lua: expose JSON parser

The JSON parser was introduced for the IPC protocol, but I guess it's
useful here too.

The motivation for this commit is the same as with 8e4fa5fc (again).

DOCS/man/lua.rst

@@ -593,6 +593,20 @@ strictly part of the guaranteed API.
 
     In all cases, ``mp.resume_all()`` is implicitly called.
 
+``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
+    reading ``error``, because there is no fine-grained error reporting of any
+    kind.)
+
+    The returned value uses similar conventions as ``mp.get_property_native()``
+    to distinguish empty objects and arrays.
+
+    If the ``trail`` parameter is ``true`` (or any value equal to ``true``),
+    then trailing non-whitespace text is tolerated by the function, and the
+    trailing text is returned as 3rd return value. (The 3rd return value is
+    always there, but with ``trail`` set, no error is raised.)
+
 Events
 ------
 

misc/json.c

@@ -70,6 +70,11 @@ static void eat_ws(char **src)
     }
 }
 
+void json_skip_whitespace(char **src)
+{
+    eat_ws(src);
+}
+
 static int read_str(void *ta_parent, struct mpv_node *dst, char **src)
 {
     if (!eat_c(src, '"'))

misc/json.h

@@ -22,6 +22,7 @@
 #include "libmpv/client.h"
 
 int json_parse(void *ta_parent, struct mpv_node *dst, char **src, int max_depth);
+void json_skip_whitespace(char **src);
 int json_write(char **s, struct mpv_node *src);
 
 #endif

player/lua.c

@@ -40,6 +40,7 @@
 #include "input/input.h"
 #include "options/path.h"
 #include "misc/bstr.h"
+#include "misc/json.h"
 #include "osdep/timer.h"
 #include "osdep/threads.h"
 #include "stream/stream.h"
@@ -1290,6 +1291,30 @@ done:
 }
 #endif
 
+static int script_parse_json(lua_State *L)
+{
+    void *tmp = mp_lua_PITA(L);
+    char *text = talloc_strdup(tmp, luaL_checkstring(L, 1));
+    bool trail = lua_toboolean(L, 2);
+    bool ok = false;
+    struct mpv_node node;
+    if (json_parse(tmp, &node, &text, 32) >= 0) {
+        json_skip_whitespace(&text);
+        ok = !text[0] || !trail;
+    }
+    if (ok) {
+        if (!pushnode(L, &node, 64))
+            luaL_error(L, "stack overflow");
+        lua_pushnil(L);
+    } else {
+        lua_pushnil(L);
+        lua_pushstring(L, "error");
+    }
+    lua_pushstring(L, text);
+    talloc_free_children(tmp);
+    return 3;
+}
+
 #define FN_ENTRY(name) {#name, script_ ## name}
 struct fn_entry {
     const char *name;
@@ -1339,6 +1364,7 @@ static const struct fn_entry utils_fns[] = {
 #if HAVE_POSIX_SPAWN
     FN_ENTRY(subprocess),
 #endif
+    FN_ENTRY(parse_json),
     {0}
 };