dpkg-buildflags: supports a prepend command

This command is particularly useful for package maintainers who don't want
their supplementary flags to take precedence over user submitted flags.

debian/changelog

@@ -65,6 +65,10 @@ dpkg (1.16.1) UNRELEASED; urgency=low
   * dpkg-buildflags parses a supplementary configuration file under the
     control of the package maintainer: debian/buildflags (or the corresponding
     architecture/os specific variants).
+  * dpkg-buildflags supports a prepend command to modify the build
+    flags. Particularly useful for package maintainers who don't want
+    their supplementary flags to take precedence over user submitted
+    flags.
 
   [ Guillem Jover ]
   * Install deb-src-control(5) man pages in dpkg-dev. Closes: #620520

man/dpkg-buildflags.1

@@ -32,8 +32,12 @@ The configuration files can contain two types of directives:
 Override the flag named \fIflag\fP to have the value \fIvalue\fP.
 .TP
 .BI APPEND " flag value"
-Extend the flag named \fIflag\fP with the options given in \fIvalue\fP.
+Extend the flag named \fIflag\fP by appending the options given in \fIvalue\fP.
 A space is prepended to the appended value if the flag's current value is non-empty.
+.TP
+.BI PREPEND " flag value"
+Extend the flag named \fIflag\fP by prepending the options given in \fIvalue\fP.
+A space is appended to the prepended value if the flag's current value is non-empty.
 .P
 The configuration files can contain comments on lines starting with a hash
 (#). Empty lines are also ignored.
@@ -128,6 +132,10 @@ This variable can be used to force the value returned for the given
 .BI DEB_ flag _APPEND
 This variable can be used to append supplementary options to the value
 returned for the given \fIflag\fP.
+.TP
+.BI DEB_ flag _PREPEND
+This variable can be used to prepend supplementary options to the value
+returned for the given \fIflag\fP.
 .
 .SH AUTHOR
 Copyright \(co 2010-2011 Rapha\[:e]l Hertzog

scripts/Dpkg/BuildFlags.pm

@@ -132,6 +132,10 @@ sub load_environment_config {
 	if (exists $ENV{$envvar}) {
 	    $self->append($flag, $ENV{$envvar}, "env");
 	}
+	$envvar = "DEB_" . $flag . "_PREPEND";
+	if (exists $ENV{$envvar}) {
+	    $self->prepend($flag, $ENV{$envvar}, "env");
+	}
     }
 }
 
@@ -196,6 +200,24 @@ sub append {
     $self->{origin}->{$flag} = $src if defined $src;
 }
 
+=item $bf->prepend($flag, $value, $source)
+
+Prepend the options listed in $value to the current value of the flag $flag.
+Record its origin as $source (if defined).
+
+=cut
+
+sub prepend {
+    my ($self, $flag, $value, $src) = @_;
+    if (length($self->{flags}->{$flag})) {
+        $self->{flags}->{$flag} = "$value " . $self->{flags}->{$flag};
+    } else {
+        $self->{flags}->{$flag} = $value;
+    }
+    $self->{origin}->{$flag} = $src if defined $src;
+}
+
+
 =item $bf->update_from_conffile($file, $source)
 
 Update the current build flags based on the configuration directives
@@ -213,7 +235,7 @@ sub update_from_conffile {
         chomp;
         next if /^\s*#/; # Skip comments
         next if /^\s*$/; # Skip empty lines
-        if (/^(append|set)\s+(\S+)\s+(\S.*\S)\s*$/i) {
+        if (/^(append|prepend|set)\s+(\S+)\s+(\S.*\S)\s*$/i) {
             my ($op, $flag, $value) = ($1, $2, $3);
             unless (exists $self->{flags}->{$flag}) {
                 warning(_g("line %d of %s mentions unknown flag %s"), $., $file, $flag);
@@ -223,6 +245,8 @@ sub update_from_conffile {
                 $self->set($flag, $value, $src);
             } elsif (lc($op) eq "append") {
                 $self->append($flag, $value, $src);
+            } elsif (lc($op) eq "prepend") {
+                $self->prepend($flag, $value, $src);
             }
         } else {
             warning(_g("line %d of %s is invalid, it has been ignored."), $., $file);
@@ -286,6 +310,9 @@ sub list {
 New method: $bf->load_package_config(). This method is called last as part
 of load_config().
 
+New method: $bf->prepend() very similar to append(). Implement support of
+the prepend operation everywhere.
+
 =head1 AUTHOR
 
 Raphaël Hertzog <hertzog@debian.org>