ftp: flush pingpong before response

Fix FTP protocol to flush the pingpong's send buffer before receiving a
response from the server, as it may never come otherwise.

Fixes FTP/FTPS tests with `CURL_DBG_SOCK_WBLOCK=90` set.

Closes curl#14452

lib/ftp.c

@@ -819,6 +819,8 @@ CURLcode Curl_GetFTPResponse(struct Curl_easy *data,
   int cache_skip = 0;
   int value_to_be_ignored = 0;
 
+  CURL_TRC_FTP(data, "getFTPResponse start");
+
   if(ftpcode)
     *ftpcode = 0; /* 0 for errors */
   else
@@ -864,21 +866,27 @@ CURLcode Curl_GetFTPResponse(struct Curl_easy *data,
        */
     }
     else if(!Curl_conn_data_pending(data, FIRSTSOCKET)) {
-      switch(SOCKET_READABLE(sockfd, interval_ms)) {
-      case -1: /* select() error, stop reading */
+      curl_socket_t wsock = Curl_pp_needs_flush(data, pp)?
+                            sockfd : CURL_SOCKET_BAD;
+      int ev = Curl_socket_check(sockfd, CURL_SOCKET_BAD, wsock, interval_ms);
+      if(ev < 0) {
         failf(data, "FTP response aborted due to select/poll error: %d",
               SOCKERRNO);
         return CURLE_RECV_ERROR;
-
-      case 0: /* timeout */
+      }
+      else if(ev == 0) {
         if(Curl_pgrsUpdate(data))
           return CURLE_ABORTED_BY_CALLBACK;
         continue; /* just continue in our loop for the timeout duration */
+      }
+    }
 
-      default: /* for clarity */
+    if(Curl_pp_needs_flush(data, pp)) {
+      result = Curl_pp_flushsend(data, pp);
+      if(result)
         break;
-      }
     }
+
     result = ftp_readresp(data, FIRSTSOCKET, pp, ftpcode, &nread);
     if(result)
       break;
@@ -897,6 +905,8 @@ CURLcode Curl_GetFTPResponse(struct Curl_easy *data,
   } /* while there is buffer left and loop is requested */
 
   pp->pending_resp = FALSE;
+  CURL_TRC_FTP(data, "getFTPResponse -> result=%d, nread=%zd, ftpcode=%d",
+               result, *nreadp, *ftpcode);
 
   return result;
 }

lib/pingpong.c

@@ -395,13 +395,23 @@ int Curl_pp_getsock(struct Curl_easy *data,
   return GETSOCK_READSOCK(0);
 }
 
+bool Curl_pp_needs_flush(struct Curl_easy *data,
+                         struct pingpong *pp)
+{
+  (void)data;
+  return pp->sendleft > 0;
+}
+
 CURLcode Curl_pp_flushsend(struct Curl_easy *data,
                            struct pingpong *pp)
 {
   /* we have a piece of a command still left to send */
   size_t written;
   CURLcode result;
 
+  if(!Curl_pp_needs_flush(data, pp))
+    return CURLE_OK;
+
   result = Curl_conn_send(data, FIRSTSOCKET,
                           pp->sendthis + pp->sendsize - pp->sendleft,
                           pp->sendleft, FALSE, &written);

lib/pingpong.h

@@ -137,6 +137,8 @@ CURLcode Curl_pp_readresp(struct Curl_easy *data,
                           int *code, /* return the server code if done */
                           size_t *size); /* size of the response */
 
+bool Curl_pp_needs_flush(struct Curl_easy *data,
+                         struct pingpong *pp);
 
 CURLcode Curl_pp_flushsend(struct Curl_easy *data,
                            struct pingpong *pp);