Enable pkg_rpm and pkg_subrpm to create empty RPMs (bazelbuild#859)

* Enable pkg_rpm and pkg_subrpm to create empty RPMs

At present we fail in two ways if we try to create empty RPMs:

- we expect srcs to be both non-empty and will fail if it is
  empty and we have no spec file

- we don't emit anything for the `%files` block in the RPM if
  there are no actual files and rpmbuild doesn't like this

This change tweaks the former condition so that srcs has to be
non-None or we have to have a specfile, but will allow us to have an
empty (`[]`) value for srcs.  Additionally, it injects
`%defattr(-,root,root)` as a reasonable default for the `%files`
blocks so as to allow rpmbuild to be happy with what we're providing.

* Inject default file mode unconditionally

We should be safe to inject this unconditionally instead of special
casing on whether or not we have no actual files.

* Fixup wrong append

pkg/rpm.bzl

@@ -53,16 +53,16 @@ def pkg_rpm(name, srcs = None, spec_file = None, subrpms = None, **kwargs):
                 depending on mode
 
     """
-    if srcs and spec_file:
+    if srcs != None and spec_file:
         fail("Cannot determine which pkg_rpm rule to use.  `srcs` and `spec_file` are mutually exclusive")
 
     if subrpms and spec_file:
         fail("Cannot build sub RPMs with a specfile.  `subrpms` and `spec_file` are mutually exclusive")
 
-    if not srcs and not spec_file:
+    if srcs == None and not spec_file:
         fail("Either `srcs` or `spec_file` must be provided.")
 
-    if srcs:
+    if srcs != None:
         pkg_rpm_pfg(
             name = name,
             srcs = srcs,

pkg/rpm_pfg.bzl

@@ -57,6 +57,9 @@ PackageSubRPMInfo = provider(
     },
 )
 
+# default mode for %files
+DEFAULT_FILE_MODE = "%defattr(-,root,root)"
+
 # TODO(nacl): __install, __cp
 # {0} is the source, {1} is the dest
 #
@@ -355,7 +358,11 @@ def _process_subrpm(ctx, rpm_name, rpm_info, rpm_ctx):
         for dep in rpm_info.srcs:
             _process_dep(dep, sub_rpm_ctx)
 
+        # rpmbuild will be unhappy if we have no files so we stick
+        # default file mode in for that scenario
+        rpm_lines += [DEFAULT_FILE_MODE]
         rpm_lines += sub_rpm_ctx.rpm_files_list
+
         rpm_lines += [""]
 
     rpm_ctx.install_script_pieces.extend(sub_rpm_ctx.install_script_pieces)
@@ -702,10 +709,11 @@ def _pkg_rpm_impl(ctx):
     rpm_files_file = ctx.actions.declare_file(
         "{}.spec.files".format(rpm_name),
     )
-    ctx.actions.write(
-        rpm_files_file,
-        "\n".join(rpm_ctx.rpm_files_list),
-    )
+
+    # rpmbuild will be unhappy if we have no files so we stick
+    # default file mode in for that scenario
+    rpm_files_contents = [DEFAULT_FILE_MODE] + rpm_ctx.rpm_files_list
+    ctx.actions.write(rpm_files_file, "\n".join(rpm_files_contents))
 
     # TreeArtifact processing work
     if rpm_ctx.packaged_directories: