From ef5f167515aed68166215e1ab02c2434d93de0f1 Mon Sep 17 00:00:00 2001
From: Matthias Kruk <m@m10k.eu>
Date: Sat, 3 Dec 2022 16:51:59 +0900
Subject: [PATCH] toolbox.sh: Implement naive interfaces and inheritance

The ipc and uipc modules provide the same API to the module user,
but the user cannot easily switch between the two implementations
because toolbox modules do not have a notion of interfaces.

This commit adds a very primitive means for modules to declare and
implement interfaces: when a module declares an interface, a vtable
is allocated in the form of an associative array, and call stubs
are generated which call the functions referenced in the vtable.
A module that implements an interface can then override entries in
the interface's vtable.
---
 toolbox.sh | 93 +++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 92 insertions(+), 1 deletion(-)

diff --git a/toolbox.sh b/toolbox.sh
index 924b11b..e9cbfd2 100644
--- a/toolbox.sh
+++ b/toolbox.sh
@@ -42,13 +42,16 @@ __toolbox_init() {
 		"$TOOLBOX_HOME/include"
 		"$toolboxroot/include"
 	)
-
+	declare -Axg __TOOLBOX_INTERFACES
 	declare -Axg __TOOLBOX_INCLUDED
 
 	readonly -f have
 	readonly -f _try_include
 	readonly -f include
 	readonly -f command_not_found_handle
+	readonly -f interface
+	readonly -f implements
+	readonly -f calling_module
 
 	return 0
 }
@@ -155,6 +158,94 @@ command_not_found_handle() {
 	return 127
 }
 
+calling_module() {
+	local modpath
+	local modfile
+	local modname
+
+	modpath="${BASH_SOURCE[2]}" # /path/to/module.sh
+	modfile="${modpath##*/}"    # module.sh
+	modname="${modfile%.*}"     # module
+
+	echo "$modname"
+	return 0
+}
+
+interface() {
+	local methods=("$@")
+
+	local name
+	local method
+	local call_stubs
+	local vtable_name
+	local -n interface
+
+	if ! name=$(calling_module); then
+		echo "ERROR: Could not determine name of module" 1>&2
+		return 1
+	fi
+
+	call_stubs=""
+	vtable_name="__TOOLBOX_INTERFACE_$name"
+
+	# Create the vtable for the interface
+	declare -Agx "$vtable_name"
+
+	interface="$vtable_name"
+
+	__TOOLBOX_INTERFACES["$name"]="$vtable_name"
+
+	# Generate and load call stubs that execute the functions referenced by the vtable
+	for method in "${methods[@]}"; do
+		# shellcheck disable=SC2016 # Reason: Don't want expansion in the format string
+		call_stubs+=$(printf '\n%s() {\n\t"${%s["%s"]}" "$@"\n\treturn "$?"\n}' \
+				     "${name}_$method" "$vtable_name" "$method")
+	done
+
+	# shellcheck disable=SC1090 # Reason: Dynamically generated call-stubs can't be checked
+	if ! source <(echo "$call_stubs"); then
+		echo "ERROR: Could not generate call stubs for interface $name" 1>&2
+		return 1
+	fi
+
+	# Set the entries in the vtable
+	for method in "${methods[@]}"; do
+		interface["$method"]="_${name}_$method"
+	done
+
+	return 0
+}
+
+implements() {
+	local iface="$1"
+
+	local module
+	local method
+	declare -n interface
+
+	if [[ -z "${__TOOLBOX_INTERFACES[$iface]}" ]]; then
+		echo "ERROR: Unknown interface: \"$iface\"" 1>&2
+		return 1
+	fi
+
+	interface="${__TOOLBOX_INTERFACES[$iface]}"
+	module=$(calling_module)
+
+	# Check if all methods are implemented before modifying the vtable
+	for method in "${!interface[@]}"; do
+		if ! declare -f "${module}_$method" &>/dev/null; then
+			echo "ERROR: Module $module does not implement the method \"$method\"" 1>&2
+			return 1
+		fi
+	done
+
+	for method in "${!interface[@]}"; do
+		interface["$method"]="${module}_$method"
+	done
+
+	return 0
+}
+
 {
 	if ! compgen -v | grep "^__TOOLBOX_INCLUDED$" &> /dev/null; then
 		if ! __toolbox_init; then
-- 
2.47.3