Make it available inside the toolbox container

Commit 5b3d234 had made the toolbox script work inside a
toolbox container, but most people didn't get to use it because it
needed extra effort to get access to the script inside the container.
One either had to grab the Toolbox sources or had to install the RPM.
Both options were inconvenient - the former needed knowing too many
technical details, while the latter would drag in dependencies like
Buildah and Podman that don't work inside the toolbox container.

This makes it easier to use the toolbox script inside a toolbox
container by bind mounting the script from the host inside the
container and keeping track of the path using the TOOLBOX_PATH
environment variable. The environment variable ensures that running
'toolbox create' from inside a toolbox container would continue to
bind mount the same script from the host that was used to create the
current container and is available inside it.

Compatibility with existing toolbox containers is broken when using
the script within a container because it insists on the TOOLBOX_PATH
environment variable being set inside. This might not be that big of a
deal because using the toolbox script inside a toolbox container wasn't
very convenient, and hence likely not used widely. In case of
complaints, this can be relaxed by falling back to "$0" when forwarding
the command to the host, but unless that happens it's better to keep
things simple to avoid a larger test matrix.

Based on an idea from Colin Walters.

https://github.com/debarshiray/toolbox/pull/126

toolbox

@@ -26,6 +26,7 @@ base_toolbox_image=""
 # https://github.com/containers/libpod/blob/master/libpod/options.go
 container_name_regexp="[a-zA-Z0-9][a-zA-Z0-9_.-]*"
 
+environment=$(set)
 environment_variables="COLORTERM \
         DBUS_SESSION_BUS_ADDRESS \
         DBUS_SYSTEM_BUS_ADDRESS \
@@ -35,6 +36,7 @@ environment_variables="COLORTERM \
         SHELL \
         SSH_AUTH_SOCK \
         TERM \
+        TOOLBOX_PATH \
         VTE_VERSION \
         WAYLAND_DISPLAY \
         XDG_CURRENT_DESKTOP \
@@ -57,6 +59,7 @@ release_default=""
 spinner_animation="[>----] [=>---] [==>--] [===>-] [====>] [----<] [---<=] [--<==] [-<===] [<====]"
 spinner_template="toolbox-spinner-XXXXXXXXXX"
 tab="$(printf '\t')"
+toolbox_command_path=""
 toolbox_container=""
 toolbox_container_default=""
 toolbox_container_old=""
@@ -322,7 +325,6 @@ create_environment_options()
     echo "$environment_variables" \
         | sed "s/ \+/\n/g" 2>&3 \
         | {
-              environment=$(set)
               environment_options=""
               echo "$base_toolbox_command: creating list of environment variables to forward" >&3
               value=""
@@ -703,6 +705,9 @@ create()
         tmpfs_size=$((total_ram * 1024 / 2)) # bytes
     fi
 
+    toolbox_path_bind="--volume $TOOLBOX_PATH:/usr/bin/toolbox:ro"
+    toolbox_path_set="--env TOOLBOX_PATH=$TOOLBOX_PATH"
+
     max_uid_count=65536
     max_minus_uid=$((max_uid_count - user_id_real))
     uid_plus_one=$((user_id_real + 1))
@@ -721,6 +726,7 @@ create()
 
     # shellcheck disable=SC2086
     $prefix_sudo podman create \
+            $toolbox_path_set \
             --group-add wheel \
             --hostname toolbox \
             --name $toolbox_container \
@@ -733,6 +739,7 @@ create()
             --uidmap 0:1:"$user_id_real" \
             --uidmap "$uid_plus_one":"$uid_plus_one":"$max_minus_uid" \
             $kcm_socket_bind \
+            $toolbox_path_bind \
             --volume "$HOME":"$HOME":rslave \
             --volume "$XDG_RUNTIME_DIR":"$XDG_RUNTIME_DIR" \
             --volume "$dbus_system_bus_path":"$dbus_system_bus_path" \
@@ -1147,13 +1154,13 @@ forward_to_host()
     set_environment=$(create_environment_options)
 
     echo "$base_toolbox_command: forwarding to host:" >&3
-    echo "$base_toolbox_command: $0" >&3
+    echo "$base_toolbox_command: $TOOLBOX_PATH" >&3
     for i in "$@"; do
         echo "$base_toolbox_command: $i" >&3
     done
 
     # shellcheck disable=SC2086
-    flatpak-spawn $set_environment --host "$0" "$@" 2>&3
+    flatpak-spawn $set_environment --host "$TOOLBOX_PATH" "$@" 2>&3
 )
 
 
@@ -1275,6 +1282,13 @@ while has_prefix "$1" -; do
     shift
 done
 
+if ! toolbox_command_path=$(realpath "$0" 2>&3); then
+    echo "$base_toolbox_command: failed to resolve absolute path to $0" >&2
+    exit 1
+fi
+
+echo "$base_toolbox_command: resolved absolute path for $0 to $toolbox_command_path" >&3
+
 if [ -f /run/.containerenv ] 2>&3; then
     if ! command -v flatpak-spawn >/dev/null 2>&3; then
         echo "$base_toolbox_command: flatpak-spawn not found" >&2
@@ -1309,8 +1323,19 @@ if [ -f /run/.containerenv ] 2>&3; then
     fi
 
     echo "$base_toolbox_command: podman PID is $podman_pid" >&3
+
+    if [ "$TOOLBOX_PATH" = "" ] 2>&3; then
+        echo "$base_toolbox_command: TOOLBOX_PATH not set" >&2
+        exit 1
+    fi
+else
+    if [ "$TOOLBOX_PATH" = "" ] 2>&3; then
+        TOOLBOX_PATH="$toolbox_command_path"
+    fi
 fi
 
+echo "$base_toolbox_command: TOOLBOX_PATH is $TOOLBOX_PATH" >&3
+
 if [ "$1" = "" ]; then
     echo "$base_toolbox_command: missing command" >&2
     echo >&2