RFR: 8275745: Reproducible copyright headers

[Emmanuel Bourg]

On Sat, 28 Nov 2020 23:14:35 GMT, Emmanuel Bourg <duke at openjdk.java.net> wrote:

> The copyright headers are generated at build time, and the year inserted in the template depends on the current date. This means the headers are not reproducible if the project is built a year later. The year in the headers could be derived from the SOURCE_DATE_EPOCH environment variable to make the build reproducible (this variable is already used in other parts of the build).

Yes that's fine. I guess this involves setting the `COPYRIGHT_YEAR` variable in `make/autoconf/jdk-options.m4` to a value derived from `SOURCE_DATE_EPOCH` (with `SOURCE_DATE_EPOCH` having the priority over `--with-copyright-year` or the opposite?), and then pick the value of `COPYRIGHT_YEAR` in `CopyrightHeaders.java` and `EquivMapsGenerator.java`, right?

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PR: https://git.openjdk.java.net/jdk/pull/1498