go/printer, cmd/gofmt: print import paths in double quotes

Fixes #9644.

Change-Id: Ia2e42befa20233107ac5409e79f9dce794983a3f
Reviewed-on: https://go-review.googlesource.com/3200Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/printer/nodes.go

@@ -12,6 +12,9 @@ import (
 	"bytes"
 	"go/ast"
 	"go/token"
+	"strconv"
+	"strings"
+	"unicode"
 	"unicode/utf8"
 )
 
@@ -1334,6 +1337,49 @@ func (p *printer) valueSpec(s *ast.ValueSpec, keepType bool) {
 	}
 }
 
+func sanitizeImportPath(lit *ast.BasicLit) *ast.BasicLit {
+	// Note: An unmodified AST generated by go/parser will already
+	// contain a backward- or double-quoted path string that does
+	// not contain any invalid characters, and most of the work
+	// here is not needed. However, a modified or generated AST
+	// may possibly contain non-canonical paths. Do the work in
+	// all cases since it's not too hard and not speed-critical.
+
+	// if we don't have a proper string, be conservative and return whatever we have
+	if lit.Kind != token.STRING {
+		return lit
+	}
+	s, err := strconv.Unquote(lit.Value)
+	if err != nil {
+		return lit
+	}
+
+	// if the string is an invalid path, return whatever we have
+	//
+	// spec: "Implementation restriction: A compiler may restrict
+	// ImportPaths to non-empty strings using only characters belonging
+	// to Unicode's L, M, N, P, and S general categories (the Graphic
+	// characters without spaces) and may also exclude the characters
+	// !"#$%&'()*,:;<=>?[\]^`{|} and the Unicode replacement character
+	// U+FFFD."
+	if s == "" {
+		return lit
+	}
+	const illegalChars = `!"#$%&'()*,:;<=>?[\]^{|}` + "`\uFFFD"
+	for _, r := range s {
+		if !unicode.IsGraphic(r) || unicode.IsSpace(r) || strings.ContainsRune(illegalChars, r) {
+			return lit
+		}
+	}
+
+	// otherwise, return the double-quoted path
+	s = strconv.Quote(s)
+	if s == lit.Value {
+		return lit // nothing wrong with lit
+	}
+	return &ast.BasicLit{ValuePos: lit.ValuePos, Kind: token.STRING, Value: s}
+}
+
 // The parameter n is the number of specs in the group. If doIndent is set,
 // multi-line identifier lists in the spec are indented when the first
 // linebreak is encountered.
@@ -1346,7 +1392,7 @@ func (p *printer) spec(spec ast.Spec, n int, doIndent bool) {
 			p.expr(s.Name)
 			p.print(blank)
 		}
-		p.expr(s.Path)
+		p.expr(sanitizeImportPath(s.Path))
 		p.setComment(s.Comment)
 		p.print(s.EndPos)
 

src/go/printer/testdata/declarations.golden

@@ -110,6 +110,12 @@ import (
 	"package_dddd"	// comment
 )
 
+// print import paths as double-quoted strings
+import (
+	"fmt"
+	"math"
+)
+
 // at least one empty line between declarations of different kind
 import _ "io"
 

src/go/printer/testdata/declarations.input

@@ -111,6 +111,15 @@ import (
 	"package_dddd" // comment
 )
 
+// print import paths as double-quoted strings
+// (we would like more test cases but the go/parser
+// already excludes most incorrect paths, and we don't
+// bother setting up test-ASTs manually)
+import (
+	`fmt`
+	"math"
+)
+
 // at least one empty line between declarations of different kind
 import _ "io"
 var _ int