build: add -Wdate-time to Werror flags #17880

-Wdate-time Warn when macros TIME, DATE or TIMESTAMP are encountered as they might prevent bit-wise-identical reproducible compilations.

This is supported by GCC and Clang.

Example output:

  CXX      bitcoind-bitcoind.o
bitcoind.cpp:48:20: warning: expansion of date or time macro is not reproducible [-Wdate-time]
    printf("%s\n", __TIMESTAMP__);
                   ^
bitcoind.cpp:49:20: warning: expansion of date or time macro is not reproducible [-Wdate-time]
    printf("%s\n", __TIME__);
                   ^
bitcoind.cpp:50:20: warning: expansion of date or time macro is not reproducible [-Wdate-time]
    printf("%s\n", __DATE__);
                   ^
3 warnings generated.

Concept ACK!

ACK b0a254019c3bc8a353c1304ee65a55cfb3d4ecac -- diff looks correct and guarding against potential non-reproducibility is good :)

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #15382 (util: add runCommandParseJSON by Sjors)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

Concept ACK.

ACK b0a254019c3bc8a353c1304ee65a55cfb3d4ecac

Last use was removed in d096d22446a1d126ea78170f37a148ce0ca43df2. I can't imagine they ever get added back in, but I don't mind this warning either if others find it useful.

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Gitian builds

Just to confirm, this change makes the compiler emit just a warning?

Just to confirm, this change makes the compiler emit just a warning?

Yes. Unless you —enable-werror.

Tested ACK b0a254019c3bc8a353c1304ee65a55cfb3d4ecac on macos with clang. Already had --enable-werror, added a wild printf("%s\n", __TIMESTAMP__) and got the following error:

error: expansion of date or time macro is not reproducible [-Werror,-Wdate-time]
printf("%s\n", __TIMESTAMP__);
               ^
1 error generated.

ACK b0a254019c3bc8a353c1304ee65a55cfb3d4ecac