iscsi-generator: add a PoC generator that brings up iscsi root device

This is not indended as a real solution. The generator should be moved to the
open-iscsi project. It is added here to show how things should be done using a
systemd generator and an initrd unit. The generator does not handle all the
initricasies of iscsi.

.github/workflows/build-fedora.sh

@@ -290,6 +290,7 @@ for phase in "${PHASES[@]}"; do
             mkosi --cache "$MKOSI_CACHE" \
                   --default fedora.mkosi \
                   --package="NetworkManager,iscsi-initiator-utils" \
+                  --extra-tree=mkosi.extra-iscsi \
                   --image-version="$KVER" \
                   --environment=KERNEL_VERSION="$KVER" \
                   --output="$INITRD" \

mkosi.extra-iscsi/usr/lib/systemd/system-generators/iscsi-generator.sh

@@ -0,0 +1,98 @@
+#!/bin/bash
+set -e
+
+# Format:
+#       root=iscsi:[<servername>]:[<protocol>]:[<port>]:[<LUN>]:<targetname>
+#       [root=*] netroot=iscsi:[<servername>]:[<protocol>]:[<port>]:[<LUN>]:<targetname>
+#
+# root= takes precedence over netroot= if root=iscsi[...]
+
+# Example:
+# netroot=iscsi:10.10.10.1::::iqn.2022-01.com.example:iscsi.initrd.test
+# /dev/disk/by-path/ip-10.10.10.1:3260-iscsi-iqn.2022-01.com.example:iscsi.initrd.test-lun-1
+# /dev/disk/by-path/ip-10.10.10.1:3260-iscsi-iqn.2022-01.com.example:iscsi.initrd.test-lun-1-part1
+# /dev/disk/by-path/ip-<servername>:<port>-iscsi-<lun>-<targetname>-lun-<lun>
+
+root=
+specs=()
+
+cmdline="${SYSTEMD_PROC_CMDLINE:-$(cat /proc/cmdline)}"
+
+for arg in $cmdline; do
+    if [[ $arg == "root="* ]]; then
+        # The last root= assignment wins.
+        root="${arg#*=}"
+    elif [[ $arg == "netroot=iscsi:"* ]]; then
+        value="${arg#*=}"
+        # All netroot= invocations matter.
+        specs+=( "$value" )
+
+        if [ -z "$root" ]; then
+            root="$value"
+        fi
+    fi
+done
+
+if [[ $root == "iscsi:"* ]]; then
+    specs+=( "$root" )
+fi
+
+if [ ${#specs[@]} -eq 0 ]; then
+    # nothing for us to do
+    exit 0
+fi
+
+make_unit() {
+    # Handle the root= argument
+    if [[ $root == "iscsi:" ]]; then
+        : # TODO
+    fi
+
+    service="${dest_normal}/initrd-iscsi-auto.service"
+    this="$(realpath "$0")"
+    cat >"${service}" <<EOF
+# generated by $this
+[Unit]
+Description=Poke iscsid to bring up iscsi devices
+
+DefaultDependencies=no
+Wants=iscsid.socket
+Wants=iscsiuio.socket
+Wants=network-online.target
+Before=initrd-root-device.target
+Conflicts=shutdown.target
+Before=shutdown.target
+After=network-online.target
+
+[Service]
+Type=oneshot
+ExecStart=$this --go
+EOF
+
+    mkdir -p "${dest_normal}/initrd-root-device.target.requires"
+    ln -s "../initrd-iscsi-auto.service" "${dest_normal}/initrd-root-device.target.requires/"
+}
+
+do_things() {
+    for spec in "${specs[@]}"; do
+        server=$(echo "$spec" | cut -d: -f2)
+        # protocol?
+        port=$(echo "$spec" | cut -d: -f4)
+        lun=$(echo "$spec" | cut -d: -f5)
+        target=$(echo "$spec" | cut -d: -f6-)
+
+        echo "$spec: will start server=$server port=$port lun=$lun target=$target"
+
+        iscsiadm -m discovery -t st -p "${server}${port:+:$port}"
+        iscsiadm -m node -T "$target" -l
+    done
+}
+
+if [ "${1:?}" == '--go' ]; then
+    do_things
+else
+    # invoked as generator
+    dest_normal=${1:?}
+
+    make_unit
+fi