math/big: special-case a 0 mantissa during Rat parsing

Previously, a 0 mantissa was special-cased during big.Float
parsing, but not during big.Rat parsing. This meant that a value
like 0e9999999999 would parse successfully in big.Float.SetString,
but would hang in big.Rat.SetString. This discrepancy became an
issue in https://golang.org/src/go/constant/value.go?#L250,
where the big.Float would report an exponent of 0, so
big.Rat.SetString would be used and would subsequently hang.

A Go Playground example of this is https://play.golang.org/p/3fy28eUJuF

The solution is to special-case a zero mantissa during big.Rat
parsing as well, so that neither big.Rat nor big.Float will hang when
parsing a value with 0 mantissa but a large exponent.

This was discovered using go-fuzz on CockroachDB:
https://github.com/cockroachdb/go-fuzz/blob/master/examples/parser/main.go

Fixes #16176

Change-Id: I775558a8682adbeba1cc9d20ba10f8ed26259c56
Reviewed-on: https://go-review.googlesource.com/24430Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/go/constant/value_test.go

@@ -244,6 +244,7 @@ var stringTests = []struct {
 	{"1e9999", "1e+9999", "0x.f8d4a9da224650a8cb2959e10d985ad92adbd44c62917e608b1f24c0e1b76b6f61edffeb15c135a4b601637315f7662f325f82325422b244286a07663c9415d2p+33216"},
 	{"1e-9999", "1e-9999", "0x.83b01ba6d8c0425eec1b21e96f7742d63c2653ed0a024cf8a2f9686df578d7b07d7a83d84df6a2ec70a921d1f6cd5574893a7eda4d28ee719e13a5dce2700759p-33215"},
 	{"2.71828182845904523536028747135266249775724709369995957496696763", "2.71828", "271828182845904523536028747135266249775724709369995957496696763/100000000000000000000000000000000000000000000000000000000000000"},
+	{"0e9999999999", "0", "0"}, // issue #16176
 
 	// Complex
 	{"0i", "(0 + 0i)", "(0 + 0i)"},

src/math/big/ratconv.go

@@ -88,6 +88,12 @@ func (z *Rat) SetString(s string) (*Rat, bool) {
 		return nil, false
 	}
 
+	// special-case 0 (see also issue #16176)
+	if len(z.a.abs) == 0 {
+		return z, true
+	}
+	// len(z.a.abs) > 0
+
 	// correct exponent
 	if ecorr < 0 {
 		exp += int64(ecorr)

src/math/big/ratconv_test.go

@@ -48,6 +48,7 @@ var setStringTests = []StringTest{
 	{"53/70893980658822810696", "53/70893980658822810696", true},
 	{"106/141787961317645621392", "53/70893980658822810696", true},
 	{"204211327800791583.81095", "4084226556015831676219/20000", true},
+	{"0e9999999999", "0", true}, // issue #16176
 	{in: "1/0"},
 }