Clarify the allowed usages of clap_plugin_entry's init() and deinit()

include/clap/entry.h

@@ -31,34 +31,109 @@ extern "C" {
 // Each directory should be recursively searched for files and/or bundles as appropriate in your OS
 // ending with the extension `.clap`.
 //
-// Every method must be thread-safe.
+// As detailed in the respective functions' documentations below, the host is forbidden from calling
+// either init() or deinit() multiple times, without calling the other first. This is to enforce
+// a "normal" initialization pattern, where one can't construct or destroy an object multiple times.
+//
+// However, there are a few cases where the host may not be able to reliably prevent this from
+// happening, such as when multiple, separate hosts co-exist within the same address space (e.g.
+// Meta-Plugins, plugins that can host other plugins inside them). It may also happen when a VST
+// compatibility layer is used to wrap a CLAP plugin, for instance.
+//
+// In those cases, the init() and deinit() pair of functions can happen to be called multiple times,
+// either in a row or even simultaneously, from different threads, by multiple non-synchronized
+// hosts, or by a single host that is presented multiple interfaces to a same DSO. This can happen
+// even though this is forbidden behavior from the host by this specification, and despite a
+// compliant host's best efforts.
+//
+// Despite those issues, hosts *must* ensure that each successful call to init() is only followed by
+// a single call to deinit() at most. As long as a single successful init() call did not have its
+// matching deinit() called, the DSO *must* remain in a fully operational state, as if deinit() had
+// not yet been called.
+//
+// Therefore, init() and deinit() functions *should* be implemented in a way that allows for init()
+// to be called any number of times in a row (possibly simultaneously, from multiple threads), and
+// for deinit() to be called at most that same number of times (also simultaneously, from multiple,
+// possibly different threads from the init() called). As of CLAP 1.11, this *must* be the case for
+// all plugin implementations.
+//
+// This is already trivially the case if there is no (de)initialization work in the entry at all,
+// or if the initialization work is thread safe and idempotent, and there is no de-initialization
+// work to be done. Those implementations already are and remain valid, and require no additional
+// consideration.
+//
+// If that is not the case, then using a global, refcount-like locked initialization counter can be
+// used to keep track of init() and deinit() calls. Then the initialization work can be done only
+// when the counter goes from 0 to 1 in init(), and de-initialization can be done when the counter
+// goes to 0 in deinit(). Any subsequent calls to init() and deinit() can just increase or decrease
+// the counter, respectively.
 typedef struct clap_plugin_entry {
    clap_version_t clap_version; // initialized to CLAP_VERSION
 
-   // This function must be called first, and can only be called once.
+   // Initializes the DSO.
+   //
+   // This function must be called first, before any-other CLAP-related function or symbol from this
+   // DSO.
+   //
+   // It also must only be called once, until a later call to deinit() is made, after which init()
+   // can be called once more to re-initialize the DSO.
+   // This enables hosts to e.g. quickly load and unload a DSO for scanning its plugins, and then
+   // load it again later to actually use the plugins if needed.
+   //
+   // As stated above, even though hosts are forbidden to do so directly, multiple calls before any
+   // deinit() call may still happen. Implementations *should* take this into account, and *must*
+   // do so as of CLAP 1.11.
    //
    // It should be as fast as possible, in order to perform a very quick scan of the plugin
    // descriptors.
    //
-   // It is forbidden to display graphical user interface in this call.
-   // It is forbidden to perform user interaction in this call.
+   // It is forbidden to display graphical user interfaces in this call.
+   // It is forbidden to perform any user interaction in this call.
    //
    // If the initialization depends upon expensive computation, maybe try to do them ahead of time
    // and cache the result.
    //
-   // If init() returns false, then the host must not call deinit() nor any other clap
-   // related symbols from the DSO.
+   // Returns true on success. If init() returns false, then the DSO must be considered
+   // uninitialized, and the host must not call deinit() nor any other CLAP-related symbols from the
+   // DSO.
+   // This function also returns true in the case where the DSO is already initialized, and no
+   // actual initialization work is done in this call, as explain above.
    //
    // plugin_path is the path to the DSO (Linux, Windows), or the bundle (macOS).
+   //
+   // This function may be called on any thread, including a different one from the one a later call
+   // to deinit() (or a later init()) can be made.
+   // However, it is forbidden to call this function simultaneously from multiple threads.
+   // It is also forbidden to call it simultaneously with *any* other CLAP-related symbols from the
+   // DSO, including (but not limited to) deinit().
    bool(CLAP_ABI *init)(const char *plugin_path);
 
-   // No more calls into the DSO must be made after calling deinit().
+   // De-initializes the DSO, freeing any resources allocated or initialized by init().
+   //
+   // After this function is called, no more calls into the DSO must be made, except calling init()
+   // again to re-initialize the DSO.
+   // This means that after deinit() is called, the DSO can be considered to be in the same state
+   // as if init() was never called at all yet, enabling it to be re-initialized as needed.
+   //
+   // As stated above, even though hosts are forbidden to do so directly, multiple calls before any
+   // new init() call may still happen. Implementations *should* take this into account, and *must*
+   // do so as of CLAP 1.11.
+   //
+   // Just like init(), this function may be called on any thread, including a different one from
+   // the one init() was called from, or from the one a later init() call can be made.
+   // However, it is forbidden to call this function simultaneously from multiple threads.
+   // It is also forbidden to call it simultaneously with *any* other CLAP-related symbols from the
+   // DSO, including (but not limited to) deinit().
    void(CLAP_ABI *deinit)(void);
 
    // Get the pointer to a factory. See factory/plugin-factory.h for an example.
    //
    // Returns null if the factory is not provided.
    // The returned pointer must *not* be freed by the caller.
+   //
+   // Unlike init() and deinit(), this function can be called simultaneously by multiple threads.
+   //
+   // [thread-safe]
    const void *(CLAP_ABI *get_factory)(const char *factory_id);
 } clap_plugin_entry_t;