dpkg-gencontrol, dpkg-distaddfile: protect update of debian/files with a lock

The lock is taken on debian/control as this is a file that we know to
always exist. Without this lock, it's possible that the file is updated
concurrently by two processes when parallel building is enabled (leading
to one of them failing unexpectedly).

Reported-by: James Vega <jamessan@debian.org>

debian/changelog

@@ -32,6 +32,11 @@ dpkg (1.16.2) UNRELEASED; urgency=low
     too. Closes: #595144
   * Rewrite architecture.mk with explicit loops instead of duplicating many
     similar lines. Based on a patch by Thorsten Glaser <tg@mirbsd.de>.
+  * Modify dpkg-gencontrol and dpkg-distaddfile to grab a write lock
+    on debian/control before updating debian/files to avoid simultaneous
+    updates. Closes: #642608
+    Add libfile-fcntllock-perl to dpkg-dev's Depends since we use this module
+    to handle the locking.
 
   [ Jonathan Nieder ]
   * Bump po4a version in Build-Depends to 0.41, since earlier versions do

debian/control

@@ -49,7 +49,7 @@ Package: dpkg-dev
 Section: utils
 Priority: optional
 Architecture: all
-Depends: libdpkg-perl (= ${source:Version}), bzip2, xz-utils,
+Depends: libdpkg-perl (= ${source:Version}), libfile-fcntllock-perl, bzip2, xz-utils,
  patch, make, binutils, base-files (>= 5.0.0), ${misc:Depends}
 Recommends: gcc | c-compiler, build-essential, fakeroot, gnupg, gpgv, libalgorithm-merge-perl
 Suggests: debian-keyring

scripts/dpkg-distaddfile.pl

@@ -18,6 +18,7 @@
 use strict;
 use warnings;
 
+use File::FcntlLock;
 use POSIX;
 use POSIX qw(:errno_h :signal_h);
 use Dpkg;
@@ -75,6 +76,15 @@ my ($file, $section, $priority) = @ARGV;
 ($file =~ m/\s/ || $section =~ m/\s/ || $priority =~ m/\s/) &&
     error(_g("filename, section and priority may contain no whitespace"));
 
+# Obtain a lock on debian/control to avoid simultaneous updates
+# of debian/files when parallel building is in use
+my $fs = File::FcntlLock->new(l_type => F_WRLCK);
+my $lockfh;
+sysopen($lockfh, "debian/control", O_WRONLY) ||
+    syserr(_g("cannot write %s"), "debian/control");
+$fs->lock($lockfh, F_SETLKW) ||
+    syserr(_("failed to get a write lock on %s"), "debian/control");
+
 $fileslistfile="./$fileslistfile" if $fileslistfile =~ m/^\s/;
 open(Y, "> $fileslistfile.new") || syserr(_g("open new files list file"));
 if (open(X,"< $fileslistfile")) {
@@ -91,3 +101,6 @@ print(Y "$file $section $priority\n")
 close(Y) || syserr(_g("close new files list file"));
 rename("$fileslistfile.new", $fileslistfile) ||
     syserr(_g("install new files list file"));
+
+# Release the lock
+close($lockfh) || syserr(_g("cannot close %s"), "debian/control");

scripts/dpkg-gencontrol.pl

@@ -18,6 +18,7 @@
 use strict;
 use warnings;
 
+use File::FcntlLock;
 use POSIX;
 use POSIX qw(:errno_h);
 use Dpkg;
@@ -333,6 +334,15 @@ for my $f (keys %remove) {
     delete $fields->{$f};
 }
 
+# Obtain a lock on debian/control to avoid simultaneous updates
+# of debian/files when parallel building is in use
+my $fs = File::FcntlLock->new(l_type => F_WRLCK);
+my $lockfh;
+sysopen($lockfh, "debian/control", O_WRONLY) ||
+    syserr(_g("cannot write %s"), "debian/control");
+$fs->lock($lockfh, F_SETLKW) ||
+    syserr(_("failed to get a write lock on %s"), "debian/control");
+
 $fileslistfile="./$fileslistfile" if $fileslistfile =~ m/^\s/;
 open(Y, ">", "$fileslistfile.new") || syserr(_g("open new files list file"));
 binmode(Y);
@@ -363,6 +373,9 @@ print(Y $substvars->substvars(sprintf("%s %s %s\n", $forcefilename,
 close(Y) || syserr(_g("close new files list file"));
 rename("$fileslistfile.new", $fileslistfile) || syserr(_g("install new files list file"));
 
+# Release the lock
+close($lockfh) || syserr(_g("cannot close %s"), "debian/control");
+
 my $cf;
 my $fh_output;
 if (!$stdout) {