Extend capabilities of zbm.prefer

Under certain circumstances, it might be preferable to ensure
ZFSBootMenu only ever interacts with a specific zpool.
Historical/default behavior has every pool being imported each boot,
with optional preference being given to one specific pool.

A new modifier to `zbm.prefer`, `!!` has been added. When this is
appended to the name of a pool, that pool is imported exclusive of all
others on the system. If the pool can't be imported, a retry/recovery
shell loop is initiated.

docs/pod/zfsbootmenu.7.pod

@@ -18,9 +18,25 @@ These options are set on the kernel command line when booting the initramfs or U
 
 When creating an initramfs or UEFI bundle, the I</etc/hostid> from the system is copied into the target. If this image will be used on another system with a different hostid, replace B<E<lt>hostidE<gt>> with the desired hostid, as an eight-digit hexadecimal number, to override the value contained within the image.
 
+=item B<zbm.prefer>
+
+ZFSBootMenu will attempt to import as many pools as possible to identify boot environments and will, by default, look for the I<bootfs> property on the first imported pool (sorted alphabetically) to select the default boot environment. This option controls this behavior.
+
+=over 2
+
 =item B<zbm.prefer=E<lt>poolE<gt>>
 
-ZFSBootMenu will attempt to import as many pools as possible to identify boot environments and will, by default, look for the I<bootfs> property on the first imported pool (sorted alphabetically) to select the default boot environment. If you have multiple pools, replace B<E<lt>poolE<gt>> with the name of your preferred pool to override the default. If B<E<lt>poolE<gt>> is the name of a pool followed by a literal I<!> character, ZFSBootMenu will insist on successfully importing the named pool before attempting to import any others.
+The simplest form attempts to import B<E<lt>poolE<gt>> before any other pool. The I<bootfs> value from this pool will control the default boot environment.
+
+=item B<zbm.prefer=E<lt>poolE<gt>I<!>>
+
+If a literal I<!> has been appended to the pool name, ZFSBootMenu will insist on successfully importing the named pool before attempting to import any others.
+
+=item B<zbm.prefer=E<lt>poolE<gt>I<!!>>
+
+If a literal I<!!> has been appended to the pool name, ZFSBootMenu will insist on successfully importing the named pool and no others. 
+
+=back
 
 =item B<zbm.import_delay=E<lt>timeE<gt>>
 

zfsbootmenu/help-files/132/zfsbootmenu.7.pod

@@ -16,11 +16,22 @@ general systems to boot without setting any values.
     on another system with a different hostid, replace <hostid> with the desired hostid, as an eight-digit hexadecimal number, to
     override the value contained within the image.
 
-zbm.prefer=<pool>
+zbm.prefer
     ZFSBootMenu will attempt to import as many pools as possible to identify boot environments and will, by default, look for the
-    bootfs property on the first imported pool (sorted alphabetically) to select the default boot environment. If you have multiple
-    pools, replace <pool> with the name of your preferred pool to override the default. If <pool> is the name of a pool followed by
-    a literal ! character, ZFSBootMenu will insist on successfully importing the named pool before attempting to import any others.
+    bootfs property on the first imported pool (sorted alphabetically) to select the default boot environment. This option controls
+    this behavior.
+
+    zbm.prefer=<pool>
+      The simplest form attempts to import <pool> before any other pool. The bootfs value from this pool will control the default
+      boot environment.
+
+    zbm.prefer=<pool>!
+      If a literal ! has been appended to the pool name, ZFSBootMenu will insist on successfully importing the named pool before
+      attempting to import any others.
+
+    zbm.prefer=<pool>!!
+      If a literal !! has been appended to the pool name, ZFSBootMenu will insist on successfully importing the named pool and no
+      others.
 
 zbm.import_delay=<time>
     Should ZFSBootMenu fail to successfully import any pool, it will repeat import attempts indefinitely until at least one pool can

zfsbootmenu/help-files/52/zfsbootmenu.7.pod

@@ -24,19 +24,30 @@ without setting any values.
     hexadecimal number, to override the value
     contained within the image.
 
