RFR(trivial): 8231024: Improve the debug info when the output is truncated

[Ioi Lam]

On 9/16/19 12:44 AM, David Holmes wrote:
> On 16/09/2019 5:09 pm, Jie Fu wrote:
>> Hi David,
>>
>> Sorry for the confusion.
>
> The confusion is mine, not from you :)
>
>> On 2019/9/16 下午2:40, David Holmes wrote:
>>> Okay I'm confused. I had assumed that the passed in buffer size must 
>>> be different in each case because of the 2014/2032/2050 values in 
>>> the message, but in fact they are all sized at 2000. 
>> According to the source code, the buffer size can be either 
>> 2000(O_BUFLEN) [1] or 2048(BUFLEN) [2].
>> So it isn't always 2000.
>
> It is 2000 is all three examples of the output that you gave.

xmlStream::va_tag() uses 2*K.

>
>>> That makes no sense
>> I think it makes sense.
>> It is still unclear which one (O_BUFLEN or BUFLEN?) should be 
>> increased if the current buffer size wasn't printed.
>
> You need to know the original caller that passed in the buffer size.
>
> But we can't just keep increasing the buffer size. We probably need to 
> examine the callers to see what amount of information is trying to be 
> passed through. Then decide whether to adjust what the callers are 
> passing through, or adjust the buffer size - perhaps on a callsite by 
> callsite basis rather than just bumping O_BUFLEN or BUFLEN.
>

I think the message should stop saying "increase O_BUFLEN" -- probably 
the reviewers will reject all future increase of O_BUFLEN and will 
suggest fixing the caller of outputStream::do_vsnprintf instead.

Callers of outputStream::do_vsnprintf don't necessarily use O_BUFLEN. 
E.g., xmlStream::va_tag allocates its own buffer. 
VMError::report_and_die() also uses its own buffer -- it uses O_BUFLEN 
today, but there's no guarantee or requirement that it always does that.

How about saying something like "outputStream::do_vsnprintf output 
truncated -- buffer length is 2000 bytes but 2100 bytes are needed."


>>> - how can we have written e.g. 2014 bytes to a buffer of only 2000 
>>> bytes? 
>>
>> The output is truncated since the current buffer size is too small 
>> (e.g., 2000 < 2014).
>
> Ah sorry - I'd missed that aspect of vsnprintf. We're calling:
>
>  102     int written = os::vsnprintf(buffer, buflen, format, ap);
>
> but it isn't "written" when it exceeds buflen, it's the required 
> buffer size.
>

I think the variable "written" should be renamed to "required".

Thanks
- Ioi

> David
> -----
>
>>
>>> Do we have a bug in os::vsnprintf?
>>
>> I don't think so.
>>
>> Thanks a lot.
>> Best regards,
>> Jie
>>
>> [1] 
>> http://hg.openjdk.java.net/jdk/jdk/file/24df796eef3d/src/hotspot/share/utilities/ostream.cpp#l124 
>>
>> [2] 
>> http://hg.openjdk.java.net/jdk/jdk/file/24df796eef3d/src/hotspot/share/utilities/xmlstream.cpp#l138 
>>
>>
>>