[PATCH] Allow embedded timestamps by C/C++ macros to be set externally (2)

[Dhole <dhole at openmailbox dot org>]

Hi!

A while back I submitted a patch to the gcc-patches mailing list
to allow the embedded timestamps by C/C++ macros to be set externally
[0]. The attached patch addresses a comment from Joseph Myers <joseph at
codesourcery dot com>: move the getenv calls into the gcc/ directory;
and it also provides better organization and style following the advice
and comments of the current Debian gcc package maintainer Matthias Klose
<doko at debian dot org> (I'm sending the patch with him as co-author).

As a reminder for the motivation behind this patch, we are working on
the reproducible builds project which aims to provide users with a way
to reproduce bit-for-bit identical binary packages from the source and
build environment. The project involves Debian as well as several other
free software projects. See <https://reproducible-builds.org/> for more
information.

Quoting from the original email:

> In order to do this, we need to make the build processes deterministic.
> As you can imagine, gcc is quite involved in producing Debian packages.
> One issue we encounter in many packages that fail to build reproducibly
> is the use of the __DATE__, __TIME__ C macros [1], right now we have 456
> affected packages that would need patching (either removing the macros,
> or passing a known date externally).

> A solution for toolchain packages that embed timestamps during the build
> process has been proposed for anyone interested and it consists of the
> following:
> The build environment can export an environment variable called
> SOURCE_DATE_EPOCH with a known timestamp in Unix epoch format (In our
> case, we use the last date of the package's debian changelog). The
> toolchain package running during the build can check if the exported
> variable is set and if so, instead of embedding the local date/time,
> embed the date/time from SOURCE_DATE_EPOCH.

The proposal to use SOURCE_DATE_EPOCH has now been gathered in a more
formal specification [2], so that any project can adhere to it to
achieve reproducible builds when dealing with timestamps.

> It would be very beneficial to our project (and other free software
> projects working on reproducible builds) if gcc supported this feature.

I'm attaching a patch for gcc-5.2.0 that enables this feature: it
modifies the behavior of the macros __DATE__ and __TIME__ when
SOURCE_DATE_EPOCH is exported.

Any comments, suggestions or other ideas to improve this patch are
mostly welcomed.

I'm willing to extend the documentation if the patch feels appropriate.

Thanks for your attention!


PD: Implementation details:

The error output in gcc/c-family/c-common.c (get_source_date_epoch) is
handled by an fprintf() to stderr followed by an exit (EXIT_FAILURE). I
am not sure this is the right approach for error handling, as I have
found many usages of the error() function in the code. If implemented
with error(), the output looks like this:

"""
$ SOURCE_DATE_EPOCH=123a123 g++ test_cpp.cpp -o test_cpp
In file included from <command-line>:0:0:
/usr/include/stdc-predef.h:1:0: error: Environment variable
$SOURCE_DATE_EPOCH: Trailing garbage: a123

 /* Copyright (C) 1991-2014 Free Software Foundation, Inc.
 ^
"""

For this reason I used fprintf(stderr, "error message") to report the
errors.


[0] https://gcc.gnu.org/ml/gcc-patches/2015-06/msg02210.html
[1] https://wiki.debian.org/ReproducibleBuilds/TimestampsFromCPPMacros
[2] https://reproducible-builds.org/specs/source-date-epoch/

Best regards,
Dhole

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