RFR: 8016780: (xs) README-builds.html misses crucial requirement on bootstrap JDK

[Erik Joelsson]

On 2013-06-18 08:57, Daniel Fuchs wrote:
> On 6/18/13 8:28 AM, David Holmes wrote:
>> On 18/06/2013 4:02 PM, Jonathan Gibbons wrote:
>>> The only problem with using N is that you don't know whether you have
>>> broken building with N-1. Therefore the general recommendation for most
>>> people should be to always use N-1.  I think Stuart is just searching
>>> for ways to make people aware that using N-1 is "the right thing to 
>>> do".
>>
>> There was certainly an issue here that caused a problem because the 
>> code used a JDK8 API that was not available when the source was 
>> compiled with JDK7. And sure compiling with 7u boot JDK would have 
>> caught that.
>>
>> But we have lots of code that only compiles with JDK8 and that is the 
>> way we want it, else JDK8 could not take advantage of any new 
>> language features or APIs in JDK8. The real problem here was that the 
>> code in question is code that is not built in a way that allows it to 
>> use the latest language features or APIs. In which case perhaps the 
>> real fix is to use build commands that enforce this restriction ie by 
>> using -source 7 -target 7 ?
>>
>> David
> Hi David,
>
> In the case of Jaxp - I'm not sure why exactly is Jaxp compiled with 
> the boot JDK.
> It comes early in the build cycle - at a time when the N JDK hasn't 
> been compiled yet.
> But is this a mere convenience - or is there a good technical reason 
> for this?
>
> Because personally - I would love to be able to use JDK N feature in 
> the JAXP source
> code.
> One could argue that using N features impairs the ability to port the 
> fixes to N-1, but
> this is already the case today anyway: for many of the fixes I ported 
> from 8 to 7 I had to
> modify my patches because I was using N-1 features in N, and therefore 
> had to convert
> the code to only use N-2 features when porting from N to N-1.
>
> So if that is possible (and it may not be - I'm not a build expert) - 
> I would argue to
> remove the restriction for Jaxp - rather than enforce it.
>
After langtools has been built (which is first), the rest, including 
jaxp, is compiled using the bootstrap javac, which is a special version 
of the newly built javac that runs on the boot jdk but acts as the JDK N 
javac. This means JDK N language features are available, but not library 
features, since there is no access to the rest of the JDK N classes. 
This separation is just convenience as far as I can tell. It's 
technically possible to work around, but the gains will have to be 
weighed against the added complexity. Easiest solution would be to get 
rid of the separate jaxp repository and just put the source back with 
the jdk.

/Erik
> best regards,
>
> -- daniel
>
>>
>>
>>> -- Jon
>>>
>>>
>>> On 06/17/2013 10:04 PM, David Holmes wrote:
>>>> I thought the only rule was "must be buildable by N-1", not that you
>>>> must not try to use N!
>>>>
>>>> Can the problem preventing a build using JDK8 as the boot JDK not be
>>>> corrected? I'm assuming it is one of the more unusual parts of the
>>>> build where we mess with bootclasspath etc?
>>>>
>>>> David
>>>>
>>>> On 18/06/2013 10:21 AM, Stuart Marks wrote:
>>>>> On 6/17/13 4:02 PM, Kelly O'Hair wrote:
>>>>>> Rule #1 Nobody reads the README
>>>>>> Rule #2 When things go wrong, blame the README
>>>>>>
>>>>>> I of course have no objection to the change, however, I'm not
>>>>>> convinced it will
>>>>>> help much the next time someone runs into this. :^(
>>>>>
>>>>> Hi Kelly! You still read this stuff here? :-)
>>>>>
>>>>> Yeah, I have no illusions that changing the README will prevent 
>>>>> many, if
>>>>> any, future occurrences of this problem. However, we had an internal
>>>>> discussion on this incident where the N-1 rule was asserted. There 
>>>>> was
>>>>> no dispute about the rule, but I went off to find where it was
>>>>> documented, and found only the fairly weak statement in the 
>>>>> README. So,
>>>>> at the very least, that ought to be fixed.
>>>>>
>>>>> A stronger step would be to modify configure to check the version 
>>>>> of the
>>>>> boot JDK and to complain if it doesn't match N-1. Or perhaps even N-1
>>>>> and update >= 7. What do you think? I was considering filing an RFE.
>>>>>
>>>>> A restriction in configure would probably be more effective at
>>>>> preventing these kinds of errors.
>>>>>
>>>>> s'marks
>>>
>