add a big comment

bin/subiquity-configure-apt

@@ -1,9 +1,38 @@
 #!/bin/bash -eux
+
+# Configure apt so that installs from the pool on the cdrom are preferred
+# during installation but not in the installed system.
+#
+# This has a few steps.
+#
+# 1. As the remaining steps mean that any changes to apt configuration are do
+#    not persist into the installed system, we get curtin to configure apt a
+#    bit earlier than it would by default.
+#
+# 2. Bind-mount the cdrom into the installed system as /run/cdrom
+#
+# 3. Set up an overlay over /target/etc/apt. This means that any changes we
+#    make will not persist into the installed system and we do not have to
+#    worry about cleaning up after ourselves.
+#
+# 4. Configure apt in /target to look at the pool on the cdrom.  This has two
+#    subcases:
+#
+#     a. if we expect the network to be working, this basically means
+#        prepending "deb file:///run/cdrom $(lsb_release -sc) main restricted"
+#        to the sources.list file.
+#
+#     b. if the network is not expected to be working, we replace the
+#        sources.list with a file just referencing the cdrom.
+#
+# 5. If the network is not expected to be working, we also set up an overlay
+#    over /target/var/lib/apt/lists (if the network is working, we'll run "apt
+#    update" after the /target/etc/apt overlay has been cleared).
+
 PY=$1
 HAS_NETWORK=$2
-mount -t tmpfs tmpfs "$TARGET_MOUNT_POINT/run"
-mkdir -p "$TARGET_MOUNT_POINT/run/cdrom"
-mount --bind /cdrom "$TARGET_MOUNT_POINT/run/cdrom"
+
+$PY -m curtin apt-config
 
 setup_overlay () {
     local dir=$1
@@ -12,10 +41,12 @@ setup_overlay () {
     mount -t overlay overlay -o lowerdir=$dir,workdir=$t/work,upperdir=$t/upper $dir
 }
 
-$PY -m curtin apt-config
-
 setup_overlay $TARGET_MOUNT_POINT/etc/apt
 
+mount -t tmpfs tmpfs "$TARGET_MOUNT_POINT/run"
+mkdir -p "$TARGET_MOUNT_POINT/run/cdrom"
+mount --bind /cdrom "$TARGET_MOUNT_POINT/run/cdrom"
+
 cat > "/tmp/sources.list" <<EOF
 deb file:///run/cdrom $(lsb_release -sc) main restricted
 EOF