepend indentation of first code line.
3. Format and write partial Go code (with all of its leading &
   trailing space trimmed).
4. Determine and append trailing space of original source.

LGTM=gri
R=golang-codereviews, bradfitz, gri
CC=golang-codereviews
https://golang.org/cl/142360043

src/cmd/gofmt/gofmt.go

@@ -87,13 +87,13 @@ func processFile(filename string, in io.Reader, out io.Writer, stdin bool) error
 		return err
 	}
 
-	file, adjust, err := parse(fileSet, filename, src, stdin)
+	file, sourceAdj, indentAdj, err := parse(fileSet, filename, src, stdin)
 	if err != nil {
 		return err
 	}
 
 	if rewrite != nil {
-		if adjust == nil {
+		if sourceAdj == nil {
 			file = rewrite(file)
 		} else {
 			fmt.Fprintf(os.Stderr, "warning: rewrite ignored for incomplete programs\n")
@@ -106,15 +106,10 @@ func processFile(filename string, in io.Reader, out io.Writer, stdin bool) error
 		simplify(file)
 	}
 
-	var buf bytes.Buffer
-	err = (&printer.Config{Mode: printerMode, Tabwidth: tabWidth}).Fprint(&buf, fileSet, file)
+	res, err := format(fileSet, file, sourceAdj, indentAdj, src)
 	if err != nil {
 		return err
 	}
-	res := buf.Bytes()
-	if adjust != nil {
-		res = adjust(src, res)
-	}
 
 	if !bytes.Equal(src, res) {
 		// formatting has changed
@@ -242,17 +237,19 @@ func diff(b1, b2 []byte) (data []byte, err error) {
 
 // parse parses src, which was read from filename,
 // as a Go source file or statement list.
-func parse(fset *token.FileSet, filename string, src []byte, stdin bool) (*ast.File, func(orig, src []byte) []byte, error) {
+func parse(fset *token.FileSet, filename string, src []byte, fragmentOk bool) (
+	file *ast.File,
+	sourceAdj func(src []byte, indent int) []byte,
+	indentAdj int,
+	err error,
+) {
 	// Try as whole source file.
-	file, err := parser.ParseFile(fset, filename, src, parserMode)
-	if err == nil {
-		return file, nil, nil
-	}
-	// If the error is that the source file didn't begin with a
-	// package line and this is standard input, fall through to
+	file, err = parser.ParseFile(fset, filename, src, parserMode)
+	// If there's no error, return.  If the error is that the source file didn't begin with a
+	// package line and source fragments are ok, fall through to
 	// try as a source fragment.  Stop and return on any other error.
-	if !stdin || !strings.Contains(err.Error(), "expected 'package'") {
-		return nil, nil, err
+	if err == nil || !fragmentOk || !strings.Contains(err.Error(), "expected 'package'") {
+		return
 	}
 
 	// If this is a declaration list, make it a source file
@@ -262,19 +259,19 @@ func parse(fset *token.FileSet, filename string, src []byte, stdin bool) (*ast.F
 	psrc := append([]byte("package p;"), src...)
 	file, err = parser.ParseFile(fset, filename, psrc, parserMode)
 	if err == nil {
-		adjust := func(orig, src []byte) []byte {
+		sourceAdj = func(src []byte, indent int) []byte {
 			// Remove the package clause.
 			// Gofmt has turned the ; into a \n.
-			src = src[len("package p\n"):]
-			return matchSpace(orig, src)
+			src = src[indent+len("package p\n"):]
+			return bytes.TrimSpace(src)
 		}
-		return file, adjust, nil
+		return
 	}
 	// If the error is that the source file didn't begin with a
 	// declaration, fall through to try as a statement list.
 	// Stop and return on any other error.
 	if !strings.Contains(err.Error(), "expected declaration") {
-		return nil, nil, err
+		return
 	}
 
