guix: SOURCE_DATE_EPOCH is already set in some environments #29935

1. 
   laanwj commented at 6:38 pm on April 22, 2024: member

   The environment variable SOURCE_DATE_EPOCH allows overriding the date that will be used inside the archives for guix-built binaries. This is an intentional feature, as documented in contrib/guix/README.md:

   • SOURCE_DATE_EPOCH

     Override the reference UNIX timestamp used for bit-for-bit reproducibility, the variable name conforms to [standard][r12e/source-date-epoch].

     (defaults to the output of $(git log --format=%at -1))

   However, some environments, as apparently Nix, set SOURCE_DATE_EPOCH by default. This can be extremely confusing, as it results in mismatches. i see three options:

   • Show a big red warning when the Guix build scripts are called with an existing SOURCE_DATE_EPOCH. Most likely it’s not what the user wanted, but they can continue if they wanted.
   • Disallow overriding SOURCE_DATE_EPOCH - unset it at start.
   • Rename our SOURCE_DATE_EPOCH to something non-standard that doesn’t conflict with Nix.

   The first one has my preference.

   See bitcoin-core/guix.sigs#1201 which prompted this.

2. laanwj added the label Build system on Apr 22, 2024
3. 
   hebasto commented at 7:30 pm on April 22, 2024: member

   • Rename our SOURCE_DATE_EPOCH to something non-standard that doesn’t conflict with Nix.

   As we run Guix shell in a container, it seems reasonable to rename SOURCE_DATE_EPOCH in the guix-build and guix-codesign scripts, and pass it to the container using its original name:

   0--env SOURCE_DATE_EPOCH="${BITCOIN_SOURCE_DATE_EPOCH:?unable to determine value}"
   

4. 
   laanwj commented at 2:19 pm on April 23, 2024: member

   Sounds good to me. I think it’s fine to rename it, I think there’s really very few edge cases in which one would really want to pass in a custom epoch to the guix build?
5. 
   fanquake commented at 10:53 am on April 24, 2024: member

   As we run Guix shell in a container, it seems reasonable to rename SOURCE_DATE_EPOCH in the guix-build and guix-codesign scripts, and pass it to the container using its original name:

   Not sure. We already have FORCE_DIRTY_WORKTREE. Seems fine to just make SOURCE_DATE_EPOCH function in the same way, rather than new variables / more build options / things being less-standard.

6. 
   laanwj commented at 11:03 am on April 24, 2024: member

   That sounds like a good approach to me too. So, fail when SOURCE_DATE_EPOCH is set, unless FORCE_SOURCE_DATE_EPOCH is also set?
7. 
   laanwj commented at 2:24 pm on April 29, 2024: member

   Or could cover SOURCE_DATE_EPOCH being set under FORCE_DIRTY_WORKTREE, it’s not strictly the same, but it’s also an intentionally introduced mismatch, and would avoid adding another build option.

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laanwj hebasto fanquake

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Build system