feat(_sh): assure proper installation of a command shell

If one of the supported shells is not in the add modules list,
use the realpath of /bin/sh, if it passes check_module. Otherwise,
use one of the supported shells. Fail if none of them passes.

This scheme assures that implicitly installed command shells pass
through their module-setup.sh with all their dependencies.

Completes fix to #1530 augmenting #1534.

dracut-init.sh

@@ -999,11 +999,13 @@ check_module() {
     [[ " $mods_to_load " == *\ $_mod\ * ]] && return 0
     [[ " $mods_checked_as_dep " == *\ $_mod\ * ]] && return 1
 
-    # This should never happen, but...
-    [[ -d $_moddir ]] || return 1
-
     [[ $2 ]] || mods_checked_as_dep+=" $_mod "
 
+    [[ -d $_moddir ]] || {
+        dwarn "Module '$_mod' cannot be installed without a source directory, '$_moddir'."
+        return 1
+    }
+
     if [[ " $omit_dracutmodules " == *\ $_mod\ * ]]; then
         ddebug "Module '$_mod' will not be installed, because it's in the list to be omitted!"
         return 1

modules.d/00_sh/module-setup.sh

@@ -0,0 +1,33 @@
+#!/bin/bash
+# This file is part of dracut.
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+# Prerequisite check(s) for module.
+check() {
+
+    local shells _shell _s
+
+    # Assure a command shell.
+    [[ " $add_dracutmodules $force_add_dracutmodules " == @(*\ dash\ *|*\ bash\ *|*\ mksh\ *|*\ busybox\ *) ]] || {
+        shells='dash bash mksh busybox'
+        _shell=$(realpath -e /bin/sh)
+        _shell=${_shell##*/}
+        { [[ "$_shell" ]] && check_module "$_shell"; } || {
+            for _s in $_shell ${shells/$_shell/}; do
+                check_module "$_s" || continue
+                # We only want to return 255 since this is a meta module.
+                return 255
+            done
+            dfatal "One of '$shells' must be made available."
+            exit 1
+            }
+        }
+    return 1
+}
+
+# Module dependency requirements.
+depends() {
+
+    return 0
+
+}

modules.d/00bash/module-setup.sh

@@ -24,8 +24,21 @@ depends() {
 # Install the required file(s) and directories for the module in the initramfs.
 install() {
 
+    local version ver0 ver1
+
     inst /bin/bash
 
+    # local - (available since bash-4.4 2016-09-15) automates the
+    #         restoration of local xtrace & other set options.
+    IFS=' ' read -r -a version <<< "$(command "$initdir"/bin/bash --version)"
+    IFS=. read -r ver0 ver1 _ <<< "${version[3]}"
+    if ((${ver0}${ver1} < 44)); then
+        dfatal "Installed Bash ${version[3]} ${version[4]}.
+        At least Bash 4.4 is required for proper xtrace logging
+        when Bash is the initramfs command shell."
+        exit 1
+    fi
+
     # Prefer bash as default shell if no other shell is preferred.
     [[ -L $initdir/bin/sh ]] || ln -sf bash "${initdir}/bin/sh"
 

modules.d/80test-root/module-setup.sh

@@ -22,6 +22,7 @@ install() {
     inst_binary "${dracutbasedir}/dracut-util" "/usr/bin/dracut-util"
     ln -s dracut-util "${initdir}/usr/bin/dracut-getarg"
     ln -s dracut-util "${initdir}/usr/bin/dracut-getargs"
+    ln -s dash /bin/sh
 
     inst_multiple -o plymouth
 }

modules.d/99base/module-setup.sh

@@ -24,11 +24,6 @@ install() {
     ln -s dracut-util "${initdir}/usr/bin/dracut-getarg"
     ln -s dracut-util "${initdir}/usr/bin/dracut-getargs"
 
-    if [ ! -e "${initdir}/bin/sh" ]; then
-        inst_multiple bash
-        (ln -s bash "${initdir}/bin/sh" || :)
-    fi
-
     # add common users in /etc/passwd, it will be used by nfs/ssh currently
     # use password for hostonly images to facilitate secure sulogin in emergency console
     [[ $hostonly ]] && pwshadow='x'