 	// If this is a statement list, make it a source file
@@ -285,65 +282,89 @@ func parse(fset *token.FileSet, filename string, src []byte, stdin bool) (*ast.F
 	fsrc := append(append([]byte("package p; func _() {"), src...), '\n', '}')
 	file, err = parser.ParseFile(fset, filename, fsrc, parserMode)
 	if err == nil {
-		adjust := func(orig, src []byte) []byte {
+		sourceAdj = func(src []byte, indent int) []byte {
+			// Cap adjusted indent to zero.
+			if indent < 0 {
+				indent = 0
+			}
 			// Remove the wrapping.
 			// Gofmt has turned the ; into a \n\n.
-			src = src[len("package p\n\nfunc _() {"):]
-			src = src[:len(src)-len("\n}\n")]
-			// Gofmt has also indented the function body one level.
-			// Remove that indent.
-			src = bytes.Replace(src, []byte("\n\t"), []byte("\n"), -1)
-			return matchSpace(orig, src)
+			// There will be two non-blank lines with indent, hence 2*indent.
+			src = src[2*indent+len("package p\n\nfunc _() {"):]
+			src = src[:len(src)-(indent+len("\n}\n"))]
+			return bytes.TrimSpace(src)
 		}
-		return file, adjust, nil
+		// Gofmt has also indented the function body one level.
+		// Adjust that with indentAdj.
+		indentAdj = -1
 	}
 
-	// Failed, and out of options.
-	return nil, nil, err
+	// Succeeded, or out of options.
+	return
 }
 
-func cutSpace(b []byte) (before, middle, after []byte) {
-	i := 0
-	for i < len(b) && (b[i] == ' ' || b[i] == '\t' || b[i] == '\n') {
-		i++
-	}
-	j := len(b)
-	for j > 0 && (b[j-1] == ' ' || b[j-1] == '\t' || b[j-1] == '\n') {
-		j--
-	}
-	if i <= j {
-		return b[:i], b[i:j], b[j:]
+func format(fset *token.FileSet, file *ast.File, sourceAdj func(src []byte, indent int) []byte, indentAdj int, src []byte) ([]byte, error) {
+	if sourceAdj == nil {
+		// Complete source file.
+		var buf bytes.Buffer
+		err := (&printer.Config{Mode: printerMode, Tabwidth: tabWidth}).Fprint(&buf, fset, file)
+		if err != nil {
+			return nil, err
+		}
+		return buf.Bytes(), nil
 	}
-	return nil, nil, b[j:]
-}
 
-// matchSpace reformats src to use the same space context as orig.
-// 1) If orig begins with blank lines, matchSpace inserts them at the beginning of src.
-// 2) matchSpace copies the indentation of the first non-blank line in orig
-//    to every non-blank line in src.
-// 3) matchSpace copies the trailing space from orig and uses it in place
-//   of src's trailing space.
-func matchSpace(orig []byte, src []byte) []byte {
-	before, _, after := cutSpace(orig)
-	i := bytes.LastIndex(before, []byte{'\n'})
-	before, indent := before[:i+1], before[i+1:]
-
-	_, src, _ = cutSpace(src)
-
-	var b bytes.Buffer
-	b.Write(before)
-	for len(src) > 0 {
-		line := src
-		if i := bytes.IndexByte(line, '\n'); i >= 0 {
-			line, src = line[:i+1], line[i+1:]
-		} else {
-			src = nil
+	// Partial source file.
+	// Determine and prepend leading space.
+	i, j := 0, 0
+	for j < len(src) && isSpace(src[j]) {
+		if src[j] == '\n' {
+			i = j + 1 // byte offset of last line in leading space
 		}
-		if len(line) > 0 && line[0] != '\n' { // not blank
-			b.Write(indent)
+		j++
+	}
+	var res []byte
+	res = append(res, src[:i]...)
+
+	// Determine and prepend indentation of first code line.
+	// Spaces are ignored unless there are no tabs,
+	// in which case spaces count as one tab.
+	indent := 0
+	hasSpace := false
+	for _, b := range src[i:j] {
+		switch b {
+		case ' ':
+			hasSpace = true
+		case '\t':
+			indent++
 		}
-		b.Write(line)
 	}
-	b.Write(after)
-	return b.Bytes()
+	if indent == 0 && hasSpace {
+		indent = 1
+	}
+	for i := 0; i < indent; i++ {
+		res = append(res, '\t')
+	}
+
+	// Format the source.
+	// Write it without any leading and trailing space.
+	cfg := &printer.Config{Mode: printerMode, Tabwidth: tabWidth}
+	cfg.Indent = indent + indentAdj
+	var buf bytes.Buffer
+	err := cfg.Fprint(&buf, fset, file)
+	if err != nil {
+		return nil, err
+	}
+	res = append(res, sourceAdj(buf.Bytes(), cfg.Indent)...)
+
+	// Determine and append trailing space.
+	i = len(src)
+	for i > 0 && isSpace(src[i-1]) {
+		i--
+	}
+	return append(res, src[i:]...), nil
+}
+
+func isSpace(b byte) bool {
+	return b == ' ' || b == '\t' || b == '\n' || b == '\r'
 }

