rc: fix rc API accepting an out of range number and overflowing 64 bits

float64(math.MaxInt64) rounds up to 2^63, so x > math.MaxInt64 in
GetInt64 lets 2^63 through to int64(x), which is out of range.

fs/rc/params.go

@@ -167,7 +167,8 @@ func (p Params) GetInt64(key string) (int64, error) {
 	case int64:
 		return x, nil
 	case float64:
-		if x > math.MaxInt64 || x < math.MinInt64 {
+		// float64(math.MaxInt64) rounds up to 2**63 which doesn't fit in an int64
+		if x >= math.MaxInt64 || x < math.MinInt64 {
 			return 0, ErrParamInvalid{fmt.Errorf("key %q (%v) overflows int64 ", key, value)}
 		}
 		return int64(x), nil

fs/rc/params_test.go

@@ -3,6 +3,7 @@ package rc
 import (
 	"errors"
 	"fmt"
+	"math"
 	"net/http"
 	"net/http/httptest"
 	"testing"
@@ -120,6 +121,9 @@ func TestParamsGetInt64(t *testing.T) {
 		{float64(14), 14, ""},
 		{float64(9.3e18), 0, "overflows int64"},
 		{float64(-9.3e18), 0, "overflows int64"},
+		{float64(math.MaxInt64), 0, "overflows int64"},
+		{math.Nextafter(float64(math.MaxInt64), 0), 9223372036854774784, ""},
+		{float64(math.MinInt64), math.MinInt64, ""},
 	} {
 		t.Run(fmt.Sprintf("%T=%v", test.value, test.value), func(t *testing.T) {
 			in := Params{