http: Implement trailing headers for chunked transfers

This adds the CURLOPT_TRAILERDATA and CURLOPT_TRAILERFUNCTION
options that allow a callback based approach to sending trailing headers
with chunked transfers.

The test server (sws) was updated to take into account the detection of the
end of transfer in the case of trailing headers presence.

Test 1591 checks that trailing headers can be sent using libcurl.

Closes #3350

1 files changed, 112 insertions, 0 deletions

diff --git a/docs/libcurl/opts/CURLOPT_TRAILERFUNCTION.3 b/docs/libcurl/opts/CURLOPT_TRAILERFUNCTION.3
new file mode 100644
index 000000000..8ca132362
--- /dev/null
+++ b/docs/libcurl/opts/CURLOPT_TRAILERFUNCTION.3
@@ -0,0 +1,112 @@
+.\" **************************************************************************
+.\" *                                  _   _ ____  _
+.\" *  Project                     ___| | | |  _ \| |
+.\" *                             / __| | | | |_) | |
+.\" *                            | (__| |_| |  _ <| |___
+.\" *                             \___|\___/|_| \_\_____|
+.\" *
+.\" * Copyright (C) 1998 - 2018, Daniel Stenberg, <daniel@haxx.se>, et al.
+.\" *
+.\" * This software is licensed as described in the file COPYING, which
+.\" * you should have received as part of this distribution. The terms
+.\" * are also available at https://curl.haxx.se/docs/copyright.html.
+.\" *
+.\" * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+.\" * copies of the Software, and permit persons to whom the Software is
+.\" * furnished to do so, under the terms of the COPYING file.
+.\" *
+.\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+.\" * KIND, either express or implied.
+.\" *
+.\" **************************************************************************
+.\"
+.TH CURLOPT_TRAILERFUNCTION 3 "14 Aug 2018" "libcurl 7.64.0" "curl_easy_setopt options"
+
+.SH NAME:
+CURLOPT_TRAILERFUNCTION \- Set callback for sending trailing headers
+
+.SH SYNOPSIS:
+#include <curl.h>
+
+int curl_trailer_callback(struct curl_slist ** list, void *userdata);
+
+CURLcode curl_easy_setopt(CURL *handle, CURLOPT_TRAILERFUNCTION, curl_trailer_callback *func);
+
+.SH DESCRIPTION:
+Pass a pointer to a callback function.
+
+This callback function will be called once right before sending the final
+CR LF in an HTTP chunked transfer to fill a list of trailing headers to be
+sent before finishing the HTTP transfer.
+
+You can set the userdata argument with the CURLOPT_TRAILERDATA option.
+
+The trailing headers included in the linked list must not be CRLF-terminated,
+because libcurl will add the appropriate line termination characters after
+each header item.
+
+If you use curl_slist_append to add trailing headers to the curl_slist then
+libcurl will duplicate the strings, and will free the curl_slist and the
+duplicates once the trailers have been sent.
+
+If one of the trailing headers is not formatted correctly
+(i.e. HeaderName: headerdata) it will be ignored and an info message
+will be emitted.
+
+The return value can either be CURL_TRAILERFUNC_OK or CURL_TRAILERFUNC_ABORT
+which would respectively instruct libcurl to either continue with sending the
+trailers or to abort the request.
+
+If you set this option to NULL, then the transfer proceeds as usual
+without any interruptions.
+
+.SH DEFAULT:
+NULL
+
+.SH PROTOCOLS:
+HTTP
+
+.SH EXAMPLE:
+#include <curl/curl.h>
+
+static int trailer_cb(struct curl_slist **tr, void *data)
+{
+    /* libcurl will free the list */
+	*tr = curl_slist_append(*tr, "My-super-awesome-trailer: trailer-stuff");
+	(void)data;
+	return CURL_TRAILERFUNC_OK;
+}
+
+CURL *curl = curl_easy_init();
+if(curl) {
+
+    /* Set the URL of the request */
+    curl_easy_setopt(curl, CURLOPT_URL, "http://example.com/");
+    /* Now set it as a put */
+    curl_easy_setopt(curl, CURLOPT_PUT, 1L);
+    
+    /* Assuming we have a function that will return the data to be pushed 
+       Let that function be read_cb */
+    curl_easy_setopt(curl, CURLOPT_READFUNCTION, read_cb);
+
+    struct curl_slist *headers = NULL;
+    headers = curl_slist_append(headers, "Trailer: My-super-awsome-trailer");
+    res = curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);
+
+    /* Set the trailers filling callback */
+    curl_easy_setopt(curl, CURLOPT_TRAILERFUNCTION, (curl_trailer_callback)trailer_cb);
+
+    /* Perform the request, res will get the return code */ 
+    res = curl_easy_perform(curl);
+
+    curl_easy_cleanup(curl);
+
+    curl_slist_free_all(headers);
+}
+
+
+.SH AVAILABILITY:
+This option was added in curl 7.64.0 and is present if HTTP support is enabled
+
+.SH "SEE ALSO"
+.BR CURLOPT_TRAILERDATA "(3), "