- avoid division-by-zero crash in tabwriter

- correct tabwidth argument for some tabwriter test cases
- catch negative tabwidth flag in gofmt w/o crashing

R=rsc
http://go/go-review/1026022

src/cmd/gofmt/gofmt.go

@@ -153,6 +153,10 @@ func walkDir(path string) {
 func main() {
 	flag.Usage = usage;
 	flag.Parse();
+	if *tabwidth < 0 {
+		fmt.Fprintf(os.Stderr, "negative tabwidth %d\n", *tabwidth);
+		os.Exit(2);
+	}
 
 	if flag.NArg() == 0 {
 		if err := processFile("/dev/stdin"); err != nil {

src/pkg/tabwriter/tabwriter.go

@@ -229,6 +229,10 @@ func (b *Writer) write0(buf []byte) os.Error {
 var newline = []byte{'\n'}
 
 func (b *Writer) writePadding(textw, cellw int) os.Error {
+	if b.cellwidth == 0 {
+		return nil;
+	}
+
 	if b.padbytes[0] == '\t' {
 		// make cell width a multiple of cellwidth
 		cellw = ((cellw + b.cellwidth - 1) / b.cellwidth) * b.cellwidth;

src/pkg/tabwriter/tabwriter_test.go

@@ -283,7 +283,7 @@ var tests = []entry{
 
 	entry{
 		"9a",
-		0, 0, '.', 0,
+		1, 0, '.', 0,
 		"1\t2\t3\t4\n"
 			"11\t222\t3333\t44444\n",
 
@@ -293,7 +293,7 @@ var tests = []entry{
 
 	entry{
 		"9b",
-		0, 0, '.', FilterHTML,
+		1, 0, '.', FilterHTML,
 		"1\t2<!---\f--->\t3\t4\n"	// \f inside HTML is ignored
 			"11\t222\t3333\t44444\n",
 
@@ -303,7 +303,7 @@ var tests = []entry{
 
 	entry{
 		"9c",
-		0, 0, '.', 0,
+		1, 0, '.', 0,
 		"1\t2\t3\t4\f"	// \f causes a newline and flush
 			"11\t222\t3333\t44444\n",
 
@@ -313,7 +313,7 @@ var tests = []entry{
 
 	entry{
 		"9c debug",
-		0, 0, '.', Debug,
+		1, 0, '.', Debug,
 		"1\t2\t3\t4\f"	// \f causes a newline and flush
 			"11\t222\t3333\t44444\n",
 
@@ -445,7 +445,7 @@ var tests = []entry{
 
 	entry{
 		"14",
-		0, 2, ' ', AlignRight,
+		1, 2, ' ', AlignRight,
 		".0\t.3\t2.4\t-5.1\t\n"
 			"23.0\t12345678.9\t2.4\t-989.4\t\n"
 			"5.1\t12.0\t2.4\t-7.0\t\n"
@@ -463,7 +463,7 @@ var tests = []entry{
 
 	entry{
 		"14 debug",
-		0, 2, ' ', AlignRight | Debug,
+		1, 2, ' ', AlignRight | Debug,
 		".0\t.3\t2.4\t-5.1\t\n"
 			"23.0\t12345678.9\t2.4\t-989.4\t\n"
 			"5.1\t12.0\t2.4\t-7.0\t\n"