[PATCH] gold: add --package-metadata

[Cary Coutant]

> Following the same format as the implementation in ld:
> 9e2bb0cb5e74aed4158f08495534922d7108f928
>
> Generate a .note.package FDO package metadata ELF note, following
> the spec: https://systemd.io/ELF_PACKAGE_METADATA/
>
> If the jansson library is available at build time (and it is explicitly
> enabled), link ld to it, and use it to validate that the input is
> correct JSON, to avoid writing garbage to the file. The
> configure option --enable-jansson has to be used to explicitly enable
> it (error out when not found). This allows bootstrappers (or others who
> are not interested) to seamlessly skip it without issues.

I reviewed the earlier discussion and I had some of the same questions
and concerns as others did there, so they've all been answered. I'd
have preferred an option syntax that would let you build up the
metadata info one key/value pair at a time. Given that it has already
been accepted for ld, however, I'll OK it for gold as well, with the
following fixes...

+  json_t *json = json_loads (desc, 0, &json_error);

C++ coding conventions here: no space before the paren in a function call.

+  if (!json)
+    gold_fatal(_("error: --package-metadata=%s does not contain valid "
+      "JSON: %s\n"),
+    desc, json_error.text);
+  else
+    json_decref (json);

Put the shorter, non-error path first (i.e., make it "if
(json)...else..."), and use { } for the longer error path (even though
it's only one statement, I think braces around a multi-line statement
help readability). Also, no space before the paren.

+  if (trailing_padding != 0)
+  {
+    posd = new Output_data_zero_fill(trailing_padding, 0);
+    os->add_output_section_data(posd);
+  }

The braces and what's inside them need an extra two spaces of indent.

-cary