RFR: 8278766: Enable OpenJDK build support for reproducible jars and jmods using --date

[Andrew Leonard]

On Fri, 17 Dec 2021 17:53:12 GMT, Erik Joelsson <erikj at openjdk.org> wrote:

>> But I think the code in InitSupport will be executed always; Init.gmk is our "bootstrapper" / "trampoline" which wraps all calls to make (and InitSupport.gmk contains gory implementation details of Init.gmk).
>
> SOURCE_DATE_EPOCH is initialized and exported in InitSupport.gmk so it's always available in the environment. We did this because we want various tools to pick this variable up from the environment, as this is a commonly expected variable name for doing so. The new variable SOURCE_DATE_ISO_8601 is a variant with a different format and there is no standard for reading this from the environment, so it's not as obvious that we should just export it the same way. 
> 
> On the other hand, we do not want to execute a shell expression every time we import MakeBase.gmk, so I would still vote for doing this in InitSupport.gmk and export SOURCE_DATE_ISO_8601.

@erikj79 Interesting, I tried adding it to InitSupport.gmk, but it wasn't always set... odd, I will revisit, I probably did something wrong...! will give that another try, thanks

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PR: https://git.openjdk.java.net/jdk/pull/6878