build: honor SOURCE_DATE_EPOCH for reproducible build timestamp - #826
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The build script baked chrono::Utc::now() into the binary (surfaced by `flyline --version`), making builds non-reproducible. Honor SOURCE_DATE_EPOCH when set, falling back to the wall clock otherwise.
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What
build.rscurrently stampsBUILD_TIMEwithchrono::Utc::now(), so everybuild embeds the wall-clock time. This makes the compiled artifact differ on
each build — the last remaining source of non-determinism when packaging
flyline (e.g. for Nix, where builds are checked for bit-for-bit
reproducibility).
This change honors
SOURCE_DATE_EPOCH:when the env var is set,
BUILD_TIMEis derived from it; otherwise thebehavior is unchanged (falls back to the current wall-clock time). A
rerun-if-env-changed=SOURCE_DATE_EPOCHline ensures the build script re-runswhen the value changes.
Why
SOURCE_DATE_EPOCHis the cross-ecosystem standard for reproducible builds andis set automatically by Nix, most distro build systems, and
cargotooling. Nobehavior changes for normal
cargo build/cargo install— the wall clock isstill used when the var is absent.
Testing
cargo buildwith no env var:BUILD_TIMEis the current time (unchanged).SOURCE_DATE_EPOCH=1700000000 cargo build:BUILD_TIMEis derived from theepoch, and repeated builds produce byte-identical output.
This is the flyline-side counterpart to HalFrgrd/flycomp#6, which applied the
same fix to flycomp's build script.