src/cmd/gofmt/long_test.go

@@ -32,7 +32,7 @@ var (
 )
 
 func gofmt(fset *token.FileSet, filename string, src *bytes.Buffer) error {
-	f, _, err := parse(fset, filename, src.Bytes(), false)
+	f, _, _, err := parse(fset, filename, src.Bytes(), false)
 	if err != nil {
 		return err
 	}
@@ -60,7 +60,7 @@ func testFile(t *testing.T, b1, b2 *bytes.Buffer, filename string) {
 
 	// exclude files w/ syntax errors (typically test cases)
 	fset := token.NewFileSet()
-	if _, _, err = parse(fset, filename, b1.Bytes(), false); err != nil {
+	if _, _, _, err = parse(fset, filename, b1.Bytes(), false); err != nil {
 		if *verbose {
 			fmt.Fprintf(os.Stderr, "ignoring %s\n", err)
 		}

src/cmd/gofmt/testdata/stdin6.golden

@@ -0,0 +1,19 @@
+	//gofmt -stdin
+
+	if err != nil {
+		source := strings.NewReader(`line 1.
+line 2.
+`)
+		return source
+	}
+
+	f := func(hat, tail string) {
+
+		fmt.Println(hat+`
+foo
+
+
+`+tail,
+			"more",
+			"and more")
+	}

src/cmd/gofmt/testdata/stdin6.input

@@ -0,0 +1,21 @@
+	//gofmt -stdin
+
+	if err != nil {
+		source := strings.NewReader(`line 1.
+line 2.
+`)
+		return source
+	}
+
+	f:=func(  hat, tail string){
+
+
+
+		   fmt.  Println  (   hat+   `
+foo
+
+
+`+ tail  ,
+ "more" ,
+		"and more"   )
+	}

src/cmd/gofmt/testdata/stdin7.golden

@@ -0,0 +1,19 @@
+															//gofmt -stdin
+
+															if err != nil {
+																source := strings.NewReader(`line 1.
+line 2.
+`)
+																return source
+															}
+
+															f := func(hat, tail string) {
+
+																fmt.Println(hat+`
+		foo
+
+
+	`+tail,
+																	"more",
+																	"and more")
+															}

src/cmd/gofmt/testdata/stdin7.input

@@ -0,0 +1,21 @@
+															//gofmt -stdin
+
+															if err != nil {
+								source := strings.NewReader(`line 1.
+line 2.
+`)
+								return source
+							}
+
+							f:=func(  hat, tail string){
+
+
+
+								   fmt.  Println  (   hat+   `
+		foo
+
+
+	`+ tail  ,
+						 "more" ,
+								"and more"   )
+							}