kernel-install: support the case /etc/machine-id is missing or empty (#5975)

Some .install plugins does not require that machine ID is set such as
20-grubby.install for Fedora and 50-depmod.install.
To support such plugins to run without valid machine-id, this commit
makes the following change:
* if /etc/machine-id is missing or empty, create temporary directory
  and set its path to BOOT_DIR_ABS,
* run the .install helpers with KERNEL_INSTALL_MACHINE_ID environment
  variable that'd be empty if /etc/machine-id is missing or empty.
This may be useful for installing kernel for e.g. stateless systems
which initialize machine-id while booting the systems.

NEWS

@@ -7,6 +7,13 @@ CHANGES WITH 234 in spe
           fallback was redundant and interfered with the [!UNAVAIL=return]
           suffix. See nss-resolve(8) for the recommended configuration.
 
+        * All kernel install plugins are called with the environment variable
+          KERNEL_INSTALL_MACHINE_ID which is set to the machine ID given by
+          /etc/machine-id. If the file is missing or empty, the variable is
+          empty and BOOT_DIR_ABS is the path of a temporary directory which is
+          removed after the all plugins exit. So, if KERNEL_INSTALL_MACHINE_ID
+          is empty, all plugins should not put anything in BOOT_DIR_ABS.
+
 CHANGES WITH 233:
 
         * This version requires at least gperf 3.1 for building, 3.0 is not

src/kernel-install/90-loaderentry.install

@@ -7,13 +7,11 @@ KERNEL_VERSION="$2"
 BOOT_DIR_ABS="$3"
 KERNEL_IMAGE="$4"
 
-if [[ -f /etc/machine-id ]]; then
+if ! [[ $KERNEL_INSTALL_MACHINE_ID ]]; then
-    read MACHINE_ID < /etc/machine-id
+    exit 0
 fi
 
-if ! [[ $MACHINE_ID ]]; then
+MACHINE_ID=$KERNEL_INSTALL_MACHINE_ID
-    exit 1
-fi
 
 BOOT_DIR="/$MACHINE_ID/$KERNEL_VERSION"
 BOOT_ROOT=${BOOT_DIR_ABS%$BOOT_DIR}

src/kernel-install/kernel-install

@@ -77,18 +77,15 @@ if [[ -f /etc/machine-id ]]; then
     read MACHINE_ID < /etc/machine-id
 fi
 
-if ! [[ $MACHINE_ID ]]; then
-    echo "Could not determine your machine ID from /etc/machine-id." >&2
-    echo "Please run 'systemd-machine-id-setup' as root. See man:machine-id(5)" >&2
-    exit 1
-fi
-
 if [[ ! $COMMAND ]] || [[ ! $KERNEL_VERSION ]]; then
     echo "Not enough arguments" >&2
     exit 1
 fi
 
-if [[ -d /efi/loader/entries ]] || [[ -d /efi/$MACHINE_ID ]]; then
+if ! [[ $MACHINE_ID ]]; then
+    BOOT_DIR_ABS=$(mktemp -d /tmp/kernel-install.XXXXX) || exit 1
+    trap "rm -rf '$BOOT_DIR_ABS'" EXIT INT QUIT PIPE
+elif [[ -d /efi/loader/entries ]] || [[ -d /efi/$MACHINE_ID ]]; then
     BOOT_DIR_ABS="/efi/$MACHINE_ID/$KERNEL_VERSION"
 elif [[ -d /boot/loader/entries ]] || [[ -d /boot/$MACHINE_ID ]]; then
     BOOT_DIR_ABS="/boot/$MACHINE_ID/$KERNEL_VERSION"
@@ -102,6 +99,8 @@ else
     BOOT_DIR_ABS="/boot/$MACHINE_ID/$KERNEL_VERSION"
 fi
 
+export KERNEL_INSTALL_MACHINE_ID=$MACHINE_ID
+
 ret=0
 
 readarray -t PLUGINS <<<"$(
@@ -127,11 +126,20 @@ case $COMMAND in
                 "$f" add "$KERNEL_VERSION" "$BOOT_DIR_ABS" "$KERNEL_IMAGE"
                 x=$?
                 if [[ $x == $SKIP_REMAINING ]]; then
-                    exit 0
+                    ret=0
+                    break
                 fi
                 ((ret+=$x))
             fi
         done
+
+        if ! [[ $MACHINE_ID ]] && ! rmdir "$BOOT_DIR_ABS"; then
+            echo "Warning: In kernel-install plugins, requiring BOOT_DIR_ABS to be preset is deprecated." >&2
+            echo "         All plugins should not put anything in BOOT_DIR_ABS if the environment" >&2
+            echo "         variable KERNEL_INSTALL_MACHINE_ID is empty." >&2
+            rm -rf "$BOOT_DIR_ABS"
+            ((ret+=$?))
+        fi
         ;;
 
     remove)
@@ -140,7 +148,8 @@ case $COMMAND in
                 "$f" remove "$KERNEL_VERSION" "$BOOT_DIR_ABS"
                 x=$?
                 if [[ $x == $SKIP_REMAINING ]]; then
-                    exit 0
+                    ret=0
+                    break
                 fi
                 ((ret+=$x))
             fi