strconv: fix for parseFloatPrefix

parseFloatPrefix accepts a string if it has a valid floating-point
number as prefix. Make sure that "infi", "infin", ... etc. are
accepted as valid numbers "inf" with suffix "i", "in", etc. This
is important for parsing complex numbers such as "0+infi".

This change does not affect the correctness of ParseFloat because
ParseFloat rejects strings that contain a suffix after a valid
floating-point number.

Updates #36771.

Change-Id: Ie1693a8ca2f8edf07b57688e0b35751b7100d39d
Reviewed-on: https://go-review.googlesource.com/c/go/+/231237
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/strconv/atof.go

@@ -55,7 +55,11 @@ func special(s string) (f float64, n int, ok bool) {
 		fallthrough
 	case 'i', 'I':
 		n := commonPrefixLenIgnoreCase(s, "infinity")
-		// both "inf" and "infinity" are ok
+		// Anything longer than "inf" is ok, but if we
+		// don't have "infinity", only consume "inf".
+		if 3 < n && n < 8 {
+			n = 3
+		}
 		if n == 3 || n == 8 {
 			return math.Inf(sign), nsign + n, true
 		}

src/strconv/atof_test.go

@@ -486,8 +486,9 @@ func TestParseFloatPrefix(t *testing.T) {
 			continue
 		}
 		// Adding characters that do not extend a number should not invalidate it.
-		// Test a few.
-		for _, suffix := range []string{" ", "q", "+", "-", "<", "=", ">", "(", ")"} {
+		// Test a few. The "i" and "init" cases test that we accept "infi", "infinit"
+		// correctly as "inf" with suffix.
+		for _, suffix := range []string{" ", "q", "+", "-", "<", "=", ">", "(", ")", "i", "init"} {
 			in := test.in + suffix
 			_, n, err := ParseFloatPrefix(in, 64)
 			if err != nil {