websocket: support CURLOPT_READFUNCTION

Add support for CURLOPT_READFUNCTION with WebSocket urls when *not* in
connect-only mode, e.g. when using curl_multi_perform.

Install the callback function and set CURLOPT_UPLOAD. Return
CURL_READFUNC_PAUSE when having nothing more to send and unpause the
transfer when more data is ready.

This will send the read bytes in a WebSocket BINARY frame.

Add support for this mode in the pytest "ws_data" client and have all
tests run in 'curl_ws_send/recv' and 'peform' mode as well.

Add `curl_ws_start_frame()`. Document, cover in libcurl-ws.md and
explain the READFUNCTION mode for websockets.

Add example `websocket-updown` for this.

Closes #17683

docs/examples/.gitignore

@@ -19,6 +19,7 @@ ephiperfifo
 evhiperfifo
 externalsocket
 fileupload
+ftp-delete
 ftp-wildcard
 ftpget
 ftpgetinfo
@@ -129,4 +130,5 @@ urlapi
 usercertinmem
 websocket
 websocket-cb
+websocket-updown
 xmlstream

docs/examples/Makefile.inc

@@ -136,7 +136,8 @@ check_PROGRAMS = \
   url2file \
   urlapi \
   websocket \
-  websocket-cb
+  websocket-cb \
+  websocket-updown
 
 # These examples require external dependencies that may not be commonly
 # available on POSIX systems, so do not bother attempting to compile them here.

docs/examples/websocket-updown.c

@@ -0,0 +1,125 @@
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ * Copyright (C) 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.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.
+ *
+ * SPDX-License-Identifier: curl
+ *
+ ***************************************************************************/
+/* <DESC>
+ * WebSocket download-only using write callback
+ * </DESC>
+ */
+#include <stdio.h>
+#include <string.h>
+#include <curl/curl.h>
+
+static size_t writecb(char *b, size_t size, size_t nitems, void *p)
+{
+  CURL *easy = p;
+  size_t i;
+  unsigned int blen = (unsigned int)(nitems * size);
+  const struct curl_ws_frame *frame = curl_ws_meta(easy);
+  fprintf(stderr, "Type: %s\n", frame->flags & CURLWS_BINARY ?
+          "binary" : "text");
+  if(frame->flags & CURLWS_BINARY) {
+    fprintf(stderr, "Bytes: %u", blen);
+    for(i = 0; i < nitems; i++)
+      fprintf(stderr, "%02x ", (unsigned char)b[i]);
+    fprintf(stderr, "\n");
+  }
+  else
+    fprintf(stderr, "Text: %.*s\n", (int)blen, b);
+  return nitems;
+}
+
+struct read_ctx {
+  CURL *easy;
+  char buf[1024];
+  size_t blen;
+  size_t nsent;
+};
+
+static size_t readcb(char *buf, size_t nitems, size_t buflen, void *p)
+{
+  struct read_ctx *ctx = p;
+  size_t len = nitems * buflen;
+  size_t left = ctx->blen - ctx->nsent;
+  CURLcode result;
+
+  if(!ctx->nsent) {
+    /* On first call, set the FRAME information to be used (it defaults
+     * to CURLWS_BINARY otherwise). */
+    result = curl_ws_start_frame(ctx->easy, CURLWS_TEXT,
+                                 (curl_off_t)ctx->blen);
+    if(result) {
+      fprintf(stderr, "error staring frame: %d\n", result);
+      return CURL_READFUNC_ABORT;
+    }
+  }
+  fprintf(stderr, "read(len=%d, left=%d)\n", (int)len, (int)left);
+  if(left) {
+    if(left < len)
+      len = left;
+    memcpy(buf, ctx->buf + ctx->nsent, len);
+    ctx->nsent += len;
+    return len;
+  }
+  return 0;
+}
+
+int main(int argc, const char *argv[])
+{
+  CURL *easy;
+  struct read_ctx rctx;
+  CURLcode res;
+  const char *payload = "Hello, friend!";
+
+  memset(&rctx, 0, sizeof(rctx));
+
+  easy = curl_easy_init();
+  if(!easy)
+    return 1;
+
+  if(argc == 2)
+    curl_easy_setopt(easy, CURLOPT_URL, argv[1]);
+  else
+    curl_easy_setopt(easy, CURLOPT_URL, "wss://example.com");
+
+  curl_easy_setopt(easy, CURLOPT_WRITEFUNCTION, writecb);
+  curl_easy_setopt(easy, CURLOPT_WRITEDATA, easy);
+  curl_easy_setopt(easy, CURLOPT_READFUNCTION, readcb);
+  /* tell curl that we want to send the payload */
+  rctx.easy = easy;
+  rctx.blen = strlen(payload);
+  memcpy(rctx.buf, payload, rctx.blen);
+  curl_easy_setopt(easy, CURLOPT_READDATA, &rctx);
+  curl_easy_setopt(easy, CURLOPT_UPLOAD, 1L);
+
+
+  /* Perform the request, res gets the return code */
+  res = curl_easy_perform(easy);
+  /* Check for errors */
+  if(res != CURLE_OK)
+    fprintf(stderr, "curl_easy_perform() failed: %s\n",
+            curl_easy_strerror(res));
+
+  /* always cleanup */
+  curl_easy_cleanup(easy);
+  return 0;
+}