https://github.com/curl/curl/commit/a33bcc9b53b9d6ea9f879624e8abea0b012b008e
From: Stefan Eissing <stefan@eissing.org>
Date: Fri, 13 Sep 2024 18:36:53 +0200
Subject: [PATCH] transfer: fix sendrecv() without interim poll

There was a "clever" optimization that skipped sendrecv() handling when
the transfer's pollset was empty. This happens for paused transfers, for
example.

Unforunately, if the libcurl application never calls curl_multi_poll(),
the pollset is and will aways remain empty, prevent the transfer from
progressing.

Remove this "optimization" and always try send/receive where applicable.

Fixes #14898
Reported-by: Victor Kislov
Closes #14901
--- a/lib/transfer.c
+++ b/lib/transfer.c
@@ -431,8 +431,6 @@ CURLcode Curl_sendrecv(struct Curl_easy *data, struct curltime *nowp)
     if(select_bits_paused(data, data->state.select_bits)) {
       /* leave the bits unchanged, so they'll tell us what to do when
        * this transfer gets unpaused. */
-      /* DEBUGF(infof(data, "sendrecv, select_bits, early return on PAUSED"));
-      */
       result = CURLE_OK;
       goto out;
     }
@@ -440,16 +438,13 @@ CURLcode Curl_sendrecv(struct Curl_easy *data, struct curltime *nowp)
     /* DEBUGF(infof(data, "sendrecv, select_bits %x, RUN", select_bits)); */
     select_bits = (CURL_CSELECT_OUT|CURL_CSELECT_IN);
   }
-  else if(data->last_poll.num) {
-    /* The transfer wanted something polled. Let's run all available
-     * send/receives. Worst case we EAGAIN on some. */
-    /* DEBUGF(infof(data, "sendrecv, had poll sockets, RUN")); */
+  else {
+    /* try both directions if the transfer wants it. We used to poll
+     * the socket here and on ran send/recv depending on POLLIN/OUT, but
+     * that does not when connections are multiplexed or handshake,
+     * or other direction reversals are happening. */
     select_bits = (CURL_CSELECT_OUT|CURL_CSELECT_IN);
   }
-  else if(data->req.keepon & KEEP_SEND_TIMED) {
-    /* DEBUGF(infof(data, "sendrecv, KEEP_SEND_TIMED, RUN ul")); */
-    select_bits = CURL_CSELECT_OUT;
-  }
 
 #ifdef USE_HYPER
   if(data->conn->datastream) {