[PATCH] Embed _psl_compile_time derived from $SOURCE_DATE_EPOCH if set

[Daniel Kahn Gillmor]

Making packages build byte-for-byte reproducibly from a given
toolchain+source makes it much easier to corroborate builds by testing
against other build infrastructure.

By default, libpsl currently embeds the current unix timestamp in
_psl_compile_time, which makes it bytewise incompatible if it is
rebuild even on the same machine one second later.

Dropping _psl_compile_time and psl_builtin_compile_time() would fix
this, but would cause an API/ABI change for the library. The next
time libpsl goes through an API/ABI change anyway, this might be worth
consideration, but maybe such a simple change doesn't warrant an
SONAME bump, so this changeset proposes to embed the environment
variable $SOURCE_DATE_EPOCH if it is set during build, falling back to
the current behavior if $SOURCE_DATE_EPOCH is unset.

See https://wiki.debian.org/ReproducibleBuilds/TimestampsProposal for
more information about $SOURCE_DATE_EPOCH.
---
src/psl2c.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/psl2c.c b/src/psl2c.c
index e634a97..1fc5877 100644
--- a/src/psl2c.c
+++ b/src/psl2c.c
@@ -157,6 +157,7 @@ int main(int argc, const char **argv)
struct stat st;
size_t cmdsize = 16 + strlen(argv[1]);
char *cmd = alloca(cmdsize), checksum[64] = "";
+ char *source_date_epoch = NULL;

#if 0
/* include library code did not generate punycode, so let's do it for the builtin data */
@@ -177,7 +178,10 @@ int main(int argc, const char **argv)
if (stat(argv[1], &st) != 0)
st.st_mtime = 0;
fprintf(fpout, "static time_t _psl_file_time = %lu;\n", st.st_mtime);
- fprintf(fpout, "static time_t _psl_compile_time = %lu;\n", time(NULL));
+ if (source_date_epoch = getenv("SOURCE_DATE_EPOCH"))
+ fprintf(fpout, "static time_t _psl_compile_time = %lu;\n", atol(source_date_epoch));
+ else
+ fprintf(fpout, "static time_t _psl_compile_time = %lu;\n", time(NULL));
fprintf(fpout, "static const char _psl_sha1_checksum[] = \"%s\";\n", checksum);
fprintf(fpout, "static const char _psl_filename[] = \"%s\";\n", argv[1]);

--
2.1.4

[Tim Rühsen]

Thanks.

Slightly amended pushed to branch 'develop'.

Regards, Tim