when the multi handle was in DO and DOING states, the HTTP and HTTPS protocol

handler functions didn't return that the socket should be waited for writing,
but instead it was treated as if no socket was needing monitoring so REMOVE
was called prematurely

lib/http.c

@@ -111,6 +111,9 @@
  * Forward declarations.
  */
 
+static int http_getsock_do(struct connectdata *conn,
+                           curl_socket_t *socks,
+                           int numsocks);
 static CURLcode https_connecting(struct connectdata *conn, bool *done);
 #ifdef USE_SSL
 static int https_getsock(struct connectdata *conn,
@@ -131,7 +134,7 @@ const struct Curl_handler Curl_handler_http = {
   ZERO_NULL,                            /* connecting */
   ZERO_NULL,                            /* doing */
   ZERO_NULL,                            /* proto_getsock */
-  ZERO_NULL,                            /* doing_getsock */
+  http_getsock_do,                      /* doing_getsock */
   ZERO_NULL,                            /* disconnect */
   PORT_HTTP,                            /* defport */
   PROT_HTTP,                            /* protocol */
@@ -151,7 +154,7 @@ const struct Curl_handler Curl_handler_https = {
   https_connecting,                     /* connecting */
   ZERO_NULL,                            /* doing */
   https_getsock,                        /* proto_getsock */
-  ZERO_NULL,                            /* doing_getsock */
+  http_getsock_do,                      /* doing_getsock */
   ZERO_NULL,                            /* disconnect */
   PORT_HTTPS,                           /* defport */
   PROT_HTTP | PROT_HTTPS | PROT_SSL     /* protocol */
@@ -1726,6 +1729,19 @@ CURLcode Curl_http_connect(struct connectdata *conn, bool *done)
   return CURLE_OK;
 }
 
+/* this returns the socket to wait for in the DO and DOING state for the multi
+   interface and then we're always _sending_ a request and thus we wait for
+   the single socket to become writable only */
+static int http_getsock_do(struct connectdata *conn,
+                           curl_socket_t *socks,
+                           int numsocks)
+{
+  /* write mode */
+  (void)numsocks; /* unused, we trust it to be at least 1 */
+  socks[0] = conn->sock[FIRSTSOCKET];
+  return GETSOCK_WRITESOCK(0);
+}
+
 static CURLcode https_connecting(struct connectdata *conn, bool *done)
 {
   CURLcode result;