-zbm.prefer=<pool>
+zbm.prefer
     ZFSBootMenu will attempt to import as many pools
     as possible to identify boot environments and
     will, by default, look for the bootfs property
     on the first imported pool (sorted
     alphabetically) to select the default boot
-    environment. If you have multiple pools, replace
-    <pool> with the name of your preferred pool to
-    override the default. If <pool> is the name of a
-    pool followed by a literal ! character,
-    ZFSBootMenu will insist on successfully
-    importing the named pool before attempting to
-    import any others.
+    environment. This option controls this behavior.
+
+    zbm.prefer=<pool>
+      The simplest form attempts to import <pool>
+      before any other pool. The bootfs value from
+      this pool will control the default boot
+      environment.
+
+    zbm.prefer=<pool>!
+      If a literal ! has been appended to the pool
+      name, ZFSBootMenu will insist on successfully
+      importing the named pool before attempting to
+      import any others.
+
+    zbm.prefer=<pool>!!
+      If a literal !! has been appended to the pool
+      name, ZFSBootMenu will insist on successfully
+      importing the named pool and no others.
 
 zbm.import_delay=<time>
     Should ZFSBootMenu fail to successfully import

zfsbootmenu/help-files/92/zfsbootmenu.7.pod

@@ -18,14 +18,23 @@ Default options were chosen to allow general systems to boot without setting any
     replace <hostid> with the desired hostid, as an eight-digit hexadecimal number, to
     override the value contained within the image.
 
-zbm.prefer=<pool>
+zbm.prefer
     ZFSBootMenu will attempt to import as many pools as possible to identify boot
     environments and will, by default, look for the bootfs property on the first imported
-    pool (sorted alphabetically) to select the default boot environment. If you have
-    multiple pools, replace <pool> with the name of your preferred pool to override the
-    default. If <pool> is the name of a pool followed by a literal ! character, ZFSBootMenu
-    will insist on successfully importing the named pool before attempting to import any
-    others.
+    pool (sorted alphabetically) to select the default boot environment. This option
+    controls this behavior.
+
+    zbm.prefer=<pool>
+      The simplest form attempts to import <pool> before any other pool. The bootfs value
+      from this pool will control the default boot environment.
+
+    zbm.prefer=<pool>!
+      If a literal ! has been appended to the pool name, ZFSBootMenu will insist on
+      successfully importing the named pool before attempting to import any others.
+
+    zbm.prefer=<pool>!!
+      If a literal !! has been appended to the pool name, ZFSBootMenu will insist on
+      successfully importing the named pool and no others.
 
 zbm.import_delay=<time>
     Should ZFSBootMenu fail to successfully import any pool, it will repeat import attempts

zfsbootmenu/hook/zfsbootmenu-parse-commandline.sh

@@ -231,13 +231,23 @@ case "${root}" in
   ;;
 esac
 
-# Pool preference ending in ! indicates a hard requirement
-bpool="${root%\!}"
-if [ "${bpool}" != "${root}" ]; then
-  # shellcheck disable=SC2034
-  zbm_require_bpool=1
-  root="${bpool}"
-fi
+# pool! : this pool must be imported before all others
+# pool!!: this pool, and no others, must be imported
+
+# shellcheck disable=SC2034
+case "${root}" in
+  *!!)
+    zbm_require_bpool="only"
+    root="${root%!!}"
+    ;;
+  *!)
+    zbm_require_bpool="yes"
+    root="${root%!}"
+    ;;
+  *)
+    zbm_require_bpool=""
+    ;;
+esac
 
 # Make sure Dracut is happy that we have a root
 if [ ${wait_for_zfs} -eq 1 ]; then

zfsbootmenu/libexec/zfsbootmenu-init

@@ -105,7 +105,10 @@ while true; do
 
   # shellcheck disable=SC2154
   if check_for_pools; then
-    if [ -n "${try_pool}" ]; then
+    if [ "${zbm_require_bpool}" = "only" ]; then
+      zdebug "only importing ${try_pool}"
+      break
+    elif [ -n "${try_pool}" ]; then
       # If a single pool was requested and imported, try importing others
       try_pool=""
       continue
@@ -127,7 +130,7 @@ while true; do
     # allow another pass to pick up others with the same hostid
     try_pool=""
     continue
-  elif [ -n "${try_pool}" ] && [ "${zbm_require_bpool:-0}" -ne 1 ]; then
+  elif [ -n "${try_pool}" ] && [ -z "${zbm_require_bpool}" ]; then
     # If a specific pool was tried unsuccessfully but is not a requirement,
     # allow another pass to try any other importable pools
     try_pool=""