http2: refactor trailers API

Rather than using the `'fetchTrailers'` event to collect trailers,
a new `getTrailers` callback option is supported. If not set, the
internals will skip calling out for trailers at all. Expands the
test to make sure trailers work from the client side also.

Backport-PR-URL: #14813
Backport-Reviewed-By: Anna Henningsen <anna@addaleax.net>
Backport-Reviewed-By: Timothy Gu <timothygu99@gmail.com>

PR-URL: #14239
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

doc/api/http2.md

@@ -346,6 +346,9 @@ added: REPLACEME
   * `weight` {number} Specifies the relative dependency of a stream in relation
     to other streams with the same `parent`. The value is a number between `1`
     and `256` (inclusive).
+  * `getTrailers` {Function} Callback function invoked to collect trailer
+    headers.
+
 * Returns: {ClientHttp2Stream}
 
 For HTTP/2 Client `Http2Session` instances only, the `http2session.request()`
@@ -371,6 +374,16 @@ req.on('response', (headers) => {
 });
 ```
 
+When set, the `options.getTrailers()` function is called immediately after
+queuing the last chunk of payload data to be sent. The callback is passed a
+single object (with a `null` prototype) that the listener may used to specify
+the trailing header fields to send to the peer.
+
+*Note*: The HTTP/1 specification forbids trailers from containing HTTP/2
+"pseudo-header" fields (e.g. `':method'`, `':path'`, etc). An `'error'` event
+will be emitted if the `getTrailers` callback attempts to set such header
+fields.
+
 #### http2session.rstStream(stream, code)
 <!-- YAML
 added: REPLACEME
@@ -617,27 +630,6 @@ added: REPLACEME
 The `'error'` event is emitted when an error occurs during the processing of
 an `Http2Stream`.
 
-#### Event: 'fetchTrailers'
-<!-- YAML
-added: REPLACEME
--->
-
-The `'fetchTrailers'` event is emitted by the `Http2Stream` immediately after
-queuing the last chunk of payload data to be sent. The listener callback is
-passed a single object (with a `null` prototype) that the listener may used
-to specify the trailing header fields to send to the peer.
-
-```js
-stream.on('fetchTrailers', (trailers) => {
-  trailers['ABC'] = 'some value to send';
-});
-```
-
-*Note*: The HTTP/1 specification forbids trailers from containing HTTP/2
-"pseudo-header" fields (e.g. `':status'`, `':path'`, etc). An `'error'` event
-will be emitted if the `'fetchTrailers'` event handler attempts to set such
-header fields.
-
 #### Event: 'frameError'
 <!-- YAML
 added: REPLACEME
@@ -991,6 +983,8 @@ added: REPLACEME
 * `options` {Object}
   * `endStream` {boolean} Set to `true` to indicate that the response will not
     include payload data.
+  * `getTrailers` {function} Callback function invoked to collect trailer
+    headers.
 * Returns: {undefined}
 
 ```js
@@ -1002,6 +996,29 @@ server.on('stream', (stream) => {
 });
 ```
 
+When set, the `options.getTrailers()` function is called immediately after
+queuing the last chunk of payload data to be sent. The callback is passed a
+single object (with a `null` prototype) that the listener may used to specify
+the trailing header fields to send to the peer.
+
+```js
+const http2 = require('http2');
+const server = http2.createServer();
+server.on('stream', (stream) => {
+  stream.respond({ ':status': 200 }, {
+    getTrailers(trailers) {
+      trailers['ABC'] = 'some value to send';
+    }
+  });
+  stream.end('some data');
+});
+```
+
+*Note*: The HTTP/1 specification forbids trailers from containing HTTP/2
+"pseudo-header" fields (e.g. `':status'`, `':path'`, etc). An `'error'` event
+will be emitted if the `getTrailers` callback attempts to set such header
+fields.
+
 #### http2stream.respondWithFD(fd[, headers[, options]])
 <!-- YAML
 added: REPLACEME
@@ -1011,6 +1028,8 @@ added: REPLACEME
 * `headers` {[Headers Object][]}
 * `options` {Object}
   * `statCheck` {Function}
+  * `getTrailers` {Function} Callback function invoked to collect trailer
+    headers.
   * `offset` {number} The offset position at which to begin reading
   * `length` {number} The amount of data from the fd to send
 
@@ -1020,8 +1039,7 @@ attempting to read data using the file descriptor, the `Http2Stream` will be
 closed using an `RST_STREAM` frame using the standard `INTERNAL_ERROR` code.
 
 When used, the `Http2Stream` object's Duplex interface will be closed
-automatically. HTTP trailer fields cannot be sent. The `'fetchTrailers'` event
-will *not* be emitted.
+automatically.
 
