fmt: don't recur if String method (etc.) misbehaves

Fixes #2555.

R=golang-dev, dsymonds, r
CC=golang-dev
https://golang.org/cl/5486076

src/pkg/fmt/fmt_test.go

@@ -813,3 +813,37 @@ func TestPanics(t *testing.T) {
 		}
 	}
 }
+
+// Test that erroneous String routine doesn't cause fatal recursion.
+var recurCount = 0
+
+type Recur struct {
+	i      int
+	failed *bool
+}
+
+func (r Recur) String() string {
+	if recurCount++; recurCount > 10 {
+		*r.failed = true
+		return "FAIL"
+	}
+	// This will call badVerb. Before the fix, that would cause us to recur into
+	// this routine to print %!p(value). Now we don't call the user's method
+	// during an error.
+	return Sprintf("recur@%p value: %d", r, r.i)
+}
+
+func TestBadVerbRecursion(t *testing.T) {
+	failed := false
+	r := Recur{3, &failed}
+	Sprintf("recur@%p value: %d\n", &r, r.i)
+	if failed {
+		t.Error("fail with pointer")
+	}
+	failed = false
+	r = Recur{4, &failed}
+	Sprintf("recur@%p, value: %d\n", r, r.i)
+	if failed {
+		t.Error("fail with value")
+	}
+}

src/pkg/fmt/print.go

@@ -74,6 +74,7 @@ type GoStringer interface {
 type pp struct {
 	n         int
 	panicking bool
+	erroring  bool // printing an error condition
 	buf       bytes.Buffer
 	// field holds the current item, as an interface{}.
 	field interface{}
@@ -124,6 +125,7 @@ var ppFree = newCache(func() interface{} { return new(pp) })
 func newPrinter() *pp {
 	p := ppFree.get().(*pp)
 	p.panicking = false
+	p.erroring = false
 	p.fmt.init(&p.buf)
 	return p
 }
@@ -299,6 +301,7 @@ func (p *pp) unknownType(v interface{}) {
 }
 
 func (p *pp) badVerb(verb rune) {
+	p.erroring = true
 	p.add('%')
 	p.add('!')
 	p.add(verb)
@@ -316,6 +319,7 @@ func (p *pp) badVerb(verb rune) {
 		p.buf.Write(nilAngleBytes)
 	}
 	p.add(')')
+	p.erroring = false
 }
 
 func (p *pp) fmtBool(v bool, verb rune) {
@@ -606,6 +610,9 @@ func (p *pp) catchPanic(field interface{}, verb rune) {
 }
 
 func (p *pp) handleMethods(verb rune, plus, goSyntax bool, depth int) (wasString, handled bool) {
+	if p.erroring {
+		return
+	}
 	// Is it a Formatter?
 	if formatter, ok := p.field.(Formatter); ok {
 		handled = true