player/command: add env property

DOCS/interface-changes/env-property.txt

@@ -0,0 +1 @@
+add env property that can be used to retrieve environment variables

DOCS/man/input.rst

@@ -2230,6 +2230,13 @@ Property list
     .. note:: This is only an estimate. (It's computed from two unreliable
               quantities: fps and possibly rounded timestamps.)
 
+``env/...``
+    Read-only property that can be used for retrieving environment variables.
+    The variable to retrieve should be set as the sub-property. E.g
+    ``${env/HOME}`` will return ``$HOME`` if set.
+
+    Note that this property cannot be used directly, a sub-property is required.
+
 ``pid``
     Process-id of mpv.
 
@@ -3643,6 +3650,8 @@ Property list
     If tracks of the requested type are selected via ``--lavfi-complex``, the
     first one is returned.
 
+    Note that this property cannot be used directly, a sub-property is required.
+
 ``chapter-list`` (RW)
     List of chapters, current entry marked.
 

player/command.c

@@ -544,6 +544,17 @@ static int mp_property_filename(void *ctx, struct m_property *prop,
     return r;
 }
 
+static int mp_property_env(void *ctx, struct m_property *prop,
+                           int action, void *arg)
+{
+    if (action == M_PROPERTY_KEY_ACTION) {
+        struct m_property_action_arg *ka = arg;
+        char *val = getenv(ka->key);
+        return m_property_strdup_ro(ka->action, ka->arg, val);
+    }
+    return M_PROPERTY_NOT_IMPLEMENTED;
+}
+
 static int mp_property_stream_open_filename(void *ctx, struct m_property *prop,
                                             int action, void *arg)
 {
@@ -4387,6 +4398,7 @@ static const struct m_property mp_properties_base[] = {
     {"video-speed-correction", mp_property_av_speed_correction, .priv = "v"},
     {"display-sync-active", mp_property_display_sync_active},
     {"filename", mp_property_filename},
+    {"env", mp_property_env},
     {"stream-open-filename", mp_property_stream_open_filename},
     {"file-size", mp_property_file_size},
     {"path", mp_property_path},