Remove Highs_compilation_date to improve build reproducibility

[rgommers]

Embedding a date/timestamp in the output of a build artifact makes a build non-reproducible. Build reproducibility matters for multiple reasons, like making security audits and build caching easier. For more on that, please see https://reproducible-builds.org/ and https://en.wikipedia.org/wiki/Reproducible_builds. The Wikipedia article note: "timestamps are "the biggest source of reproducibility issues."

Highs_compilation_date also doesn't seem to add much value, because the git hash is also included in the build output, and that is more informative (it uniquely identifies the exact version of the code being built, while build date does not). Hence, it would be good to remove the compilation date info completely.

I'll note that the Meson build is currently checking for SOURCE_DATE_EPOCH, which is already an improvement (distros often set this to make timestamps reproducible), but the CMake and Bazel builds do not. Adding this to the CMake/Bazel builds probably isn't worthwhile though, it makes the build more than less complex.

[jajhall]

HIGHS_COMPILATION_DATE is no longer printed in the logging header at run-time - only HIGHS_GITHASH - but are you referring to it appearing in build files like HConfig.h?

I've got no attachment to HIGHS_COMPILATION_DATE, as I've never used it, so was happy to remove it from the logging header.

Is it useful to have it in the build @galabovaa?

[rgommers]

but are you referring to it appearing in build files like HConfig.h?

Yes indeed. src/lp_data/Highs.cpp contains:

const char* highsCompilationDate() { return HIGHS_COMPILATION_DATE; }

The effect of that is that it changes the created binary that will be installed each day, for the exact same version of the source code. Caching systems tend to hash the build output and if it doesn't change, then it knows that nothing that depends on HiGHS has to be rebuilt. So an effect of inserting a compilation date is to invalidate build caches for HiGHS every day.

[jajhall]

OK, I think I understand. Although a hard-coded date is only created in the text file HConfig.h, the actual value of HIGHS_COMPILATION_DATE is in the binary, so this changes if HiGHS is re-built.

Hence we need to remove HIGHS_COMPILATION_DATE completely. from the source code.

@odow : do you make use of HIGHS_COMPILATION_DATE appearing in the build files?

[odow]

Nope

👍 for reproducible builds.

[jajhall]

Thanks @odow, @galabovaa will remove all reference to HIGHS_COMPILATION_DATE , and

const char* highsCompilationDate()

will return a "Not known" message, and be deprecated

[jajhall]

Closed by #1747

[rgommers]

Thank you!