 ```js
 const http2 = require('http2');
@@ -1052,6 +1070,39 @@ The `offset` and `length` options may be used to limit the response to a
 specific range subset. This can be used, for instance, to support HTTP Range
 requests.
 
+When set, the `options.getTrailers()` function is called immediately after
+queuing the last chunk of payload data to be sent. The callback is passed a
+single object (with a `null` prototype) that the listener may used to specify
+the trailing header fields to send to the peer.
+
+```js
+const http2 = require('http2');
+const fs = require('fs');
+
+const fd = fs.openSync('/some/file', 'r');
+
+const server = http2.createServer();
+server.on('stream', (stream) => {
+  const stat = fs.fstatSync(fd);
+  const headers = {
+    'content-length': stat.size,
+    'last-modified': stat.mtime.toUTCString(),
+    'content-type': 'text/plain'
+  };
+  stream.respondWithFD(fd, headers, {
+    getTrailers(trailers) {
+      trailers['ABC'] = 'some value to send';
+    }
+  });
+});
+server.on('close', () => fs.closeSync(fd));
+```
+
+*Note*: The HTTP/1 specification forbids trailers from containing HTTP/2
+"pseudo-header" fields (e.g. `':status'`, `':path'`, etc). An `'error'` event
+will be emitted if the `getTrailers` callback attempts to set such header
+fields.
+
 #### http2stream.respondWithFile(path[, headers[, options]])
 <!-- YAML
 added: REPLACEME
@@ -1061,15 +1112,16 @@ added: REPLACEME
 * `headers` {[Headers Object][]}
 * `options` {Object}
   * `statCheck` {Function}
+  * `getTrailers` {Function} Callback function invoked to collect trailer
+    headers.
   * `offset` {number} The offset position at which to begin reading
   * `length` {number} The amount of data from the fd to send
 
 Sends a regular file as the response. The `path` must specify a regular file
 or an `'error'` event will be emitted on the `Http2Stream` object.
 
 When used, the `Http2Stream` object's Duplex interface will be closed
-automatically. HTTP trailer fields cannot be sent. The `'fetchTrailers'` event
-will *not* be emitted.
+automatically.
 
 The optional `options.statCheck` function may be specified to give user code
 an opportunity to set additional content headers based on the `fs.Stat` details
@@ -1120,6 +1172,29 @@ The `offset` and `length` options may be used to limit the response to a
 specific range subset. This can be used, for instance, to support HTTP Range
 requests.
 
+When set, the `options.getTrailers()` function is called immediately after
+queuing the last chunk of payload data to be sent. The callback is passed a
+single object (with a `null` prototype) that the listener may used to specify
+the trailing header fields to send to the peer.
+
+```js
+const http2 = require('http2');
+const server = http2.createServer();
+server.on('stream', (stream) => {
+  function getTrailers(trailers) {
+    trailers['ABC'] = 'some value to send';
+  }
+  stream.respondWithFile('/some/file',
+                         { 'content-type': 'text/plain' },
+                         { getTrailers });
+});
+```
+
+*Note*: The HTTP/1 specification forbids trailers from containing HTTP/2
+"pseudo-header" fields (e.g. `':status'`, `':path'`, etc). An `'error'` event
+will be emitted if the `getTrailers` callback attempts to set such header
+fields.
+
 ### Class: Http2Server
 <!-- YAML
 added: REPLACEME