lua: add mp.get_env_list() function

Because Lua is too stupid to provide this directly, and I sort of need
it.

DOCS/man/input.rst

@@ -539,6 +539,9 @@ Remember to quote string arguments in input.conf (see `Flat command syntax`_).
         useless.) The format of the list is as in the ``execle()`` syscall. Each
         string item defines an environment variable as in ``NANME=VALUE``.
 
+        On Lua, you may use ``utils.get_env_list()`` to retrieve the current
+        environment if you e.g. simply want to add a new variable.
+
     The command returns the following result (as ``MPV_FORMAT_NODE_MAP``):
 
     ``status`` (``MPV_FORMAT_INT64``)

DOCS/man/lua.rst

@@ -826,6 +826,10 @@ strictly part of the guaranteed API.
     Returns the process ID of the running mpv process. This can be used to identify
     the calling mpv when launching (detached) subprocesses.
 
+``utils.get_env_list()``
+    Returns the C environment as a list of strings. (Do not confuse this with
+    the Lua "environment", which is an unrelated concept.)
+
 ``utils.parse_json(str [, trail])``
     Parses the given string argument as JSON, and returns it as a Lua table. On
     error, returns ``nil, error``. (Currently, ``error`` is just a string

player/lua.c

@@ -53,6 +53,8 @@
 #include "client.h"
 #include "libmpv/client.h"
 
+extern char **environ;
+
 // List of builtin modules and their contents as strings.
 // All these are generated from player/lua/*.lua
 static const char * const builtin_lua_scripts[][2] = {
@@ -1185,6 +1187,16 @@ static int script_format_json(lua_State *L, void *tmp)
     return 2;
 }
 
+static int script_get_env_list(lua_State *L)
+{
+    lua_newtable(L); // table
+    for (int n = 0; environ && environ[n]; n++) {
+        lua_pushstring(L, environ[n]); // table str
+        lua_rawseti(L, -2, n + 1); // table
+    }
+    return 1;
+}
+
 #define FN_ENTRY(name) {#name, script_ ## name, 0}
 #define AF_ENTRY(name) {#name, 0, script_ ## name}
 struct fn_entry {
@@ -1226,6 +1238,7 @@ static const struct fn_entry main_fns[] = {
     FN_ENTRY(get_wakeup_pipe),
     FN_ENTRY(raw_hook_add),
     FN_ENTRY(raw_hook_continue),
+    FN_ENTRY(get_env_list),
     {0}
 };