command: add property returning current working directory

Requested; fixes #1717.

DOCS/man/input.rst

@@ -1693,6 +1693,10 @@ Property list
     Return the audio device selected by the AO driver (only implemented for
     some drivers: currently only ``coreaudio``).
 
+``working-directory``
+    Return the working directory of the mpv process. Can be useful for JSON IPC
+    users, because the command line player usually works with relative paths.
+
 ``mpv-version``
     Return the mpv version/copyright string. Depending on how the binary was
     built, it might contain either a release version, or just a git hash.

player/command.c

@@ -3149,6 +3149,24 @@ static int mp_property_packet_bitrate(void *ctx, struct m_property *prop,
     return m_property_int64_ro(action, arg, r[type] * 8 / 1000.0 + 0.5);
 }
 
+static int mp_property_cwd(void *ctx, struct m_property *prop,
+                           int action, void *arg)
+{
+    switch (action) {
+    case M_PROPERTY_GET: {
+        char *cwd = mp_getcwd(NULL);
+        if (!cwd)
+            return M_PROPERTY_ERROR;
+        *(char **)arg = cwd;
+        return M_PROPERTY_OK;
+    }
+    case M_PROPERTY_GET_TYPE:
+        *(struct m_option *)arg = (struct m_option){.type = CONF_TYPE_STRING};
+        return M_PROPERTY_OK;
+    }
+    return M_PROPERTY_NOT_IMPLEMENTED;
+}
+
 static int mp_property_version(void *ctx, struct m_property *prop,
                                int action, void *arg)
 {
@@ -3524,6 +3542,8 @@ static const struct m_property mp_properties[] = {
     {"display-names", mp_property_display_names},
     {"display-fps", mp_property_display_fps},
 
+    {"working-directory", mp_property_cwd},
+
     {"mpv-version", mp_property_version},
     {"mpv-configuration", mp_property_configuration},