RFR 8170155: StringBuffer and StringBuilder stream methods are not late-binding

[Xueming Shen]

On 12/01/2016 12:05 PM, Paul Sandoz wrote:
>> On 1 Dec 2016, at 11:17, Xueming Shen<xueming.shen at oracle.com>  wrote:
>>
>> On 11/28/2016 11:27 AM, Paul Sandoz wrote:
>>>> On 25 Nov 2016, at 02:47, Tobias Hartmann<tobias.hartmann at oracle.com>   wrote:
>>>>
>>>>> I'm not sure if it is still desired to do the same boundary check in case of LATIN1
>>>>> for the benefit of consistency.  Assume there might be concurrent access/operation
>>>>> between val = this.value and count = this.count; (for StringBuilder) for example,
>>>>> the this.value got doubled and the this.count got increased. One will end up with
>>>>> StringIndexOutOfBoundsException() from checkOffset, but the other will be ioobe
>>>>> from vm?
>>>> You mean when hitting a check in an intrinsic compared to when hitting a check in the Java wrapper?
>>>>
>>> Not quite. There is a spliterator implementation for each coder, in the case of the LATIN1 coder there are no associated intrinsics. I think it’s ok just to perform the explicit bounds check for the UTF16 coder, since for the LATIN1 bounds checks will be performed by baloads.
>>>
>>> However, i think there is bug. The coder could change from LATIN1 to UTF16, while holding a reference to a byte[] value as LATIN1.
>>>
>>> For StringBuilder i could fix that by atomically reading the value/count/coder, but there is still an issue with StringBuffer. Thus, in the UTF16 coder spliterator we need to perform the explicit bounds before *every* StringUTF16.getChar().
>>>
>>> Webrev updated:
>>>
>>>    http://cr.openjdk.java.net/~psandoz/jdk9/JDK-8170155-string-buffer-builder-late-binding/webrev/
>>>
>>> Paul.
>>>
>> Hi Paul,
>>
>> Seems like we do have a bug here. But I think to use "charAt()" might not be the
>> desired/correct approach here?
>>
> I think the patch is correct for both StringBuffer and StringBuilder.
>
> It is specified that buffer must remain constant during execution of the stream terminal operation otherwise the results are undefined. Such an undefined result might be the throwing of an IndexOutOfBounds.


Ok, if the spec does not require to guarantee the correctness in , then the
patch is fine. It might be a small surprise  for some of the StringBuffer users
with the false assumption that StringBuffer is thread-safe so the returned
stream is kinda thread-safe as well.

I'm fine with either way you think is appropriate from "stream" point of
view.

Sherman


>
>> For StringBuffer, since we have to guarantee the correctness in concurrent use scenario,
>> I would assume we need a synchronized version of getSupplier(...) to return a late binding
>> and thread-safe Supplier in StringBuffer, similar to "getBytes(...)" (we have a un-synced
>> version of getBytes() in AbstractStringBuilder and a synced version at the bottom of the
>> StringBuffer.java). This should be for the latin1 case as well.
>>
> I did ponder that, and moving the stream returning methods into the concrete builder classes. I opted for the sneakier approach :-) what do you prefer?
>
>
>> For StringBuilder, since the only concern is the bounds check, I think the existing
>> checkOffset(...) should be good enough, even in case the coder changes from latin1
>> to utf16, as the checkOffset(...) checks the "count" against "val.length>>  coder" on
>> the local copy,  the getChar() access after that should be safe (though might not be
>> correct, if it's a utf16 coder on a latin1 byte[]).
>>
> Yes, i was being very conservative (I mistakenly thought the original code read the coder field twice).
>
> If we go with the single wider bounds check i think we should place it in the a root spliterator constructor if possible.
>
> Paul.