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add scope based resource cleanup

Barnabás Pőcze requested to merge pobrn/pipewire:raii into master

systemd, dbus-broker, and many glib applications heavily utilize the GNU C attribute "cleanup" to achieve C++ RAII-like semantics for local resource management. This commit introduces essentialy same mechanism into pipewire.

At the moment, this is inteded to be a strictly private API.

free() and close() as cleanup targets are sufficiently common to warrant their own special macros:

  spa_autofree char *s = strdup(p);
  // will call `free(s)` at the end of the lifetime of `s`

  spa_autoclose int fd = openat(...);
  // will call `close(fd)` if `fd >= 0` at the end of the lifetime of `fd`

However, with spa_auto() or spa_autoptr() it is possible to define other variables that will be cleaned up properly. Currently four are supported:

  spa_autoptr(FILE) f = fopen(...);
  // `f` has type `FILE *`
  // will call `fclose(f)` if `f != NULL`

  spa_autoptr(DIR) d = opendir(...);
  // `d` has type `DIR *`
  // will call `closedir(d)` if `d != NULL`

  spa_autoptr(pw_properties) p = pw_properties_new(NULL, NULL);
  // `p` has type `struct pw_properties *`
  // will call `pw_properties_free(p)`

  spa_auto(pw_strv) v = pw_split_strv(...);
  // `v` has type `char **`
  // will call `pw_strv_free(v)`

It is possible to add support for other types, e.g.

  SPA_DEFINE_AUTOPTR_CLEANUP(pw_main_loop, struct pw_main_loop, {
    // the pointer can be accessed using `*thing`
    // `thing` has type `struct pw_main_loop **`
    spa_clear_ptr(*thing, pw_main_loop_destroy);
  })

  spa_autoptr(pw_main_loop) l = ...;
  // `l` has type `struct pw_main_loop *`
  // will call `pw_main_loop_destroy(l)`

or

  SPA_DEFINE_AUTO_CLEANUP(spa_pod_dynamic_builder, struct spa_pod_dynamic_builder, {
    // `thing` has type `struct spa_pod_dynamic_builder *`
    spa_pod_dynamic_builder_clean(thing);
  })

  spa_auto(spa_pod_dynamic_builder) builder = ...
  // `builder` has type `struct spa_pod_dynamic_builder`
  // will call `spa_pod_dynamic_builder_clean(&builder)`

The first argument is always an arbitrary name. This name must be passed to spa_auto() and spa_autoptr() as it is what determines the actual type and destructor used. The second parameter is the concrete type. For SPA_DEFINE_AUTO_CLEANUP() this is the concrete type to be used, while for SPA_DEFINE_AUTOPTR_CLEANUP() it is the concrete type without the outermost pointer. That is,

  SPA_DEFINE_AUTOPTR_CLEANUP(A, foo, ...)
  SPA_DEFINE_AUTO_CLEANUP(B, foo, ...)

  spa_autoptr(A) x; // `x` has type `foo *`
  spa_auto(B) y; // `y` has type `foo`

A couple other macros are also added:

  spa_clear_ptr(ptr, destructor)
  // calls `destructor(ptr)` if `ptr != NULL` and sets `ptr` to `NULL`

  spa_clear_fd(fd)
  // calls `close(fd)` if `fd >= 0` and sets `fd` to -1

  spa_steal_ptr(ptr)
  // sets `ptr` to `NULL` and returns the old value,
  // useful for preventing the auto cleanup mechanism from kicking in
  // when returning the pointer from a function

  spa_steal_fd(fd)
  // sets `fd` to -1 and returns the old value

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