cmd/yacc: always import fmt, safely

The parser depends on it but the client might not import it, so make sure it's there.
Fixes #4038.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6497094

src/cmd/yacc/yacc.go

@@ -51,6 +51,7 @@ import (
 	"fmt"
 	"os"
 	"strings"
+	"unicode"
 )
 
 // the following are adjustable
@@ -153,6 +154,8 @@ var ftable *bufio.Writer    // y.go file
 var fcode = &bytes.Buffer{} // saved code
 var foutput *bufio.Writer   // y.output file
 
+var fmtImported bool // output file has recorded an import of "fmt"
+
 var oflag string  // -o [y.go]		- y.go file
 var vflag string  // -v [y.output]	- y.output file
 var lflag bool    // -l			- disable line directives
@@ -1073,6 +1076,7 @@ out:
 
 //
 // saves code between %{ and %}
+// adds an import for __fmt__ the first time
 //
 func cpycode() {
 	lno := lineno
@@ -1085,15 +1089,18 @@ func cpycode() {
 	if !lflag {
 		fmt.Fprintf(ftable, "\n//line %v:%v\n", infile, lineno)
 	}
+	// accumulate until %}
+	code := make([]rune, 0, 1024)
 	for c != EOF {
 		if c == '%' {
 			c = getrune(finput)
 			if c == '}' {
+				emitcode(code, lno+1)
 				return
 			}
-			ftable.WriteRune('%')
+			code = append(code, '%')
 		}
-		ftable.WriteRune(c)
+		code = append(code, c)
 		if c == '\n' {
 			lineno++
 		}
@@ -1103,6 +1110,107 @@ func cpycode() {
 	errorf("eof before %%}")
 }
 
+//
+// emits code saved up from between %{ and %}
+// called by cpycode
+// adds an import for __yyfmt__ after the package clause
+//
+func emitcode(code []rune, lineno int) {
+	for i, line := range lines(code) {
+		writecode(line)
+		if !fmtImported && isPackageClause(line) {
+			fmt.Fprintln(ftable, `import __yyfmt__ "fmt"`)
+			fmt.Fprintf(ftable, "//line %v:%v\n\t\t", infile, lineno+i)
+			fmtImported = true
+		}
+	}
+}
+
+//
+// does this line look like a package clause?  not perfect: might be confused by early comments.
+//
+func isPackageClause(line []rune) bool {
+	line = skipspace(line)
+
+	// must be big enough.
+	if len(line) < len("package X\n") {
+		return false
+	}
+
+	// must start with "package"
+	for i, r := range []rune("package") {
+		if line[i] != r {
+			return false
+		}
+	}
+	line = skipspace(line[len("package"):])
+
+	// must have another identifier.
+	if len(line) == 0 || (!unicode.IsLetter(line[0]) && line[0] != '_') {
+		return false
+	}
+	for len(line) > 0 {
+		if !unicode.IsLetter(line[0]) && !unicode.IsDigit(line[0]) && line[0] != '_' {
+			break
+		}
+		line = line[1:]
+	}
+	line = skipspace(line)
+
+	// eol, newline, or comment must follow
+	if len(line) == 0 {
+		return true
+	}
+	if line[0] == '\r' || line[0] == '\n' {
+		return true
+	}
+	if len(line) >= 2 {
+		return line[0] == '/' && (line[1] == '/' || line[1] == '*')
+	}
+	return false
+}
+
+//
+// skip initial spaces
+//
+func skipspace(line []rune) []rune {
+	for len(line) > 0 {
+		if line[0] != ' ' && line[0] != '\t' {
+			break
+		}
+		line = line[1:]
+	}
+	return line
+}
+
+//
+// break code into lines
+//
+func lines(code []rune) [][]rune {
+	l := make([][]rune, 0, 100)
+	for len(code) > 0 {
+		// one line per loop
+		var i int
+		for i = range code {
+			if code[i] == '\n' {
+				break
+			}
+		}
+		l = append(l, code[:i+1])
+		code = code[i+1:]
+	}
+	return l
+}
+
+//
+// writes code to ftable
+//
+func writecode(code []rune) {
+	for _, r := range code {
+		ftable.WriteRune(r)
+	}
+}
+
 //
 // skip over comments
 // skipcom is called after reading a '/'
@@ -3115,7 +3223,7 @@ func $$Tokname(c int) string {
 			return $$Toknames[c-1]
 		}
 	}
-	return fmt.Sprintf("tok-%v", c)
+	return __yyfmt__.Sprintf("tok-%v", c)
 }
 
 func $$Statname(s int) string {
@@ -3124,7 +3232,7 @@ func $$Statname(s int) string {
 			return $$Statenames[s]
 		}
 	}
-	return fmt.Sprintf("state-%v", s)
+	return __yyfmt__.Sprintf("state-%v", s)
 }
 
 func $$lex1(lex $$Lexer, lval *$$SymType) int {
@@ -3157,7 +3265,7 @@ out:
 		c = $$Tok2[1] /* unknown char */
 	}
 	if $$Debug >= 3 {
-		fmt.Printf("lex %U %s\n", uint(char), $$Tokname(c))
+		__yyfmt__.Printf("lex %U %s\n", uint(char), $$Tokname(c))
 	}
 	return c
 }
@@ -3184,7 +3292,7 @@ ret1:
 $$stack:
 	/* put a state and value onto the stack */
 	if $$Debug >= 4 {
-		fmt.Printf("char %v in %v\n", $$Tokname($$char), $$Statname($$state))
+		__yyfmt__.Printf("char %v in %v\n", $$Tokname($$char), $$Statname($$state))
 	}
 
 	$$p++
@@ -3253,8 +3361,8 @@ $$default:
 			$$lex.Error("syntax error")
 			Nerrs++
 			if $$Debug >= 1 {
-				fmt.Printf("%s", $$Statname($$state))
-				fmt.Printf("saw %s\n", $$Tokname($$char))
+				__yyfmt__.Printf("%s", $$Statname($$state))
+				__yyfmt__.Printf("saw %s\n", $$Tokname($$char))
 			}
 			fallthrough
 
@@ -3273,7 +3381,7 @@ $$default:
 
 				/* the current p has no shift on "error", pop stack */
 				if $$Debug >= 2 {
-					fmt.Printf("error recovery pops state %d\n", $$S[$$p].yys)
+					__yyfmt__.Printf("error recovery pops state %d\n", $$S[$$p].yys)
 				}
 				$$p--
 			}
@@ -3282,7 +3390,7 @@ $$default:
 
 		case 3: /* no shift yet; clobber input char */
 			if $$Debug >= 2 {
-				fmt.Printf("error recovery discards %s\n", $$Tokname($$char))
+				__yyfmt__.Printf("error recovery discards %s\n", $$Tokname($$char))
 			}
 			if $$char == $$EofCode {
 				goto ret1
@@ -3294,7 +3402,7 @@ $$default:
 
 	/* reduction by production $$n */
 	if $$Debug >= 2 {
-		fmt.Printf("reduce %v in:\n\t%v\n", $$n, $$Statname($$state))
+		__yyfmt__.Printf("reduce %v in:\n\t%v\n", $$n, $$Statname($$state))
 	}
 
 	$$nt := $$n