flag: recognize "0s" as the zero value for a flag.Duration

Implemented by using a reflect-based approach to recognize the zero
value of any non-interface type that implements flag.Value.  Interface
types will fall back to the old code.

Fixes #15904.

Change-Id: I594c3bfb30e9ab1aca3e008ef7f70be20aa41a0b
Reviewed-on: https://go-review.googlesource.com/23581
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Rob Pike <[email protected]>

src/flag/flag.go

@@ -68,6 +68,7 @@ import (
  "fmt"
  "io"
  "os"
+ "reflect"
  "sort"
  "strconv"
  "time"
@@ -378,7 +379,21 @@ func Set(name, value string) error {
 
 // isZeroValue guesses whether the string represents the zero
 // value for a flag. It is not accurate but in practice works OK.
-func isZeroValue(value string) bool {
+func isZeroValue(flag *Flag, value string) bool {
+ // Build a zero value of the flag's Value type, and see if the
+ // result of calling its String method equals the value passed in.
+ // This works unless the Value type is itself an interface type.
+ typ := reflect.TypeOf(flag.Value)
+ var z reflect.Value
+ if typ.Kind() == reflect.Ptr {
+ z = reflect.New(typ.Elem())
+ } else {
+ z = reflect.Zero(typ)
+ }
+ if value == z.Interface().(Value).String() {
+ return true
+ }
+
  switch value {
  case "false":
  return true
@@ -449,7 +464,7 @@ func (f *FlagSet) PrintDefaults() {
  s += "\n \t"
  }
  s += usage
- if !isZeroValue(flag.DefValue) {
+ if !isZeroValue(flag, flag.DefValue) {
  if _, ok := flag.Value.(*stringValue); ok {
  // put quotes on the value
  s += fmt.Sprintf(" (default %q)", flag.DefValue)

src/flag/flag_test.go

@@ -393,7 +393,7 @@ const defaultOutput = ` -A for bootstrapping, allow 'any' type
  -Z int
  an int that defaults to zero
  -maxT timeout
- set timeout for dial (default 0s)
+ set timeout for dial
 `
 
 func TestPrintDefaults(t *testing.T) {