Build flags: Add support for extension builders (#1543394)

buildflags.md

@@ -332,3 +332,44 @@ linker flags:
   security exploits, redirecting execution).  Therefore, it is
   preferable to turn of lazy binding, although it increases startup
   time.
+
+# Support for extension builders
+
+Some packages include extension builders that allow users to build
+extension modules (which are usually written in C and C++) under the
+control of a special-purpose build system.  This is a common
+functionality provided by scripting languages such as Python and Perl.
+Traditionally, such extension builders captured the Fedora build flags
+when these extension were built.  However, these compiler flags are
+adjusted for a specific Fedora release and toolchain version and
+therefore do not work with a custom toolchain (e.g., different C/C++
+compilers), and users might want to build their own extension modules
+with such toolchains.
+
+The macros `%{extension_cflags}`, `%{extension_cxxflags}`,
+`%{extension_fflags}`, `%{extension_ldflags}` contain a subset of
+flags that have been adjusted for compatibility with alternative
+toolchains, while still preserving some of the compile-time security
+hardening that the standard Fedora build flags provide.
+
+The current set of differences are:
+
+* No GCC plugins (such as annobin) are activated.
+* No GCC spec files (`-specs=` arguments) are used.
+
+Additional flags may be removed in the future if they prove to be
+incompatible with alternative toolchains.
+
+Extension builders should detect whether they are performing a regular
+RPM build (e.g., by looking for an `RPM_OPT_FLAGS` variable).  In this
+case, they should use the *current* set of Fedora build flags (that
+is, the output from `rpm --eval '%{build_cflags}'` and related
+commands).  Otherwise, when not performing an RPM build, they can
+either use hard-coded extension builder flags (thus avoiding a
+run-time dependency on `redhat-rpm-config`), or use the current
+extension builder flags (with a run-time dependency on
+`redhat-rpm-config`).
+
+As a result, extension modules built for Fedora will use the official
+Fedora build flags, while users will still be able to build their own
+extension modules with custom toolchains.

macros

@@ -57,6 +57,22 @@
   FCFLAGS="${FCFLAGS:-%{build_fflags}}" ; export FCFLAGS ; \
   LDFLAGS="${LDFLAGS:-%{build_ldflags}}" ; export LDFLAGS
 
+# Internal-only.  Do not use.  Expand a variable and strip the flags
+# not suitable to extension builders.
+%__extension_strip_flags() %{lua:
+local name = rpm.expand("%{1}")
+local value = " " .. rpm.expand("%{build_" .. name .. "}")
+local result = string.gsub(value, "%s+-specs=[^%s]+", " ")
+print(result)
+}
+
+# Variants of CFLAGS, CXXFLAGS, FFLAGS, LDFLAGS for use within
+# extension builders.
+%extension_cflags %{__extension_strip_flags cflags}
+%extension_cxxflags %{__extension_strip_flags cxxflags}
+%extension_fflags %{__extension_strip_flags fflags}
+%extension_ldflags %{__extension_strip_flags ldflags}
+
 # Deprecated names.  For backwards compatibility only.
 %__global_cflags %{build_cflags}
 %__global_cxxflags %{build_cxxflags}

redhat-rpm-config.spec

@@ -6,7 +6,7 @@
 
 Summary: Red Hat specific rpm configuration files
 Name: redhat-rpm-config
-Version: 126
+Version: 127
 Release: 1%{?dist}
 # No version specified.
 License: GPL+
@@ -199,6 +199,9 @@ install -p -m 644 -t %{buildroot}%{_rpmluadir}/fedora/srpm forge.lua
 %{_rpmconfigdir}/macros.d/macros.kmp
 
 %changelog
+* Thu Dec 20 2018 Florian Weimer <fweimer@redhat.com> - 127-1
+- Build flags: Add support for extension builders (#1543394)
+
 * Mon Dec 17 2018 Panu Matilainen <pmatilai@redhat.com> - 126-1
 - Silence the annoying warning from ldconfig brp-script (#1540971)