Fix /dev/ptmx rather than excluding makedev

Revert exclusion of makedev from buildd chroots, as it turned out not to
be the problem.  Instead, fix up /dev/ptmx to be a character device node
rather than a symlink to /dev/pts/ptmx, in line with the discussion in
https://bugs.debian.org/817236; I think this is safer than
cherry-picking the fix to debootstrap at this point in a stable release
cycle.

LP: #1844504

debian/changelog

@@ -1,3 +1,14 @@
+livecd-rootfs (2.408.54) UNRELEASED; urgency=medium
+
+  * Revert exclusion of makedev from buildd chroots, as it turned out not to
+    be the problem.  Instead, fix up /dev/ptmx to be a character device node
+    rather than a symlink to /dev/pts/ptmx, in line with the discussion in
+    https://bugs.debian.org/817236; I think this is safer than
+    cherry-picking the fix to debootstrap at this point in a stable release
+    cycle (LP: #1844504).
+
+ -- Colin Watson <cjwatson@ubuntu.com>  Thu, 26 Sep 2019 10:31:55 +0100
+
 livecd-rootfs (2.408.53) xenial; urgency=medium
 
   * Fix exclusion of makedev from buildd chroots; debootstrap doesn't

live-build/buildd/hooks/00-no-makedev.chroot_early

@@ -1,7 +0,0 @@
-#! /bin/sh
-set -e
-
-# makedev is "Priority: required" in xenial and so unavoidably installed by
-# debootstrap, but it isn't truly required and has historically been
-# excluded from buildd chroots.
-apt-get -y purge makedev

live-build/buildd/hooks/00-ptmx-chardev.chroot_early

@@ -0,0 +1,19 @@
+#! /bin/sh
+set -e
+
+# debootstrap 1.0.76 started creating /dev/ptmx as a symlink to
+# /dev/pts/ptmx.  Unfortunately, this doesn't work with sbuild, because it
+# leaves the ptmxmode mount option at its default of 000, which causes
+# builds to be unable to open /dev/pts/ptmx.  To avoid this, debootstrap
+# 1.0.89 switched to creating it as a device node where possible.  See
+# https://bugs.debian.org/817236 for details and analysis.
+#
+# xenial has a version of debootstrap in the range that contains this bug.
+# It seems too risky to try to cherry-pick the debootstrap change in
+# question in an SRU at this point; instead, just fix things up here for
+# buildd images.
+
+if [ -h /dev/ptmx ]; then
+	mknod -m 666 /dev/ptmx.new c 5 2
+	mv /dev/ptmx.new /dev/ptmx
+fi