dpkg-buildflags: implement support of DEB_<flag>_MAINT_<op> env variables

Those variables are meant to be used by the package maintainer within
debian/rules to alter the resulting build flags if needed.

debian/changelog

@@ -92,7 +92,8 @@ dpkg (1.16.1) UNRELEASED; urgency=low
     Closes: #615899
   * dpkg-buildflags now supports "--export=configure" to output compilation
     flags on a single line with double quotes as delimiter of the various
-    values.
+    values. It also uses DEB_<flag>_MAINT_<op> to let the maintainer
+    extend the build flags to use.
 
   [ Guillem Jover ]
   * Install deb-src-control(5) man pages in dpkg-dev. Closes: #620520

man/dpkg-buildflags.1

@@ -18,7 +18,11 @@ system-wide with \fB/etc/dpkg/buildflags.conf\fP;
 for the current user with \fB$XDG_CONFIG_HOME/dpkg/buildflags.conf\fP
 where \fB$XDG_CONFIG_HOME\fP defaults to \fB$HOME/.config\fP;
 .IP 3.
-temporarily by the user with environment variables (see section \fBENVIRONMENT\fP).
+temporarily by the user with environment variables (see section
+\fBENVIRONMENT\fP);
+.IP 4.
+dynamically by the package maintainer with environment variables set via
+\fBdebian/rules\fP (see section \fBENVIRONMENT\fP).
 .P
 The configuration files can contain two types of directives:
 .TP
@@ -119,19 +123,28 @@ System wide configuration file.
 .BR $XDG_CONFIG_HOME/dpkg/buildflags.conf " or " $HOME/.config/dpkg/buildflags.conf
 User configuration file.
 .SH ENVIRONMENT
-Those environment variables should not be set by official packages, they
-are meant for users who are recompiling a Debian package and would like to
-change the compilation flags.
+There are 2 sets of environment variables doing the same operations, the
+first one (DEB_\fIflag\fP_\fIop\fP) should never be used within
+\fBdebian/rules\fP. It's meant for any user that wants to rebuild the
+source package with different build flags. The second set
+(DEB_\fIflag\fP_MAINT_\fIop\fP) should only be used in \fBdebian/rules\fP
+by package maintainers to change the resulting build flags.
 .TP
 .BI DEB_ flag _SET
+.TQ
+.BI DEB_ flag _MAINT_SET
 This variable can be used to force the value returned for the given
 \fIflag\fP.
 .TP
 .BI DEB_ flag _APPEND
+.TQ
+.BI DEB_ flag _MAINT_APPEND
 This variable can be used to append supplementary options to the value
 returned for the given \fIflag\fP.
 .TP
 .BI DEB_ flag _PREPEND
+.TQ
+.BI DEB_ flag _MAINT_PREPEND
 This variable can be used to prepend supplementary options to the value
 returned for the given \fIflag\fP.
 .

scripts/Dpkg/BuildFlags.pm

@@ -115,7 +115,7 @@ sub load_user_config {
 
 =item $bf->load_environment_config()
 
-Update flags based on directives stored in the environment. See
+Update flags based on user directives stored in the environment. See
 dpkg-buildflags(1) for details.
 
 =cut
@@ -138,11 +138,37 @@ sub load_environment_config {
     }
 }
 
+=item $bf->load_maintainer_config()
+
+Update flags based on maintainer directives stored in the environment. See
+dpkg-buildflags(1) for details.
+
+=cut
+
+sub load_maintainer_config {
+    my ($self) = @_;
+    foreach my $flag (keys %{$self->{flags}}) {
+	my $envvar = "DEB_" . $flag . "_MAINT_SET";
+	if (exists $ENV{$envvar}) {
+	    $self->set($flag, $ENV{$envvar}, undef);
+	}
+	$envvar = "DEB_" . $flag . "_MAINT_APPEND";
+	if (exists $ENV{$envvar}) {
+	    $self->append($flag, $ENV{$envvar}, undef);
+	}
+	$envvar = "DEB_" . $flag . "_MAINT_PREPEND";
+	if (exists $ENV{$envvar}) {
+	    $self->prepend($flag, $ENV{$envvar}, undef);
+	}
+    }
+}
+
+
 =item $bf->load_config()
 
-Call successively load_system_config(), load_user_config() and
-load_environment_config() to update the default build flags
-defined by the vendor.
+Call successively load_system_config(), load_user_config(),
+load_environment_config() and load_maintainer_config() to update the
+default build flags defined by the vendor.
 
 =cut
 
@@ -151,24 +177,26 @@ sub load_config {
     $self->load_system_config();
     $self->load_user_config();
     $self->load_environment_config();
+    $self->load_maintainer_config();
 }
 
 =item $bf->set($flag, $value, $source)
 
-Update the build flag $flag with value $value and record its origin as $source.
+Update the build flag $flag with value $value and record its origin as
+$source (if defined).
 
 =cut
 
 sub set {
     my ($self, $flag, $value, $src) = @_;
     $self->{flags}->{$flag} = $value;
-    $self->{origin}->{$flag} = $src;
+    $self->{origin}->{$flag} = $src if defined $src;
 }
 
 =item $bf->append($flag, $value, $source)
 
 Append the options listed in $value to the current value of the flag $flag.
-Record its origin as $source.
+Record its origin as $source (if defined).
 
 =cut
 
@@ -179,7 +207,7 @@ sub append {
     } else {
         $self->{flags}->{$flag} = $value;
     }
-    $self->{origin}->{$flag} = $src;
+    $self->{origin}->{$flag} = $src if defined $src;
 }
 
 =item $bf->prepend($flag, $value, $source)
@@ -292,6 +320,9 @@ sub list {
 New method: $bf->prepend() very similar to append(). Implement support of
 the prepend operation everywhere.
 
+New method: $bf->load_maintainer_config() that update the build flags
+based on the package maintainer directives.
+
 =head1 AUTHOR
 
 Raphaël Hertzog <hertzog@debian.org>