code review 7196050: encoding/json: properly unmarshal empty arrays.

encoding/json: properly unmarshal empty arrays.

The JSON unmarshaller failed to allocate an array when there
are no values for the input causing the `[]` unmarshalled
to []interface{} to generate []interface{}(nil) rather than
[]interface{}{}. This wasn't caught in the tests because Decode()
works correctly and because jsonBig never generated zero-sized
arrays. The modification to scanner_test.go quickly triggers
the error:

without the change to decoder.go, but with the change to scanner_test.go:

$ go test
--- FAIL: TestUnmarshalMarshal (0.10 seconds)
decode_test.go:446:		Marshal jsonBig
scanner_test.go:206:	diverge at 70: «03c1OL6$":null},{"[=» vs «03c1OL6$":[]},{"[=^\»
FAIL
exit status 1
FAIL	encoding/json	0.266s

Also added a simple regression to decode_test.go.

[aam, 2013-01-24 19:33:32]

Hello golang-dev@googlegroups.com,

I'd like you to review this change to
https://go.googlecode.com/hg/

[adg, 2013-01-24 23:23:18]

What happens if you unmarshal null into an []interface{} value? Can you add a
test for that? I would expect a nil []interface{}.

[aam, 2013-01-24 23:41:24]

Hello golang-dev@googlegroups.com, adg@golang.org (cc:
golang-dev@googlegroups.com),

Please take another look.

[adg, 2013-01-24 23:45:54]

LGTM

But I'd like a second opinion.

[dave_cheney.net, 2013-01-25 00:09:22]

On 2013/01/24 23:45:54, adg wrote:
> LGTM
> 
> But I'd like a second opinion.

I'd feel more comfortable if this was fixing an existing issue.

[aam, 2013-01-25 00:13:10]

Hello adg@golang.org, dave@cheney.net (cc: golang-dev@googlegroups.com),

Please take another look.

[aam, 2013-01-25 00:15:39]

the issue appeared on IRC. in its shortest form it's a difference
between an original JSON and one that was unmarshalled, then
marshalled again:

http://play.golang.org/p/_QCrIIT0nN

the issue was always there, the tests just didn't cover that
particular case (zero-length json arrays).

On Thu, Jan 24, 2013 at 5:13 PM,  <mirtchovski@gmail.com> wrote:
> Hello adg@golang.org, dave@cheney.net (cc: golang-dev@googlegroups.com),
>
>
> Please take another look.
>
>
> https://codereview.appspot.com/7196050/

[rsc, 2013-01-30 17:06:43]

LGTM

Sure.

[rsc, 2013-01-30 17:10:35]

*** Submitted as https://code.google.com/p/go/source/detail?r=49d533bb7cd3 ***

encoding/json: properly unmarshal empty arrays.

The JSON unmarshaller failed to allocate an array when there
are no values for the input causing the `[]` unmarshalled
to []interface{} to generate []interface{}(nil) rather than
[]interface{}{}. This wasn't caught in the tests because Decode()
works correctly and because jsonBig never generated zero-sized
arrays. The modification to scanner_test.go quickly triggers
the error:

without the change to decoder.go, but with the change to scanner_test.go:

$ go test
--- FAIL: TestUnmarshalMarshal (0.10 seconds)
decode_test.go:446:		Marshal jsonBig
scanner_test.go:206:	diverge at 70: «03c1OL6$":null},{"[=» vs
«03c1OL6$":[]},{"[=^\»
FAIL
exit status 1
FAIL	encoding/json	0.266s

Also added a simple regression to decode_test.go.

R=adg, dave, rsc
CC=golang-dev
https://codereview.appspot.com/7196050

Committer: Russ Cox <rsc@golang.org>

[remyoudompheng, 2013-07-20 21:24:49]

R=close