ftp: treat a 226 arriving before data as a signal to read data

For active mode transfers.

Due to some interesting timing, curl can sometimes get the 226 (transfer
complete) over the control channel first, before the data connection
signals readability. If this happens, use that as a signal to check the
data connection.

Additionally, set the socket filter in listen mode *before* the
PORT/EPRT command is issued, to reduce the risk that the little time gap
could interfere.

This issue never reproduced for me on Debian and takes several hundred
rounds for me to trigger on my mac.

Reported-by: Stefan Eissing
Fixes #12823
Closes #12841

lib/ftp.c

@@ -85,6 +85,14 @@
 #define INET_ADDRSTRLEN 16
 #endif
 
+/* macro to check for a three-digit ftp status code at the start of the
+   given string */
+#define STATUSCODE(line) (ISDIGIT(line[0]) && ISDIGIT(line[1]) &&       \
+                          ISDIGIT(line[2]))
+
+/* macro to check for the last line in an FTP server response */
+#define LASTLINE(line) (STATUSCODE(line) && (' ' == line[3]))
+
 #ifdef CURL_DISABLE_VERBOSE_STRINGS
 #define ftp_pasv_verbose(a,b,c,d)  Curl_nop_stmt
 #endif
@@ -412,8 +420,32 @@ static CURLcode ReceivedServerConnect(struct Curl_easy *data, bool *received)
   }
   if(response) {
     infof(data, "Ctrl conn has data while waiting for data conn");
+    if(pp->overflow > 3) {
+      char *r = Curl_dyn_ptr(&pp->recvbuf);
+
+      DEBUGASSERT((pp->overflow + pp->nfinal) <=
+                  Curl_dyn_len(&pp->recvbuf));
+      /* move over the most recently handled response line */
+      r += pp->nfinal;
+
+      if(LASTLINE(r)) {
+        int status = curlx_sltosi(strtol(r, NULL, 10));
+        if(status == 226) {
+          /* funny timing situation where we get the final message on the
+             control connection before traffic on the data connection has been
+             noticed. Leave the 226 in there and use this as a trigger to read
+             the data socket. */
+          infof(data, "Got 226 before data activity");
+          *received = TRUE;
+          return CURLE_OK;
+        }
+      }
+    }
+
     (void)Curl_GetFTPResponse(data, &nread, &ftpcode);
 
+    infof(data, "FTP code: %03d", ftpcode);
+
     if(ftpcode/100 > 3)
       return CURLE_FTP_ACCEPT_FAILED;
 
@@ -525,14 +557,6 @@ out:
   return result;
 }
 
-/* macro to check for a three-digit ftp status code at the start of the
-   given string */
-#define STATUSCODE(line) (ISDIGIT(line[0]) && ISDIGIT(line[1]) &&       \
-                          ISDIGIT(line[2]))
-
-/* macro to check for the last line in an FTP server response */
-#define LASTLINE(line) (STATUSCODE(line) && (' ' == line[3]))
-
 static bool ftp_endofresp(struct Curl_easy *data, struct connectdata *conn,
                           char *line, size_t len, int *code)
 {
@@ -1179,6 +1203,12 @@ static CURLcode ftp_state_use_port(struct Curl_easy *data,
     conn->bits.ftp_use_eprt = TRUE;
 #endif
 
+  /* Replace any filter on SECONDARY with one listening on this socket */
+  result = Curl_conn_tcp_listen_set(data, conn, SECONDARYSOCKET, &portsock);
+  if(result)
+    goto out;
+  portsock = CURL_SOCKET_BAD; /* now held in filter */
+
   for(; fcmd != DONE; fcmd++) {
 
     if(!conn->bits.ftp_use_eprt && (EPRT == fcmd))
@@ -1252,11 +1282,6 @@ static CURLcode ftp_state_use_port(struct Curl_easy *data,
   /* store which command was sent */
   ftpc->count1 = fcmd;
 
-  /* Replace any filter on SECONDARY with one listening on this socket */
-  result = Curl_conn_tcp_listen_set(data, conn, SECONDARYSOCKET, &portsock);
-  if(result)
-    goto out;
-  portsock = CURL_SOCKET_BAD; /* now held in filter */
   ftp_state(data, FTP_PORT);
 
 out: