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doc: clarifying thread safe function references #33871

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doc: clarifying thread safe function references
legendecas Jun 14, 2020
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fixup! doc: clarifying thread safe function references
legendecas Jun 21, 2020
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27 changes: 23 additions & 4 deletions doc/api/n-api.md
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Expand Up @@ -5119,7 +5119,9 @@ The `context` given during the call to `napi_create_threadsafe_function()` can
be retrieved from any thread with a call to
`napi_get_threadsafe_function_context()`.

`napi_call_threadsafe_function()` can then be used for initiating a call into
### Calling a thread-safe function

`napi_call_threadsafe_function()` can be used for initiating a call into
JavaScript. `napi_call_threadsafe_function()` accepts a parameter which controls
whether the API behaves blockingly. If set to `napi_tsfn_nonblocking`, the API
behaves non-blockingly, returning `napi_queue_full` if the queue was full,
Expand Down Expand Up @@ -5151,6 +5153,8 @@ Node.js process exits while there is a thread-safe function still active.
It is not necessary to call into JavaScript via `napi_make_callback()` because
N-API runs `call_js_cb` in a context appropriate for callbacks.

### Reference counting of thread-safe functions

Threads can be added to and removed from a `napi_threadsafe_function` object
during its existence. Thus, in addition to specifying an initial number of
threads upon creation, `napi_acquire_threadsafe_function` can be called to
Expand All @@ -5170,7 +5174,10 @@ reason, do not make use of a thread-safe function
after receiving a return value of `napi_closing` in response to a call to
`napi_call_threadsafe_function`. Data associated with the
`napi_threadsafe_function` can be freed in its `napi_finalize` callback which
was passed to `napi_create_threadsafe_function()`.
was passed to `napi_create_threadsafe_function()`. The parameter
`initial_thread_count` of `napi_create_threadsafe_function` marks the initial
number of aquisitions of the thread-safe functions, instead of calling
`napi_acquire_threadsafe_function` multiple times at creation.

Once the number of threads making use of a `napi_threadsafe_function` reaches
zero, no further threads can start making use of it by calling
Expand All @@ -5190,13 +5197,19 @@ of `napi_closing` from `napi_call_threadsafe_function()` a thread must make no
further use of the thread-safe function because it is no longer guaranteed to
be allocated.**

### Deciding whether to keep the process running

Similarly to libuv handles, thread-safe functions can be "referenced" and
"unreferenced". A "referenced" thread-safe function will cause the event loop on
the thread on which it is created to remain alive until the thread-safe function
is destroyed. In contrast, an "unreferenced" thread-safe function will not
prevent the event loop from exiting. The APIs `napi_ref_threadsafe_function` and
`napi_unref_threadsafe_function` exist for this purpose.

Neither does `napi_unref_threadsafe_function` mark the thread-safe functions as
able to be destroyed nor does `napi_ref_threadsafe_function` prevent it from
being destroyed.

### napi_create_threadsafe_function

<!-- YAML
Expand Down Expand Up @@ -5234,8 +5247,9 @@ napi_create_threadsafe_function(napi_env env,
the kind of resource that is being provided for diagnostic information exposed
by the `async_hooks` API.
* `[in] max_queue_size`: Maximum size of the queue. `0` for no limit.
* `[in] initial_thread_count`: The initial number of threads, including the main
thread, which will be making use of this function.
* `[in] initial_thread_count`: The initial number of acquisitions, i.e. initial
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number of threads, including the main thread, which will be making use of this
function.
* `[in] thread_finalize_data`: Optional data to be passed to `thread_finalize_cb`.
* `[in] thread_finalize_cb`: Optional function to call when the
`napi_threadsafe_function` is being destroyed.
Expand Down Expand Up @@ -5377,6 +5391,11 @@ This API is used to indicate that the event loop running on the main thread
should not exit until `func` has been destroyed. Similar to [`uv_ref`][] it is
also idempotent.

Neither does `napi_unref_threadsafe_function` mark the thread-safe functions as
able to be destroyed nor does `napi_ref_threadsafe_function` prevent it from
being destroyed. `napi_acquire_threadsafe_function` and
`napi_release_threadsafe_function` are available for that purpose.

This API may only be called from the main thread.

### napi_unref_threadsafe_function
Expand Down