path-unix: support ~~desktop path expansion

xdg-user-dirs is basically the standard for defining user desktop
directories on linux/bsd. To account for locales and languages, we
basically have to parse the config file (~/.config/user-dirs.dirs)
directly. That or call xdg-user-dir. Either way, it's a little more
complicated than the others, but still doable enough.

DOCS/man/mpv.rst

@@ -590,7 +590,7 @@ Name             Meaning
 ``~~global/``    The global config path (such as ``/etc/mpv``), if available
                  (not on win32).
 ``~~osxbundle/`` The macOS bundle resource path (macOS only).
-``~~desktop/``   The path to the desktop (win32, macOS).
+``~~desktop/``   The path to the desktop.
 ``~~exe_dir/``   The path to the directory containing ``mpv.exe`` (for config
                  file purposes, ``$MPV_HOME`` will override this) (win32 only).
 ``~~cache/``     The path to application cache data (``~/.cache/mpv/``).

osdep/path-unix.c

@@ -17,6 +17,7 @@
 
 #include <string.h>
 
+#include "misc/bstr.h"
 #include "options/path.h"
 #include "osdep/threads.h"
 #include "path.h"
@@ -30,6 +31,7 @@ static char mpv_home[CONF_MAX];
 static char old_home[CONF_MAX];
 static char mpv_cache[CONF_MAX];
 static char old_cache[CONF_MAX];
+static char mpv_desktop[CONF_MAX];
 static char mpv_state[CONF_MAX];
 #define MKPATH(BUF, ...) (snprintf((BUF), CONF_MAX, __VA_ARGS__) >= CONF_MAX)
 
@@ -65,6 +67,38 @@ static void path_init(void)
         err = err || MKPATH(mpv_state, "%s/.local/state/mpv", home);
     }
 
+    char xdg_user_dirs[CONF_MAX];
+    if (xdg_config && xdg_config[0]) {
+        err = err || MKPATH(xdg_user_dirs, "%s/user-dirs.dirs", xdg_config);
+    } else if (home && home[0]) {
+        err = err || MKPATH(xdg_user_dirs, "%s/.config/user-dirs.dirs", home);
+    }
+
+    // Attempt to read user-dirs for XDG_DESKTOP_DIR
+    mpv_desktop[0] = '\0';
+    if (mp_path_exists(xdg_user_dirs)) {
+        char line[4096];
+        FILE *user_dirs = fopen(xdg_user_dirs, "r");
+        while (fgets(line, sizeof(line), user_dirs)) {
+            bstr data = bstr0(line);
+            if (bstr_eatstart0(&data, "XDG_DESKTOP_DIR=")) {
+                bstr value = bstr_strip_linebreaks(data);
+                if (bstr_eatstart0(&value, "\"") && bstr_eatend0(&value, "\"")) {
+                    bool home_prefix = bstr_eatstart0(&value, "$HOME/");
+                    if (home_prefix) {
+                        err = err || MKPATH(mpv_desktop, "%s/%.*s", home, BSTR_P(value));
+                    } else {
+                        err = err || MKPATH(mpv_desktop, "%.*s", BSTR_P(value));
+                    }
+                }
+                break;
+            }
+        }
+    }
+
+    if (!mpv_desktop[0])
+        err = err || MKPATH(mpv_desktop, "%s/%s", home, "Desktop");
+
     // If the old ~/.mpv exists, and the XDG config dir doesn't, use the old
     // config dir only. Also do not use any other XDG directories.
     if (mp_path_exists(old_home) && !mp_path_exists(mpv_home)) {
@@ -95,6 +129,6 @@ const char *mp_get_platform_path_unix(void *talloc_ctx, const char *type)
     if (strcmp(type, "global") == 0)
         return MPV_CONFDIR;
     if (strcmp(type, "desktop") == 0)
-        return getenv("HOME");
+        return mpv_desktop;
     return NULL;
 }